Ocean planet

From Wikipedia, the free encyclopedia

 

Diagram of the interior of Europa

 

Artist's illustration of a hypothetical ocean planet with two natural satellites

An ocean planet, ocean world, water world, aquaplanet or panthalassic planet is a type of terrestrial planet that contains a substantial amount of water either at its surface or subsurface. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid, such as lava (the case of Io) or ammonia (the case of Titan's inner ocean).

Earth is the only known astronomical object to have bodies of liquid water on its surface, although several exoplanets have been found with the right conditions to support liquid water. For exoplanets, current technology cannot directly observe liquid surface water, so atmospheric water vapor may be used as a proxy. The characteristics of ocean worlds —or ocean planets— provide clues to their history, and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to originate and host life.

Overview

Exoplanets containing water (artist concept; 17 August 2018)

Water worlds are of extreme interest to astrobiologists for their potential to develop life and sustain biological activity over geological timescales. The five best established water worlds in the Solar System include Europa, Enceladus, Ganymede, and Callisto. A host of other bodies in the outer Solar System are inferred by a single type of observation or by theoretical modeling to have subsurface oceans, and these include: Dione, Pluto, Triton, and Ceres, as well as Mimas, Eris, and Oberon.

History

Important preliminary theoretical work was carried prior to the planetary missions launched starting in the 1970s. In particular, Lewis showed in 1971 that radioactive decay alone was likely sufficient to produce subsurface oceans in large moons, especially if ammonia (NH
₃) was present. Peale and Cassen figured out in 1979 the important role of tidal heating (aka: tidal flexing) on satellite evolution and structure. The first confirmed detection of an exoplanet was in 1992. Alain Léger et al figured in 2004 that a small number of icy planets that form in the region beyond the snow line can migrate inward to ∼1 AU, where the outer layers subsequently melt.

The cumulative evidence collected by the Hubble Space Telescope, as well as Pioneer, Galileo, Voyager, Cassini–Huygens, and New Horizons missions, strongly indicate that several outer Solar System bodies harbour internal liquid water oceans under an insulating ice shell. Meanwhile, the Kepler space observatory, launched in March 7, 2009, has discovered thousands of exoplanets, about 50 of them of Earth-size in or near habitable zones.

Planets of almost all masses, sizes, and orbits have been detected, illustrating not only the variable nature of planet formation but also a subsequent migration through the circumstellar disc from the planet's place of origin. As of 1 September 2018, there are 3,823 confirmed planets in 2,860 systems, with 632 systems having more than one planet.

Formation

Atacama Large Millimeter Array image of HL Tauri, a protoplanetary disk

Planetary objects that form in the outer Solar System begin as a comet-like mixture of roughly half water and half rock by mass, displaying a density lower than that of rocky planets. Icy planets and moons that form near the frost line should contain mostly H
₂O and silicates. Those that form farther out can acquire ammonia (NH
₃) and methane (CH
₄) as hydrates, together with CO, N
₂, and CO
₂.

Planets that form prior to the dissipation of the gaseous circumstellar disk experience strong torques that can induce rapid inward migration into the habitable zone, especially for planets in the terrestrial mass range.^] Since water is highly soluble in magma, a large fraction of the planet's water content will initially be trapped in the mantle. As the planet cools and the mantle begins to solidify from the bottom up, large amounts of water (between 60% and 99% of the total amount in the mantle) are exsolved to form a steam atmosphere, which may eventually condense to form an ocean. Ocean formation requires differentiation, and a heat source, either radioactive decay, tidal heating, or the early luminosity of the parent body. Unfortunately, the initial conditions following accretion are theoretically incomplete.

Planets that formed in the outer, water-rich regions of a disk and migrated inward are more likely to have abundant water. Conversely, planets that formed close to their host stars are less likely to have water because the primordial disks of gas and dust are thought to have hot and dry inner regions. So if a water world is found close to a star, it would be strong evidence for migration and ex situ formation, because insufficient volatiles exist near the star for in situ formation. Simulations of Solar System formation and of extra-solar system formation have shown that planets are likely to migrate inward (i.e., toward the star) as they form. Outward migration may also occur under particular conditions. Inward migration presents the possibility that icy planets could move to orbits where their ice melts into liquid form, turning them into ocean planets. This possibility was first discussed in the astronomical literature by Marc Kuchner and Alain Léger in 2004.

