Artist's impression of a planet in a far-off system
Planets outside of the Solar System have appeared in fiction
since at least the 1850s, long before the first real ones were
discovered in the 1990s. Most of these fictional planets do not differ
significantly from the Earth and serve only as settings for the narrative. The majority host native lifeforms, sometimes with humans integrated into the ecosystems. Fictional planets that are not Earth-like vary in many different ways. They may have significantly stronger or weaker gravity on their surfaces, or have a particularly hot or cold climate. Both desert planets and ocean planets appear, as do planets with unusual chemical conditions. Various peculiar planetary shapes have been depicted, including flattened, cubic, and toroidal. Some fictional planets exist in multiple-star systems where the orbital mechanics can lead to exotic day–night or seasonal cycles, while others do not orbit any star at all. More fancifully, planets are occasionally portrayed as having sentience, though this is less common than stars receiving the same treatment or a planet's lifeforms having a collective consciousness.
General characteristics
[S]ince information about extrasolar planets remains limited and
incomplete, science fiction writers can freely imagine various sorts of
alien worlds where their heroes might experience different sorts of
adventures and encounter exotic aliens. Indeed, one activity associated with hard science fiction is "world building", meticulously crafting bizarre planets that nonetheless accord with all scientific laws.
Most extrasolar planets in fiction are similar to Earth—referred to in the Star Trek franchise as Class M planets—and serve only as settings for the narrative. One reason for this, writes Stephen L. Gillett [Wikidata] in The Greenwood Encyclopedia of Science Fiction and Fantasy, is to enable satire. Nevertheless, there are also many fictional planets that differ significantly from Earth. Earth-like planets have become less common in fiction following the first detection of an exoplanet around a Sun-like star in 1995, reflecting the scarcity of such worlds among the thousands discovered since. The majority of extrasolar planets in fiction are inhabited by native species, and humans are variously depicted as being integrated into or remaining apart from such alien ecosystems. Some fictional planets are described as orbiting real stars; a 2024 article in the Journal of Science Communication
analysed a sample of 142 fictional exoplanets, of which nearly a third
fulfilled this criterion, and found "an absence of influence of whether
or not the planet setting is in a real star system on other
worldbuilding characteristics".
Schematic diagram of the orbits in a binary star system. One planet is in a P-type, or circumbinary,
orbit around both stars. Another planet is in an S-type, or
circumstellar, orbit around only one of the two stars. Circumbinary
planets are sometimes nicknamed "Tatooine worlds" after the Star Wars planet.
Planets in multiple star systems have attracted attention from science fiction writers, especially in terms of what kind of life would exist on planets with more than one sun and how history might be cyclical as a result of the "long year" that occurs if the orbital period around one of the stars is very lengthy. A particularly early example of this is C. I. Defontenay's 1854 novel Star ou Psi de Cassiopée (English title: Star: Psi Cassiopeia), described by science fiction editorDavid Pringle as "the first detailed evocation of an alien solar system", which depicts various alien species inhabiting the planets orbiting the stars. Isaac Asimov's 1941 short story "Nightfall"
portrays a planet which is in constant daylight from at least one of
its six suns for millennia at a time before a single night of true
darkness, which is a much-anticipated event;the 1963 The Twilight Zone episode "On Thursday We Leave for Home" depicts a planet that is challenging for humans to inhabit due to the unending heat and light from a pair of suns; and Mark Hodder's 2012 novel A Red Sun Also Rises is set on a planet where a dim red sun rises at the same time as the planet's twin white suns set. Hal Clement's 1957 novel Cycle of Fire depicts a planet in a binary star system where the seasons last for decades and different species dominate the hot and cold parts of the year, Poul Anderson's 1974 novel Fire Time
portrays a planet where the majority of the surface becomes
uninhabitable approximately once a millennium when it makes a close
approach to one of its stars and mass migration of the native lifeforms ensues, and Brian Aldiss's 1982–1985 Helliconia trilogy is set on a planet where the orbital mechanics lead to century-long seasons and there are two distinct ecosystems—one adapted to the short period around the closer star and another adapted to the long year around the more distant one. A similar effect appears in Aldiss's 1977 short story "Creatures of Apogee", albeit here as a result of a highly eccentric orbit around a single star where the distance to the star thus varies greatly between the nearest and farthest points in the orbit. The 1985 anthology Medea: Harlan's World is a collaborative effort between Harlan Ellison and several other science fiction writers consisting of several stories set on the same planet in a multiple star system. The 2002 television series Firefly is set in a system of five stars each orbited by its own planetary system, all close enough to each other to permit easy travel between the worlds.
Planets that do not orbit any star, known as rogue planets, appear in several works. In the 1977 novel Dying of the Light by George R. R. Martin, such a planet becomes a temporary tourist destination as it passes by a star before leaving the star's vicinity and becoming uninhabitable again. Hal Clement's 1974 short story "The Logical Life" explores what kind of life could exist on a planet without a star, while the 2002 Star Trek: Enterprise episode "Rogue Planet" depicts how the lifeforms on a world of perpetual night might be exploited by outsiders.
Portraying planets with conditions that differ significantly from
Earth's in terms of physical environment has been a recurring practice
since the middle of the 1900s. Many of these stories imagine how indigenous lifeforms might be adapted to those conditions, with Hal Clement and Poul Anderson being particularly prolific exponents of this craft. The high gravity of Mesklin in Clement's Mission of Gravity thus results in its inhabitants having a centipede-like body structure, while the low gravity yet dense atmosphere in Anderson's 1958 novel War of the Wing-Men (a.k.a. The Man Who Counts) makes it possible for humanoid creatures to fly using their own wings. Desert planets are common; astrophysicist Elizabeth Stanwayposits that this is because the setting strikes the right balance
between novelty and familiarity to most audiences, in addition to the
relative inhospitality providing a survival aspect to the narrative. One of the most prominent examples thereof is Arrakis in Frank Herbert's 1965 novel Dune, where the extreme scarcity of water influences all aspects of the planet's ecology and society. Less extreme desert conditions are found on the Star Wars planet Tatooine, with more plentiful and varied lifeforms as a result. At the other end of the spectrum are planets covered entirely by water, an early example of which appears in Neil R. Jones's 1933 short story "Into the Hydrosphere". Joan Slonczewski's 1986 novel A Door into Ocean is a piece of feminist science fiction set on an ocean world with an all-female population, while Ursula K. Le Guin's 1969 novel The Left Hand of Darkness is set on a frigid world of perpetual winter where the inhabitants do not have a fixed sex. One of the planets in the 2014 film Interstellar is covered by a shallow ocean and orbits so closely around a black hole that there are both tidal waves the height of mountains and extreme time dilation. Other fictional planets differ in their chemical rather than physical environment. Chlorine planets appear in Isaac Asimov's 1951 short story "C-Chute" and the 1976 Space: 1999 episode "The AB Chrysalis", while C. J. Cherryh's 1988 novel Cyteen depicts a planet dominated by silicon-based life whose biochemistry creates byproducts extremely hazardous to human health.
Arecibo Telescope
in Puerto Rico with its 300 m (980 ft) dish was one of the world's
largest filled-aperture (i.e. full dish) radio telescopes and conducted
some SETI searches.
The search for extraterrestrial intelligence (usually shortened as SETI)
is an expression that refers to the diverse efforts and scientific
projects intended to detect extraterrestrial signals, or any evidence of
intelligent life beyond Earth.
Researchers use methods such as monitoring electromagnetic
radiation, searching for optical signals, and investigating potential
extraterrestrial artifacts for any signs of transmission from
civilizations present on other planets. Some initiatives have also attempted to send messages to hypothetical alien civilizations, such as NASA's Golden Record.
Modern SETI research began in the early 20th century after the advent of radio, expanding with projects like Project Ozma, the Wow! signal detection, and the Breakthrough Listen initiative; a $100 million, 10-year attempt to detect signals from nearby stars, announced in 2015 by Stephen Hawking, and Yuri Milner.
Since the 1980s, international efforts have been ongoing, with
community led projects such as SETI@home and Project Argus, engaging in
analyzing data.
While SETI remains a respected scientific field, it often gets compared
to conspiracy theory, UFO research, bringing unawarrented skepticism
from the public, despite its reliance on rigorous scientific methods and
verifiable data and research. Similar studies on Unidentified Aerial Phenomena (UAP) such as the Avi Loeb's Galileo Project have brought further attention to SETI research.
Despite decades of searching, no confirmed evidence of alien
intelligence has been found, bringing criticism onto SETI for being
'overly hopeful'. Critics argue that SETI is speculative and
unfalsifiable, while supporters see it as a crucial step in addressing
the Fermi Paradox and understanding extraterrestrial technosignature.
History
Early work
There have been many earlier searches for extraterrestrial intelligence within the Solar System. In 1896, Nikola Tesla suggested that an extreme version of his wireless electrical transmission system could be used to contact beings on Mars. In 1899, while conducting experiments at his Colorado Springs experimental station,
he thought he had detected a signal from Mars since an odd repetitive
static signal seemed to cut off when Mars set in the night sky. Analysis
of Tesla's research has led to a range of explanations including:
Tesla simply misunderstood the new technology he was working with,
that he may have been observing signals from Marconi's European radio experiments,
and even speculation that he could have picked up naturally occurring radio noise caused by a moon of Jupiter (Io) moving through the magnetosphere of Jupiter.
In the early 1900s, Guglielmo Marconi, Lord Kelvin and David Peck Todd also stated their belief that radio could be used to contact Martians, with Marconi stating that his stations had also picked up potential Martian signals.
On August 21–23, 1924, Mars entered an opposition closer to Earth than at any time in the century before or the next 80 years.
In the United States, a "National Radio Silence Day" was promoted
during a 36-hour period from August 21–23, with all radios quiet for
five minutes on the hour, every hour. At the United States Naval Observatory, a radio receiver was lifted 3 kilometres (1.9 miles) above the ground in a dirigible tuned to a wavelength between 8 and 9 km, using a "radio-camera" developed by Amherst College and Charles Francis Jenkins. The program was led by David Peck Todd with the military assistance of Admiral Edward W. Eberle (Chief of Naval Operations), with William F. Friedman (chief cryptographer of the United States Army), assigned to translate any potential Martian messages.
In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named "Project Ozma" after the Queen of Oz in L. Frank Baum's fantasy books. Drake used a radio telescope 26 metres (85 ft) in diameter at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency, a region of the radio spectrum dubbed the "water hole" due to its proximity to the hydrogen and hydroxyl radical
spectral lines. A 400 kilohertz band around the marker frequency was
scanned using a single-channel receiver with a bandwidth of 100 hertz.
He found nothing of interest.
Soviet scientists took a strong interest in SETI during the 1960s and performed a number of searches with omnidirectional antennas in the hope of picking up powerful radio signals. Soviet astronomer Iosif Shklovsky wrote the pioneering book in the field, Universe, Life, Intelligence (1962), which was expanded upon by American astronomer Carl Sagan as the best-selling book Intelligent Life in the Universe (1966).
In the March 1955 issue of Scientific American, John D. Kraus described an idea to scan the cosmos for natural radio signals using a flat-plane radio telescope equipped with a parabolic reflector. Within two years, his concept was approved for construction by Ohio State University. With a total of US$71,000 (equivalent to $794,880 in 2024)
in grants from the National Science Foundation, construction began on an 8-hectare (20-acre) plot in Delaware, Ohio.
This Ohio State University Radio Observatory telescope was called "Big
Ear". Later, it began the world's first continuous SETI program, called
the Ohio State University SETI program.
In 1971, NASA funded a SETI study that involved Drake, Barney Oliver of Hewlett-Packard Laboratories,
and others. The resulting report proposed the construction of an
Earth-based radio telescope array with 1,500 dishes known as "Project Cyclops". The price tag for the Cyclops array was US$10 billion. Cyclops was not built, but the report formed the basis of much SETI work that followed.
The Ohio State SETI program gained fame on August 15, 1977, when Jerry Ehman,
a project volunteer, witnessed a startlingly strong signal received by
the telescope. He quickly circled the indication on a printout and
scribbled the exclamation "Wow!" in the margin. Dubbed the Wow! signal, it is considered by some to be the best candidate for a radio signal from an artificial, extraterrestrial source ever discovered, but it has not been detected again in several additional searches.
On 24 May 2023, a test extraterrestrial signal, in the form of a
"coded radio signal from Mars", was transmitted to radio telescopes on
Earth, according to a report in The New York Times.
In the early 1980s, Harvard University physicist Paul Horowitz took the next step and proposed the design of a spectrum analyzer
specifically intended to search for SETI transmissions. Traditional
desktop spectrum analyzers were of little use for this job, as they
sampled frequencies using banks of analog filters and so were restricted
in the number of channels they could acquire. However, modern
integrated-circuit digital signal processing (DSP) technology could be used to build autocorrelation
receivers to check far more channels. This work led in 1981 to a
portable spectrum analyzer named "Suitcase SETI" that had a capacity of
131,000 narrow band channels. After field tests that lasted into 1982,
Suitcase SETI was put into use in 1983 with the 26-meter (85 ft)
Harvard/Smithsonian radio telescope at Oak Ridge Observatory in Harvard, Massachusetts. This project was named "Sentinel" and continued into 1985.