Structure

The internal structure of an icy astronomical body is generally deduced from measurements of its bulk density, gravity moments, and shape. Determining the moment of inertia of a body can help assess whether it has undergone differentiation (separation into rock-ice layers) or not. Shape or gravity measurements can in some cases be used to infer the moment of inertia – if the body is in hydrostatic equilibrium (i.e. behaving like a fluid on long timescales). However, proving that a body is in hydrostatic equilibrium is extremely difficult, but by using a combination of shape and gravity data, the hydrostatic contributions can be deduced. Specific techniques to detect inner oceans include magnetic induction, geodesy, librations, axial tilt, tidal response, radar sounding, compositional evidence, and surface features.

Artist's cut-away representation of the internal structure of Ganymede, with a liquid water ocean "sandwiched" between two ice layers. Layers drawn to scale.

A generic icy moon will consist of a water layer sitting atop a silicate core. For a small satellite like Enceladus, an ocean will sit directly above the silicates and below a solid icy shell, but for a larger ice-rich body like Ganymede, pressures are sufficiently high that the ice at depth will transform to higher pressure phases, effectively forming a "water sandwich" with an ocean located between ice shells. An important difference between these two cases is that for the small satellite the ocean is in direct contact with the silicates, which may provide hydrothermal and chemical energy and nutrients to simple life forms. Because of the varying pressure at depth, models of a water world may include "steam, liquid, superfluid, high-pressure ices, and plasma phases" of water. Some of the solid-phase water could be in the form of ice VII.

Maintaining a subsurface ocean depends on the rate of internal heating compared with the rate at which heat is removed, and the freezing point of the liquid. Ocean survival and tidal heating are thus intimately linked.

Smaller ocean planets would have less dense atmospheres and lower gravity; thus, liquid could evaporate much more easily than on more massive ocean planets. Simulations suggest that planets and satellites of less than one Earth mass could have liquid oceans driven by hydrothermal activity, radiogenic heating, or tidal flexing. Where fluid-rock interactions propagate slowly into a deep brittle layer, thermal energy from serpentinization may be the primary cause of hydrothermal activity in small ocean planets. The dynamics of global oceans beneath tidally flexing ice shells represents a significant set of challenges which have barely begun to be explored. The extent to which cryovolcanism occurs is a subject of some debate, as water, being denser than ice by about 8%, has difficulty erupting under normal circumstances.

Atmospheric models

To allow water to be liquid for long periods of time, a planet —or moon— must orbit within the habitable zone (HZ), possess a protective magnetic field, and have the gravitational pull needed to retain an ample amount of atmospheric pressure. If the planet gravity cannot sustain that, then all the water will eventually evaporate into the outer space. A strong planetary magnetosphere, maintained by internal dynamo action in an electrically conducting fluid layer, is helpful for shielding the upper atmosphere from stellar wind mass loss and retaining water over long geological time scales.

A planet's atmosphere forms from outgassing during planet formation or is gravitationally captured from the surrounding protoplanetary nebula. The surface temperature on an exoplanet is governed by the atmosphere's greenhouse gases (or lack thereof), so an atmosphere can be detectable in the form of upwelling infrared radiation because the greenhouse gases absorb and re-radiate energy from the host star. Ice-rich planets that have migrated inward into orbit too close to their host stars may develop thick steamy atmospheres but still retain their volatiles for billions of years, even if their atmospheres undergo slow hydrodynamic escape. Ultraviolet photons are not only biologically harmful but can drive fast atmospheric escape that leads to the erosion of planetary atmospheres; photolysis of water vapor, and hydrogen/oxygen escape to space can lead to the loss of several Earth oceans of water from planets throughout the habitable zone, regardless of whether the escape is energy-limited or diffusion-limited. The amount of water lost seems proportional with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to the planet surface gravity.
During a runaway greenhouse effect, water vapor reaches the stratosphere, where it is easily broken down (photolyzed) by ultraviolet radiation (UV). Heating of the upper atmosphere by UV radiation can then drive a hydrodynamic wind that carries the hydrogen (and potentially some of the oxygen) to space, leading to the irreversible loss of a planet's surface water, oxidation of the surface, and possible accumulation of oxygen in the atmosphere. The fate of a given planet's atmosphere strongly depends on the extreme ultraviolet flux, the duration of the runaway regime, the initial water content, and the rate at which oxygen is absorbed by the surface. Volatile-rich planets should be more common in the habitable zones of young stars and M-type stars.