Even 131,000 channels were not enough to search the sky in detail
at a fast rate, so Suitcase SETI was followed in 1985 by Project
"META", for "Megachannel Extra-Terrestrial Assay". The META spectrum
analyzer had a capacity of 8.4 million channels and a channel resolution
of 0.05 hertz. An important feature of META was its use of frequency Doppler shift
to distinguish between signals of terrestrial and extraterrestrial
origin. The project was led by Horowitz with the help of the Planetary
Society, and was partly funded by movie maker Steven Spielberg. A second such effort, META II, was begun in Argentina in 1990, to search the southern sky, receiving an equipment upgrade in 1996–1997.
The follow-on to META was named "BETA", for "Billion-channel
Extraterrestrial Assay", and it commenced observation on October 30,
1995. The heart of BETA's processing capability consisted of 63
dedicated fast Fourier transform (FFT) engines, each capable of performing a 222-point complex FFTs in two seconds, and 21 general-purpose personal computers equipped with custom digital signal processing
boards. This allowed BETA to receive 250 million simultaneous channels
with a resolution of 0.5 hertz per channel. It scanned through the
microwave spectrum
from 1.400 to 1.720 gigahertz in eight hops, with two seconds of
observation per hop. An important capability of the BETA search was
rapid and automatic re-observation of candidate signals, achieved by
observing the sky with two adjacent beams, one slightly to the east and
the other slightly to the west. A successful candidate signal would
first transit the east beam, and then the west beam and do so with a
speed consistent with Earth's sidereal
rotation rate. A third receiver observed the horizon to veto signals of
obvious terrestrial origin. On March 23, 1999, the 26-meter radio
telescope on which Sentinel, META and BETA were based was blown over by
strong winds and seriously damaged. This forced the BETA project to cease operation.
MOP and Project Phoenix
Sensitivity
vs range for SETI radio searches. The diagonal lines show transmitters
of different effective powers. The x-axis is the sensitivity of the
search. The y-axis on the right is the range in light-years,
and on the left is the number of Sun-like stars within this range. The
vertical line labeled SS is the typical sensitivity achieved by a full
sky search, such as BETA above. The vertical line labeled TS is the
typical sensitivity achieved by a targeted search such as Phoenix.
In 1978, the NASA SETI program had been heavily criticized by Senator William Proxmire, and funding for SETI research was removed from the NASA budget by Congress in 1981; however, funding was restored in 1982, after Carl Sagan talked with Proxmire and convinced him of the program's value.
In 1992, the U.S. government funded an operational SETI program, in the
form of the NASA Microwave Observing Program (MOP). MOP was planned as a
long-term effort to conduct a general survey of the sky and also carry
out targeted searches of 800 specific nearby stars. MOP was to be
performed by radio antennas associated with the NASA Deep Space Network, as well as the 140-foot (43 m) radio telescope of the National Radio Astronomy Observatory at Green Bank, West Virginia and the 1,000-foot (300 m) radio telescope at the Arecibo Observatory
in Puerto Rico. The signals were to be analyzed by spectrum analyzers,
each with a capacity of 15 million channels. These spectrum analyzers
could be grouped together to obtain greater capacity. Those used in the
targeted search had a bandwidth of 1 hertz per channel, while those used
in the sky survey had a bandwidth of 30 hertz per channel.
MOP drew the attention of the United States Congress, where the program met opposition and canceled one year after its start.[26] SETI advocates continued without government funding, and in 1995 the nonprofit SETI Institute of Mountain View, California
resurrected the MOP program under the name of Project "Phoenix", backed
by private sources of funding. In 2012 it cost around $2 million per
year to maintain SETI research at the SETI Institute and around 10 times
that to support different SETI activities globally. Project Phoenix, under the direction of Jill Tarter, was a continuation of the targeted search program from MOP and studied roughly 1,000 nearby Sun-like stars until approximately 2015. From 1995 through March 2004, Phoenix conducted observations at the 64-meter (210 ft) Parkes radio telescope in Australia, the 140-foot (43 m) radio telescope of the National Radio Astronomy Observatory
in Green Bank, West Virginia, and the 1,000-foot (300 m) radio
telescope at the Arecibo Observatory in Puerto Rico. The project
observed the equivalent of 800 stars over the available channels in the
frequency range from 1200 to 3000 MHz. The search was sensitive enough
to pick up transmitters with 1 GW EIRP to a distance of about 200 light-years.
Ongoing radio searches
Microwave window as seen by a ground based system. From NASA report SP-419: SETI – the Search for Extraterrestrial Intelligence
Many radio frequencies penetrate Earth's atmosphere quite well, and this led to radio telescopes
that investigate the cosmos using large radio antennas. Furthermore,
human endeavors emit considerable electromagnetic radiation as a
byproduct of communications such as television and radio. These signals
would be easy to recognize as artificial due to their repetitive nature
and narrow bandwidths. Earth has been sending radio waves from broadcasts into space for over 100 years. These signals have reached over 1,000 stars, most notably Vega, Aldebaran, Barnard's Star, Sirius, and Proxima Centauri.
If intelligent alien life exists on any planet orbiting these nearby
stars, these signals could be heard and deciphered, even though some of
the signal is garbled by the Earth's ionosphere.
The SETI Institute collaborated with the Radio Astronomy Laboratory at the Berkeley SETI Research Center
to develop a specialized radio telescope array for SETI studies,
similar to a mini-cyclops array. Formerly known as the One Hectare
Telescope (1HT), the concept was renamed the "Allen Telescope Array"
(ATA) after the project's benefactor, Paul Allen.
Its sensitivity is designed to be equivalent to a single large dish
more than 100 meters in diameter, if fully completed. Presently, the array has 42 operational dishes at the Hat Creek Radio Observatory in rural northern California.
The full array (ATA-350) is planned to consist of 350 or more offset-Gregorian
radio dishes, each 6.1 meters (20 feet) in diameter. These dishes are
the largest producible with commercially available satellite television
dish technology. The ATA was planned for a 2007 completion date, at a
cost of US$25 million. The SETI Institute provided money for building
the ATA while University of California, Berkeley designed the telescope
and provided operational funding. The first portion of the array
(ATA-42) became operational in October 2007 with 42 antennas. The DSP
system planned for ATA-350 is extremely ambitious. Completion of the
full 350 element array will depend on funding and the technical results
from ATA-42.
ATA-42 (ATA) is designed to allow multiple observers simultaneous access to the interferometer output at the same time. Typically, the ATA snapshot imager (used for astronomical surveys and SETI) is run in parallel with a beamforming system (used primarily for SETI).
ATA also supports observations in multiple synthesized pencil beams at
once, through a technique known as "multibeaming". Multibeaming provides
an effective filter for identifying false positives in SETI, since a
very distant transmitter must appear at only one point on the sky.
SETI Institute's Center for SETI Research (CSR) uses ATA in the
search for extraterrestrial intelligence, observing 12 hours a day, 7
days a week. From 2007 to 2015, ATA identified hundreds of millions of
technological signals. So far, all these signals have been assigned the
status of noise or radio frequency interference because a) they appear
to be generated by satellites or Earth-based transmitters, or b) they
disappeared before the threshold time limit of ~1 hour.
Researchers in CSR are working on ways to reduce the threshold time
limit, and to expand ATA's capabilities for detection of signals that
may have embedded messages.
Berkeley astronomers used the ATA to pursue several science topics, some of which might have transient SETI signals, until 2011, when the collaboration between the University of California, Berkeley and the SETI Institute was terminated.
CNET published an article and pictures about the Allen Telescope Array (ATA) on December 12, 2008.
In April 2011, the ATA entered an 8-month "hibernation" due to
funding shortfalls. Regular operation of the ATA resumed on December 5,
2011.
In 2012, the ATA was revitalized with a $3.6 million donation by Franklin Antonio, co-founder and Chief Scientist of QUALCOMM Incorporated.
This gift supported upgrades of all the receivers on the ATA dishes to
have (2× to 10× over the range 1–8 GHz) greater sensitivity than before
and supporting observations over a wider frequency range from 1–18 GHz,
though initially the radio frequency electronics only go to 12 GHz. As
of July 2013, the first of these receivers was installed and proven,
with full installation on all 42 antennas being expected for June 2017.
ATA is well suited to the search for extraterrestrial intelligence
(SETI) and to discovery of astronomical radio sources, such as heretofore unexplained non-repeating, possibly extragalactic, pulses known as fast radio bursts or FRBs.
SERENDIP (Search for Extraterrestrial Radio Emissions from Nearby
Developed Intelligent Populations) is a SETI program launched in 1979 by
the Berkeley SETI Research Center.SERENDIP takes advantage of ongoing "mainstream" radio telescope observations as a "piggy-back" or "commensal" program, using large radio telescopes including the NRAO 90m telescope at Green Bank and, formerly, the Arecibo 305m telescope. Rather than having its own observation program, SERENDIP analyzes deep space radio telescope data that it obtains while other astronomers are using the telescopes. The most recently deployed SERENDIP spectrometer, SERENDIP VI, was installed at both the Arecibo Telescope and the Green Bank Telescope in 2014–2015.
Breakthrough Listen
is a ten-year initiative with $100 million funding begun in July 2015
to actively search for intelligent extraterrestrial communications in
the universe, in a substantially expanded way, using resources that had not previously been extensively used for the purpose. It has been described as the most comprehensive search for alien communications to date. The science program for Breakthrough Listen is based at Berkeley SETI Research Center, located in the Astronomy Department at the University of California, Berkeley.
Announced in July 2015, the project is observing for thousands of hours every year on two major radio telescopes, the Green Bank Observatory in West Virginia, and the Parkes Observatory in Australia. Previously, only about 24 to 36 hours of telescope time per year were used in the search for alien life. Furthermore, the Automated Planet Finder at Lick Observatory
is searching for optical signals coming from laser transmissions. The
massive data rates from the radio telescopes (24 GB/s at Green Bank)
necessitated the construction of dedicated hardware at the telescopes to
perform the bulk of the analysis. Some of the data are also analyzed by volunteers in the SETI@home volunteer computing network. Founder of modern SETI Frank Drake was one of the scientists on the project's advisory committee.
In October 2019, Breakthrough Listen started a collaboration with scientists from the TESS team (Transiting Exoplanet Survey Satellite) to look for signs of advanced extraterrestrial life. Thousands of new planets found by TESS will be scanned for technosignatures
by Breakthrough Listen partner facilities across the globe. Data from
TESS monitoring of stars will also be searched for anomalies.
China's 500 meter Aperture Spherical Telescope (FAST) lists detecting interstellar communication signals as part of its science mission. It is funded by the National Development and Reform Commission
(NDRC) and managed by the National Astronomical observatories (NAOC) of
the Chinese Academy of Sciences (CAS). FAST is the first radio
observatory built with SETI as a core scientific goal. FAST consists of a fixed 500 m (1,600 ft) diameter spherical dish constructed in a natural depression sinkhole caused by karst processes in the region. It is the world's largest filled-aperture radio telescope.
According to its website, FAST can search to 28 light-years, and is able
to reach 1,400 stars. If the transmitter's radiated power were to be
increased to 1,000,000 MW, FAST would be able to reach one million
stars. This is compared to the former Arecibo 305 meter telescope
detection distance of 18 light-years.
On 14 June 2022, astronomers, working with China's FAST telescope,
reported the possibility of having detected artificial (presumably
alien) signals, but cautioned that further studies were required to
determine if a natural radio interference may be the source. More recently, on 18 June 2022, Dan Werthimer, chief scientist for several SETI-related
projects, reportedly noted, "These signals are from radio interference;
they are due to radio pollution from earthlings, not from E.T.".
UCLA
Since
2016, University of California Los Angeles (UCLA) undergraduate and
graduate students have been participating in radio searches for
technosignatures with the Green Bank Telescope. Targets include the Kepler field, TRAPPIST-1, and solar-type stars.
The search is sensitive to Arecibo-class transmitters located within
420 light years of Earth and to transmitters that are 1,000 times more
powerful than Arecibo located within 13,000 light years of Earth.
Community SETI projects
Screen shot of the screensaver for SETI@home, a former volunteer computing project in which volunteers donated idle computer power to analyze radio signals for signs of extraterrestrial intelligence.
SETI@home was conceived by David Gedye along with Craig Kasnoff and is a popular volunteer computing project that was launched by the Berkeley SETI Research Center at the University of California, Berkeley, in May 1999. It was originally funded by The Planetary Society and Paramount Pictures, and later by the state of California. The project is run by director David P. Anderson and chief scientist Dan Werthimer. Any individual could become involved with SETI research by downloading the Berkeley Open Infrastructure for Network Computing
(BOINC) software program, attaching to the SETI@home project, and
allowing the program to run as a background process that uses idle
computer power. The SETI@home program itself ran signal analysis on a
"work unit" of data recorded from the central 2.5 MHz wide band of the
SERENDIP IV instrument. After computation on the work unit was complete,
the results were then automatically reported back to SETI@home servers
at University of California, Berkeley. By June 28, 2009, the SETI@home
project had over 180,000 active participants volunteering a total of
over 290,000 computers. These computers gave SETI@home an average
computational power of 617 teraFLOPS. In 2004 radio source SHGb02+14a
set off speculation in the media that a signal had been detected but
researchers noted the frequency drifted rapidly and the detection on
three SETI@home computers fell within random chance.