Composition models

There are challenges in examining an exoplanetary surface and its atmosphere, as cloud coverage influences the atmospheric temperature, structure as well as the observability of spectral features. However, planets composed of large quantities of water that reside in the habitable zone (HZ) are expected to have distinct geophysics and geochemistry of their surface and atmosphere. For example, in the case of exoplanets Kepler-62e and -62f, they could possess a liquid ocean outer surface, a steam atmosphere, or a full cover of surface Ice I, depending on their orbit within the HZ and the magnitude of their greenhouse effect. Several other surface and interior processes affect the atmospheric composition, including but not limited to the ocean fraction for dissolution of CO
₂ and for atmospheric relative humidity, redox state of the planetary surface and interior, acidity levels of the oceans, planetary albedo, and surface gravity.

The atmospheric structure, as well as the resulting HZ limits, depend on the density of a planet's atmosphere, shifting the HZ outward for lower mass and inward for higher mass planets. Theory, as well as computer models suggest that atmospheric composition for water planets in the habitable zone (HZ) should not differ substantially from those of land-ocean planets. For modeling purposes, it is assumed that the initial composition of icy planetesimals that assemble into water planets is similar to that of comets: mostly water (H
₂O), and some ammonia (NH
₃), and carbon dioxide (CO
₂). An initial composition of ice similar to that of comets leads to an atmospheric model composition of 90% H
₂O, 5% NH
₃, and 5% CO
₂.

Atmospheric models for Kepler-62f show that an atmospheric pressure of between 1.6 bar and 5 bar of CO
₂ are needed to warm the surface temperature above freezing, leading to a scaled surface pressure of 0.56–1.32 times Earth's.

Exoplanets

Artist's illustration of a hypothetical ocean planet with two natural satellites

Outside the Solar System, Kepler-11, GJ 1214 b, Kepler-22b, Kepler-62f, Kepler-62e and the planets of TRAPPIST-1 are some of most likely known candidates for an extrasolar ocean planet. Many more such objects are expected to be discovered by Kepler.

Although 70.8% of all Earth's surface is covered in water, water accounts for only 0.05% of Earth's mass. An extraterrestrial ocean's depth would be so deep and dense that even at high temperatures the pressure would turn the water into ice. The immense pressures in the lower regions of these oceans could lead to the formation of a mantle of exotic forms of ice such as ice V. This ice would not necessarily be as cold as conventional ice. If the planet is close enough to its star that the water reaches its boiling point, the water will become supercritical and lack a well-defined surface. Even on cooler water-dominated planets, the atmosphere can be much thicker than that of Earth, and composed largely of water vapor, producing a very strong greenhouse effect. Such planets would have to be small enough not to be able to retain a thick envelope of hydrogen and helium, or be close enough to their primary star to be stripped of these light elements. Otherwise, they would form a warmer version of an ice giant instead, like Uranus and Neptune.

Astrobiology

The characteristics of ocean worlds or ocean planets provide clues to their history, and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to form and host life. Life as we know it requires liquid water, a source of energy and nutrients, and all three key requirements can potentially be satisfied within some of these bodies, that may offer the possibility for sustaining simple biological activity over geological timescales. In August 2018, researchers reported that water worlds could support life.

An ocean world's habitation by Earth-like life is limited if the planet is completely covered by liquid water at the surface, even more restricted if a pressurized, solid ice layer is located between the global ocean and the lower rocky mantle. Simulations of a hypothetical ocean world covered by 5 Earth oceans' worth of water indicate the water would not contain enough phosphorus and other nutrients for Earth like oxygen-producing ocean organisms such as plankton to evolve. On Earth, phosphorus is washed into the oceans by rainwater hitting rocks on exposed land so the mechanism would not work on an ocean world. Simulations of ocean planets with 50 Earth oceans' worth of water indicate the pressure on the sea floor would be so immense that the planet's interior would not sustain plate tectonics, volcanism to provide the right chemical environment for terrestrial life.

On the other hand, small bodies such as Europa and Enceladus are regarded as particularly habitable environments because their oceans are in direct contact with the underlying silicate core, a potential source of both heat, and biologically important chemical elements. The surface geological activity of these bodies may also lead to the transport to the oceans of biologically-important building blocks implanted at the surface, such as organic molecules from comets or tholins —formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane, often in combination with nitrogen.