By 2010, after 10 years of data collection, SETI@home had
listened to that one frequency at every point of over 67 percent of the
sky observable from Arecibo with at least three scans (out of the goal
of nine scans), which covers about 20 percent of the full celestial
sphere.
On March 31, 2020, with 91,454 active users, the project stopped
sending out new work to SETI@home users, bringing this particular SETI
effort to an indefinite hiatus.
SETI Net
SETI Network was the only fully operational private search system.
The SETI Net station consisted of off-the-shelf, consumer-grade
electronics to minimize cost and to allow this design to be replicated
as simply as possible. It had a 3-meter parabolic antenna that could be
directed in azimuth and elevation, an LNA that covered 100 MHz of the
1420 MHz spectrum, a receiver to reproduce the wideband audio, and a
standard personal computer
as the control device and for deploying the detection algorithms. The
antenna could be pointed and locked to one sky location in Ra and DEC
which enabling the system to integrate on it for long periods. The Wow! signal area was monitored for many long periods. All search data was collected and is available on the Internet archive.
SETI Net started operation in the early 1980s as a way to learn
about the science of the search, and developed several software packages
for the amateur SETI community. It provided an astronomical clock, a
file manager to keep track of SETI data files, a spectrum analyzer
optimized for amateur SETI, remote control of the station from the
Internet, and other packages.
SETI Net went dark and was decommissioned on 2021-12-04. The collected data is available on their website.
The SETI League and Project Argus
Founded
in 1994 in response to the United States Congress cancellation of the
NASA SETI program, The SETI League, Incorporated is a
membership-supported nonprofit organization with 1,500 members in 62
countries. This grass-roots alliance of amateur and professional radio
astronomers is headed by executive director emeritus H. Paul Shuch,
the engineer credited with developing the world's first commercial home
satellite TV receiver. Many SETI League members are licensed radio
amateurs and microwave experimenters. Others are digital signal
processing experts and computer enthusiasts.
The SETI League pioneered the conversion of backyard satellite TV
dishes 3 to 5 m (10–16 ft) in diameter into research-grade radio
telescopes of modest sensitivity.
The organization concentrates on coordinating a global network of
small, amateur-built radio telescopes under Project Argus, an all-sky
survey seeking to achieve real-time coverage of the entire sky.
Project Argus was conceived as a continuation of the all-sky survey
component of the late NASA SETI program (the targeted search having been
continued by the SETI Institute's Project Phoenix). There are currently
143 Project Argus radio telescopes operating in 27 countries. Project
Argus instruments typically exhibit sensitivity on the order of 10−23
Watts/square metre, or roughly equivalent to that achieved by the Ohio
State University Big Ear radio telescope in 1977, when it detected the
landmark "Wow!" candidate signal.
The name "Argus" derives from the mythical Greek guard-beast
who had 100 eyes, and could see in all directions at once. In the SETI
context, the name has been used for radio telescopes in fiction (Arthur
C. Clarke, "Imperial Earth"; Carl Sagan, "Contact"),
was the name initially used for the NASA study ultimately known as
"Cyclops," and is the name given to an omnidirectional radio telescope
design being developed at the Ohio State University.
Optical experiments
While
most SETI sky searches have studied the radio spectrum, some SETI
researchers have considered the possibility that alien civilizations
might be using powerful lasers for interstellar communications at optical wavelengths. The idea was first suggested by R. N. Schwartz and Charles Hard Townes in a 1961 paper published in the journal Nature
titled "Interstellar and Interplanetary Communication by Optical
Masers". However, the 1971 Cyclops study discounted the possibility of
optical SETI, reasoning that construction of a laser system that could
outshine the bright central star of a remote star system would be too
difficult. In 1983, Townes published a detailed study of the idea in the
United States journal Proceedings of the National Academy of Sciences, which was met with interest by the SETI community.
There are two problems with optical SETI. The first problem is
that lasers are highly "monochromatic", that is, they emit light only on
one frequency, making it troublesome to figure out what frequency to
look for.
However, emitting light in narrow pulses results in a broad spectrum of
emission; the spread in frequency becomes higher as the pulse width
becomes narrower, making it easier to detect an emission.
The other problem is that while radio transmissions can be
broadcast in all directions, lasers are highly directional. Interstellar
gas and dust is almost transparent to near infrared, so these signals
can be seen from greater distances, but the extraterrestrial laser
signals would need to be transmitted in the direction of Earth in order
to be detected.
Optical SETI supporters have conducted paper studies
of the effectiveness of using contemporary high-energy lasers and a
ten-meter diameter mirror as an interstellar beacon. The analysis shows
that an infrared pulse from a laser, focused into a narrow beam by such a
mirror, would appear thousands of times brighter than the Sun to a
distant civilization in the beam's line of fire. The Cyclops study
proved incorrect in suggesting a laser beam would be inherently hard to
see.
Such a system could be made to automatically steer itself through
a target list, sending a pulse to each target at a constant rate. This
would allow targeting of all Sun-like stars within a distance of 100
light-years. The studies have also described an automatic laser pulse
detector system with a low-cost, two-meter mirror made of carbon
composite materials, focusing on an array of light detectors. This
automatic detector system could perform sky surveys to detect laser
flashes from civilizations attempting contact.
Several optical SETI experiments are now in progress. A
Harvard-Smithsonian group that includes Paul Horowitz designed a laser
detector and mounted it on Harvard's 155-centimeter (61-inch) optical
telescope. This telescope is currently being used for a more
conventional star survey, and the optical SETI survey is "piggybacking"
on that effort. Between October 1998 and November 1999, the survey
inspected about 2,500 stars. Nothing that resembled an intentional laser
signal was detected, but efforts continue. The Harvard-Smithsonian
group is now working with Princeton University
to mount a similar detector system on Princeton's 91-centimeter
(36-inch) telescope. The Harvard and Princeton telescopes will be
"ganged" to track the same targets at the same time, with the intent
being to detect the same signal in both locations as a means of reducing
errors from detector noise.
The Harvard-Smithsonian SETI group led by Professor Paul Horowitz
built a dedicated all-sky optical survey system along the lines of that
described above, featuring a 1.8-meter (72-inch) telescope. The new
optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.
The University of California, Berkeley, home of SERENDIP and SETI@home, is also conducting optical SETI searches and collaborates with the NIROSETI program. The optical SETI program at Breakthrough Listen was initially directed by Geoffrey Marcy, an extrasolar planet hunter, and it involves examination of records of spectra taken during extrasolar planet hunts for a continuous, rather than pulsed, laser signal. This survey uses the Automated Planet Finder 2.4-m telescope at the Lick Observatory, situated on the summit of Mount Hamilton, east of San Jose, California. The other Berkeley optical SETI effort is being pursued by the Harvard-Smithsonian group and is being directed by Dan Werthimer
of Berkeley, who built the laser detector for the Harvard-Smithsonian
group. This survey uses a 76-centimeter (30-inch) automated telescope at
Leuschner Observatory and an older laser detector built by Werthimer.