Oxygen

Molecular oxygen (O
₂) can be produced by geophysical processes, as well as a byproduct of photosynthesis by life forms, so although encouraging, O
₂ is not a reliable biosignature. In fact, planets with high concentration of O
₂ in their atmosphere may be uninhabitable. Abiogenesis in the presence of massive amounts of atmospheric oxygen could be difficult because early organisms relied on the free energy available in redox reactions involving a variety of hydrogen compounds; on an O
₂-rich planet, organisms would have to compete with the oxygen for this free energy.

Super-Earth

From Wikipedia, the free encyclopedia

 

Illustration of the inferred size of the super-Earth COROT-7b (center) in comparison with Earth and Neptune

A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 15 and 17 times Earth's, respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, as suggested by MIT professor Sara Seager, although "mini-Neptunes" is a more common term.

Definition

Artist’s impression of the super-Earth exoplanet LHS 1140b.

 

In general, super-Earths are defined by their masses, and the term does not imply temperatures, compositions, orbital properties, habitability, or environments. While sources generally agree on an upper bound of 10 Earth masses (~69% of the mass of Uranus, which is the Solar System's giant planet with the least mass), the lower bound varies from 1 or 1.9 to 5, with various other definitions appearing in the popular media. The term "super-Earth" is also used by astronomers to refer to planets bigger than Earth-like planets (from 0.8 to 1.25 Earth-radii), but smaller than mini-Neptunes (from 2 to 4 Earth-radii). This definition was made by the Kepler Mission. Some authors further suggest that the term Super-Earth might be limited to rocky planets without a significant atmosphere, or planets that have not just atmospheres but also solid surfaces or oceans with a sharp boundary between liquid and atmosphere, which the four giant planets in the Solar System do not have. Planets above 10 Earth masses are termed massive solid planets/mega-Earths or gas giant planets depending on whether they are mostly rock and ice or mostly gas.

Discoveries

Illustration of the inferred size of the super-Earth Kepler-10b (right) in comparison with Earth

First

Sizes of Kepler Planet Candidates – based on 2,740 candidates orbiting 2,036 stars as of November 4, 2013 (NASA)

The first super-Earths were discovered by Aleksander Wolszczan and Dale Frail around the pulsar PSR B1257+12 in 1992. The two outer planets of the system have masses approximately four times Earth—too small to be gas giants.

The first super-Earth around a main-sequence star was discovered by a team under Eugenio Rivera in 2005. It orbits Gliese 876 and received the designation Gliese 876 d (two Jupiter-sized gas giants had previously been discovered in that system). It has an estimated mass of 7.5 Earth masses and a very short orbital period of just about 2 days. Due to the proximity of Gliese 876 d to its host star (a red dwarf), it may have a surface temperature of 430–650 kelvin and too hot to support liquid water.^[17]

First in habitable zone

In April 2007, a team headed by Stéphane Udry based in Switzerland announced the discovery of two new super-Earths within the Gliese 581 planetary system, both on the edge of the habitable zone around the star where liquid water may be possible on the surface. With Gliese 581c having a mass of at least 5 Earth masses and a distance from Gliese 581 of 0.073 astronomical units (AU; 6.8 million mi, 11 million km), it is on the "warm" edge of the habitable zone around Gliese 581 with an estimated mean temperature (without taking into consideration effects from an atmosphere) of −3 degrees Celsius with an albedo comparable to Venus and 40 degrees Celsius with an albedo comparable to Earth. Subsequent research suggested Gliese 581c had likely suffered a runaway greenhouse effect like Venus.

Mass and radius values for transiting super-Earths in context of other detected exoplanets and selected composition models. The "Fe" line defines planets made purely of iron, and "H₂O" for those made of water. Those between the two lines, and closer to the Fe line, are most likely solid rocky planets, while those near or above the water line are more likely gas and/or liquid. Planets in the Solar System are on the chart, labeled with their astronomical symbols.

Others by year

2006

Two further super-Earths were discovered in 2006: OGLE-2005-BLG-390Lb with a mass of 5.5 Earth masses, which was found by gravitational microlensing, and HD 69830 b with a mass of 10 Earth masses.

2008

The smallest super-Earth found as of 2008 was MOA-2007-BLG-192Lb. The planet was announced by astrophysicist David P. Bennett for the international MOA collaboration on June 2, 2008. This planet has approximately 3.3 Earth masses and orbits a brown dwarf. It was detected by gravitational microlensing.