The SETI Institute also runs a program called 'Laser SETI'
with an instrument composed of several cameras that continuously survey
the entire night sky searching for millisecond singleton laser pulses
of extraterrestrial origin.
In January 2020, two Pulsed All-sky Near-infrared Optical SETI
(PANOSETI) project telescopes were installed in the Lick Observatory
Astrograph Dome. The project aims to commence a wide-field optical SETI
search and continue prototyping designs for a full observatory. The
installation can offer an "all-observable-sky" optical and wide-field
near-infrared pulsed technosignature and astrophysical transient search
for the northern hemisphere.
In May 2017, astronomers reported studies related to laser light
emissions from stars, as a way of detecting technology-related signals
from an alien civilization. The reported studies included Tabby's Star (designated KIC 8462852 in the Kepler Input Catalog),
an oddly dimming star in which its unusual starlight fluctuations may
be the result of interference by an artificial megastructure, such as a Dyson swarm, made by such a civilization. No evidence was found for technology-related signals from KIC 8462852 in the studies.
Quantum communications
In
a 2020 paper, Berera examined sources of decoherence in the
interstellar medium and made the observation that quantum coherence
of photons in certain frequency bands could be sustained to interstellar
distances.
It was suggested this would allow for quantum communication at these
distances.
In a 2021 preprint, astronomer Michael Hipke described for the first time how one could search for quantum communication transmissions sent by ETI
using existing telescope and receiver technology. He also provides
arguments for why future searches of ETI should also target interstellar
quantum communication networks.
A 2022 paper by Arjun Berera and Jaime Calderón-Figueroa noted
that interstellar quantum communication by other civilizations could be
possible and may be advantageous, identifying some potential challenges
and factors for detecting technosignatures. They may, for example, use X-ray photons for remotely established quantum communication and quantum teleportation as the communication mode.
Search for extraterrestrial artifacts
The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960 (see Bracewell probe), and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals (see Voyager Golden Record).
In recognition that any sufficiently advanced interstellar probe in the vicinity of Earth could easily monitor the terrestrial Internet, 'Invitation to ETI' was established by Allen Tough
in 1996, as a Web-based SETI experiment inviting such spacefaring
probes to establish contact with humanity. The project's 100 signatories
includes prominent physical, biological, and social scientists, as well
as artists, educators, entertainers, philosophers and futurists. H. Paul Shuch, executive director emeritus of The SETI League, serves as the project's Principal Investigator.
Inscribing a message in matter and transporting it to an
interstellar destination can be enormously more energy efficient than
communication using electromagnetic waves if delays larger than light
transit time can be tolerated. That said, for simple messages such as "hello," radio SETI could be far more efficient.
If energy requirement is used as a proxy for technical difficulty, then
a solarcentric Search for Extraterrestrial Artifacts (SETA) may be a useful supplement to traditional radio or optical searches.
Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth libration orbits
might therefore constitute the most universally convenient parking
places for automated extraterrestrial spacecraft exploring arbitrary
stellar systems. A viable long-term SETI program may be founded upon a
search for these objects.
In 1979, Freitas and Valdes conducted a photographic search of the vicinity of the Earth-Moon triangular libration points L4 and L5,
and of the solar-synchronized positions in the associated halo orbits,
seeking possible orbiting extraterrestrial interstellar probes, but
found nothing to a detection limit of about 14th magnitude. The authors conducted a second, more comprehensive photographic search for probes in 1982 that examined the five Earth-Moon Lagrangian positions
and included the solar-synchronized positions in the stable L4/L5
libration orbits, the potentially stable nonplanar orbits near L1/L2,
Earth-Moon L3, and also L2
in the Sun-Earth system. Again no extraterrestrial probes were found to
limiting magnitudes of 17–19th magnitude near L3/L4/L5, 10–18th
magnitude for L1/L2, and 14–16th magnitude for Sun-Earth L2.
In June 1983, Valdes and Freitas used the 26 m radiotelescope at
Hat Creek Radio Observatory to search for the tritium hyperfine line at
1516 MHz from 108 assorted astronomical objects, with emphasis on 53
nearby stars including all visible stars within a 20 light-year radius.
The tritium frequency was deemed highly attractive for SETI work because
(1) the isotope is cosmically rare, (2) the tritium hyperfine line is
centered in the SETI water hole
region of the terrestrial microwave window, and (3) in addition to
beacon signals, tritium hyperfine emission may occur as a byproduct of
extensive nuclear fusion
energy production by extraterrestrial civilizations. The wideband- and
narrowband-channel observations achieved sensitivities of 5–14×10−21 W/m2/channel and 0.7–2×10−24 W/m2/channel, respectively, but no detections were made.
Others have speculated, that we might find traces of past civilizations in our very own Solar System, on planets like Venus or Mars, although the traces would be found most likely underground.
Technosignatures, including all signs of technology, are a recent avenue in the search for extraterrestrial intelligence. Technosignatures may originate from various sources, from megastructures such as Dyson spheres and space mirrors or space shaders to the atmospheric contamination created by an industrial civilization, or city lights on extrasolar planets, and may be detectable in the future with large hypertelescopes.
Technosignatures can be divided into three broad categories: astroengineering projects, signals of planetary origin, and spacecraft within and outside the Solar System.
An astroengineering installation such as a Dyson sphere,
designed to convert all of the incident radiation of its host star into
energy, could be detected through the observation of an infrared excess
from a solar analog star, or by the star's apparent disappearance in the visible spectrum over several years.
After examining some 100,000 nearby large galaxies, a team of
researchers has concluded that none of them display any obvious signs of
highly advanced technological civilizations.
Another hypothetical form of astroengineering, the Shkadov thruster, moves its host star by reflecting some of the star's light back on itself, and would be detected by observing if its transits across the star abruptly end with the thruster in front. Asteroid mining within the Solar System is also a detectable technosignature of the first kind.
Individual extrasolar planets can be analyzed for signs of technology. Avi Loeb of the Center for Astrophysics | Harvard & Smithsonian
has proposed that persistent light signals on the night side of an
exoplanet can be an indication of the presence of cities and an advanced
civilization. In addition, the excess infrared radiation and chemicals produced by various industrial processes or terraforming efforts may point to intelligence.
Light and heat detected from planets need to be distinguished
from natural sources to conclusively prove the existence of civilization
on a planet. However, as argued by the Colossus team, a civilization heat signature should be within a "comfortable" temperature range, like terrestrial urban heat islands,
i.e., only a few degrees warmer than the planet itself. In contrast,
such natural sources as wild fires, volcanoes, etc. are significantly
hotter, so they will be well distinguished by their maximum flux at a
different wavelength.
Other than astroengineering, technosignatures such as artificial satellites around exoplanets, particularly such in geostationary orbit, might be detectable even with today's technology and data, and would allow, similar to fossils on Earth, to find traces of extrasolar life from long ago.
Extraterrestrial craft are another target in the search for technosignatures. Magnetic sailinterstellar spacecraft should be detectable over thousands of light-years of distance through the synchrotron radiation they would produce through interaction with the interstellar medium; other interstellar spacecraft designs may be detectable at more modest distances. In addition, robotic probes within the Solar System are also being sought with optical and radio searches.
For a sufficiently advanced civilization, hyper energetic
neutrinos from Planck scale accelerators should be detectable at a
distance of many Mpc.
Advances for Bio and Technosignature Detection
A
notable advancement in technosignature detection is the development of
an algorithm for signal reconstruction in zero-knowledge one-way
communication channels. This algorithm decodes signals from unknown sources without prior knowledge of the encoding scheme, using principles from Algorithmic Information Theory
to identify the geometric and topological dimensions of the encoding
space. It successfully reconstructed the Arecibo message despite
significant noise. The work establishes a connection between syntax and
semantics in SETI and technosignature detection, enhancing fields like cryptography and Information Theory.
Based on fractal theory and the Weierstrass function,
a known fractal, another method authored by the same group called
fractal messaging offers a framework for space-time scale-free
communication. This method leverages properties of self-similarity and
scale invariance, enabling spatio-temporal scale-independent and
parallel infinite-frequency communication. It also embodies the concept
of sending a self-encoding/self-decoding signal as a mathematical
formula, equivalent to self-executable computer code that unfolds to
read a message at all possible time scales and in all possible channels
simultaneously.
Italian physicist Enrico Fermi
suggested in the 1950s that if technologically advanced civilizations
are common in the universe, then they should be detectable in one way or
another. According to those who were there, Fermi either asked "Where
are they?" or "Where is everybody?"
The Fermi paradox is commonly understood as asking why extraterrestrials have not visited Earth,
but the same reasoning applies to the question of why signals from
extraterrestrials have not been heard. The SETI version of the question
is sometimes referred to as "the Great Silence".
The Fermi paradox can be stated more completely as follows:
The size and age of the universe
incline us to believe that many technologically advanced civilizations
must exist. However, this belief seems logically inconsistent with our
lack of observational evidence to support it. Either (1) the initial
assumption is incorrect and technologically advanced intelligent life is
much rarer than we believe, or (2) our current observations are
incomplete, and we simply have not detected them yet, or (3) our search
methodologies are flawed and we are not searching for the correct
indicators, or (4) it is the nature of intelligent life to destroy
itself.
There are multiple explanations proposed for the Fermi paradox, ranging from analyses suggesting that intelligent life is rare (the "Rare Earth hypothesis"),
to analyses suggesting that although extraterrestrial civilizations may
be common, they would not communicate with us, would communicate in a
way we have not discovered yet, could not travel across interstellar
distances, or destroy themselves before they master the technology of
either interstellar travel or communication.
The German astrophysicist and radio astronomer Sebastian von Hoerner suggested
that the average duration of civilization was 6,500 years. After this
time, according to him, it disappears for external reasons (the
destruction of life on the planet, the destruction of only rational
beings) or internal causes (mental or physical degeneration). According
to his calculations, on a habitable planet (one in three million stars)
there is a sequence of technological species over a time distance of
hundreds of millions of years, and each of them "produces" an average of
four technological species. With these assumptions, the average
distance between civilizations in the Milky Way is 1,000 light years.
Science writer Timothy Ferris
has posited that since galactic societies are most likely only
transitory, an obvious solution is an interstellar communications
network, or a type of library consisting mostly of automated systems.
They would store the cumulative knowledge of vanished civilizations and
communicate that knowledge through the galaxy. Ferris calls this the
"Interstellar Internet", with the various automated systems acting as
network "servers". If such an Interstellar Internet exists, the
hypothesis states, communications between servers are mostly through
narrow-band, highly directional radio or laser links. Intercepting such
signals is, as discussed earlier, very difficult. However, the network
could maintain some broadcast nodes in hopes of making contact with new
civilizations.
Although somewhat dated in terms of "information culture"
arguments, not to mention the obvious technological problems of a system
that could work effectively for billions of years and requires multiple
lifeforms agreeing on certain basics of communications technologies,
this hypothesis is actually testable (see below).
Difficulty of detection
A
significant problem is the vastness of space. Despite piggybacking on
the world's most sensitive radio telescope, astronomer and initiator of
SERENDIP Charles Stuart Bowyer noted the then world's largest instrument could not detect random radio noise emanating from a civilization like ours, which has been leaking radio and TV signals for less than 100 years. For SERENDIP
and most other SETI projects to detect a signal from an
extraterrestrial civilization, the civilization would have to be beaming
a powerful signal directly at us. It also means that Earth civilization
will only be detectable within a distance of 100 light-years.
Post-detection disclosure protocol
The International Academy of Astronautics
(IAA) has a long-standing SETI Permanent Study Group (SPSG, formerly
called the IAA SETI Committee), which addresses matters of SETI science, technology, and international policy. The SPSG meets in conjunction with the International Astronautical Congress (IAC),
held annually at different locations around the world, and sponsors two
SETI Symposia at each IAC. In 2005, the IAA established the SETI:
Post-Detection Science and Technology Taskgroup (chairman, Professor Paul Davies)
"to act as a Standing Committee to be available to be called on at any
time to advise and consult on questions stemming from the discovery of a
putative signal of extraterrestrial intelligent (ETI) origin."
However, the protocols mentioned apply only to radio SETI rather than for METI (Active SETI).
The intention for METI is covered under the SETI charter "Declaration
of Principles Concerning Sending Communications with Extraterrestrial
Intelligence".
In October 2000 astronomers Iván Almár and Jill Tarter presented a paper to The SETI Permanent Study Group in Rio de Janeiro, Brazil which proposed a scale (modelled after the Torino scale)
which is an ordinal scale between zero and ten that quantifies the
impact of any public announcement regarding evidence of extraterrestrial
intelligence; the Rio scale has since inspired the 2005 San Marino Scale
(in regard to the risks of transmissions from Earth) and the 2010
London Scale (in regard to the detection of extraterrestrial life). The Rio scale itself was revised in 2018.
The SETI Institute does not officially recognize the Wow! signal
as of extraterrestrial origin as it was unable to be verified, although
in a 2020 Twitter post the organization stated that ''an astronomer
might have pinpointed the host star''. The SETI Institute has also publicly denied that the candidate signal Radio source SHGb02+14a is of extraterrestrial origin. Although other volunteering projects such as Zooniverse credit users for discoveries, there is currently no crediting or early notification by SETI@Home following the discovery of a signal.
Some people, including Steven M. Greer,
have expressed cynicism that the general public might not be informed
in the event of a genuine discovery of extraterrestrial intelligence due
to significant vested interests. Some, such as Bruce Jakosky
have also argued that the official disclosure of extraterrestrial life
may have far reaching and as yet undetermined implications for society,
particularly for the world's religions.
Active SETI,
also known as messaging to extraterrestrial intelligence (METI),
consists of sending signals into space in the hope that they will be
detected by an alien intelligence.
Example
of a high-resolution pictorial message to potential ETI. These messages
usually contain information about the location of the solar system in
the Milky Way.
Whether or not to attempt to contact extraterrestrials has attracted significant academic debate in the fields of space ethics and space policy. Physicist Stephen Hawking, in his book A Brief History of Time,
suggested that "alerting" extraterrestrial intelligences to our
existence is foolhardy, citing humankind's history of treating its own
kind harshly in meetings of civilizations with a significant technology
gap, e.g., the extermination of Tasmanian aborigines. He suggests, in
view of this history, that we "lay low". In one response to Hawking, in September 2016, astronomer Seth Shostak sought to allay such concerns. Astronomer Jill Tarter
also disagrees with Hawking, arguing that aliens developed and
long-lived enough to communicate and travel across interstellar
distances would have evolved a cooperative and less violent
intelligence. She however thinks it is too soon for humans to attempt
active SETI and that humans should be more advanced technologically
first but keep listening in the meantime.
As various SETI projects have progressed, some have criticized early
claims by researchers as being too "euphoric". For example, Peter
Schenkel, while remaining a supporter of SETI projects, wrote in 2006
that:
[i]n light of new findings and
insights, it seems appropriate to put excessive euphoria to rest and to
take a more down-to-earth view[...] We
should quietly admit that the early estimates—that there may be a
million, a hundred thousand, or ten thousand advanced extraterrestrial
civilizations in our galaxy—may no longer be tenable.
Critics claim that the existence of extraterrestrial intelligence has no good Popperian criteria for falsifiability, as explained in a 2009 editorial in Nature, which said:
Seti... has always sat at the edge
of mainstream astronomy. This is partly because, no matter how
scientifically rigorous its practitioners try to be, SETI can't escape
an association with UFO believers and other such crackpots. But it is
also because SETI is arguably not a falsifiable experiment. Regardless
of how exhaustively the Galaxy is searched, the null result of radio
silence doesn't rule out the existence of alien civilizations. It means
only that those civilizations might not be using radio to communicate.
Nature added that SETI was "marked by a hope, bordering on
faith" that aliens were aiming signals at us, that a hypothetical alien
SETI project looking at Earth with "similar faith" would be "sorely
disappointed", despite our many untargeted radar and TV signals, and our
few targeted Active SETI radio signals denounced by those fearing
aliens, and that it had difficulties attracting even sympathetic working
scientists and government funding because it was "an effort so likely
to turn up nothing".
However, Nature also added, "Nonetheless, a small SETI
effort is well worth supporting, especially given the enormous
implications if it did succeed" and that "happily, a handful of wealthy
technologists and other private donors have proved willing to provide
that support".
Supporters of the Rare Earth Hypothesis argue that advanced lifeforms are likely to be very rare, and that, if that is so, then SETI efforts will be futile. However, the Rare Earth Hypothesis itself faces many criticisms.
In 1993, Roy Mash
stated that "Arguments favoring the existence of extraterrestrial
intelligence nearly always contain an overt appeal to big numbers, often
combined with a covert reliance on generalization from a single
instance" and concluded that "the dispute between believers and skeptics
is seen to boil down to a conflict of intuitions which can barely be
engaged, let alone resolved, given our present state of knowledge". In response, in 2012, Milan M. Ćirković, then research professor at the Astronomical Observatory of Belgrade and a research associate of the Future of Humanity Institute at the University of Oxford, said that Mash was unrealistically over-reliant on excessive abstraction that ignored the empirical information available to modern SETI researchers.
George Basalla, Emeritus Professor of History at the University of Delaware,
is a critic of SETI who argued in 2006 that "extraterrestrials
discussed by scientists are as imaginary as the spirits and gods of
religion or myth" and was in turn criticized by Milan M. Ćirković
for, among other things, being unable to distinguish between "SETI
believers" and "scientists engaged in SETI", who are often sceptical
(especially about quick detection), such as Freeman Dyson
and, at least in their later years, Iosif Shklovsky and Sebastian von
Hoerner, and for ignoring the difference between the knowledge
underlying the arguments of modern scientists and those of ancient Greek
thinkers.
Massimo Pigliucci, Professor of Philosophy at CUNY – City College, asked in 2010 whether SETI is "uncomfortably close to the status of pseudoscience" due to the lack of any clear point at which negative results cause the hypothesis of Extraterrestrial Intelligence to be abandoned, before eventually concluding that SETI is "almost-science", which is described by Milan M. Ćirković as Pigliucci putting SETI in "the illustrious company of string theory, interpretations of quantum mechanics, evolutionary psychology and history (of the 'synthetic' kind done recently by Jared Diamond)",
while adding that his justification for doing so with SETI "is weak,
outdated, and reflecting particular philosophical prejudices similar to
the ones described above in Mash and Basalla".
UfologistStanton Friedman has often criticized SETI researchers for, among other reasons, what he sees as their unscientific criticisms of Ufology, but, unlike SETI, Ufology has generally not been embraced by academia as a scientific field of study, and it is usually characterized as a partial or total pseudoscience. In a 2016 interview, Jill Tarter pointed out that it is still a misconception that SETI and UFOs are related.
She states, "SETI uses the tools of the astronomer to attempt to find
evidence of somebody else's technology coming from a great distance. If
we ever claim detection of a signal, we will provide evidence and data
that can be independently confirmed. UFOs—none of the above." The Galileo Project headed by Harvard astronomer Avi Loeb is one of the few scientific efforts to study UFOs or UAPs.
Loeb criticized that the study of UAP is often dismissed and not
sufficiently studied by scientists and should shift from "occupying the
talking points of national security administrators and politicians" to
the realm of science. The Galileo Project's position after the publication of the 2021 UFO Report by the U.S. Intelligence community is that the scientific community
needs to "systematically, scientifically and transparently look for
potential evidence of extraterrestrial technological equipment".