In June 2008, European researchers announced the discovery of three super-Earths around the star HD 40307, a star that is only slightly less massive than our Sun. The planets have at least the following minimum masses: 4.2, 6.7, and 9.4 times Earth's. The planets were detected by the radial velocity method by the HARPS (High Accuracy Radial Velocity Planet Searcher) in Chile.

In addition, the same European research team announced a planet 7.5 times the mass of Earth orbiting the star HD 181433. This star also has a Jupiter-like planet that orbits every three years.

2009

Planet COROT-7b, with a mass estimated at 4.8 Earth masses and an orbital period of only 0.853 days, was announced on 3 February 2009. The density estimate obtained for COROT-7b points to a composition including rocky silicate minerals, similar to the four inner planets of the Solar System, a new and significant discovery. COROT-7b, discovered right after HD 7924 b, is the first super-Earth discovered that orbits a main sequence star that is G class or larger.

The discovery of Gliese 581e with a minimum mass of 1.9 Earth masses was announced on 21 April 2009. It was at the time the smallest extrasolar planet discovered around a normal star and the closest in mass to Earth. Being at an orbital distance of just 0.03 AU and orbiting its star in just 3.15 days, it is not in the habitable zone, and may have 100 times more tidal heating than Jupiter's volcanic satellite Io.

A planet found in December 2009, GJ 1214 b, is 2.7 times as large as Earth and orbits a star much smaller and less luminous than our Sun. "This planet probably does have liquid water," said David Charbonneau, a Harvard professor of astronomy and lead author of an article on the discovery. However, interior models of this planet suggest that under most conditions it does not have liquid water.

By November 2009, a total of 30 super-Earths had been discovered, 24 of which were first observed by HARPS.

2010

Discovered on 5 January 2010, a planet HD 156668 b with a minimum mass of 4.15 Earth masses, is the second least massive planet detected by the radial velocity method. The only confirmed radial velocity planet smaller than this planet is Gliese 581e at 1.9 Earth masses (see above). On 24 August, astronomers using ESO's HARPS instrument announced the discovery of a planetary system with up to seven planets orbiting a Sun-like star, HD 10180, one of which, although not yet confirmed, has an estimated minimum mass of 1.35 ± 0.23 times that of Earth, which would be the lowest mass of any exoplanet found to date orbiting a main-sequence star. Although unconfirmed, there is 98.6% probability that this planet does exist.

The National Science Foundation announced on 29 September the discovery of a fourth super-Earth (Gliese 581g) orbiting within the Gliese 581 planetary system. The planet has a minimum mass 3.1 times that of Earth and a nearly circular orbit at 0.146 AU with a period of 36.6 days, placing it in the middle of the habitable zone where liquid water could exist and midway between the planets c and d. It was discovered using the radial velocity method by scientists at the University of California at Santa Cruz and the Carnegie Institution of Washington. However, the existence of Gliese 581 g has been questioned by another team of astronomers, and it is currently listed as unconfirmed at The Extrasolar Planets Encyclopaedia.

2011

On 2 February, the Kepler Space Observatory Mission team released a list of 1235 extrasolar planet candidates, including 68 candidates of approximately "Earth-size" (Rp < 1.25 Re) and 288 candidates of "super-Earth-size" (1.25 Re < Rp < 2 Re). In addition, 54 planet candidates were detected in the "habitable zone." Six candidates in this zone were less than twice the size of the Earth [namely: KOI 326.01 (Rp=0.85), KOI 701.03 (Rp=1.73), KOI 268.01 (Rp=1.75), KOI 1026.01 (Rp=1.77), KOI 854.01 (Rp=1.91), KOI 70.03 (Rp=1.96) – Table 6] A more recent study found that one of these candidates (KOI 326.01) is in fact much larger and hotter than first reported. Based on the latest Kepler findings, astronomer Seth Shostak estimates "within a thousand light-years of Earth" there are "at least 30,000 of these habitable worlds." Also based on the findings, the Kepler Team has estimated "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.

On 17 August, a potentially habitable super-Earth HD 85512 b was found using the HARPS as well as a three super-Earth system 82 G. Eridani.^[42] On HD 85512 b, it would be habitable if it exhibits more than 50% cloud cover. Then less than a month later, a flood of 41 new exoplanets including 10 super-Earths were announced.

On 5 December 2011, the Kepler space telescope discovered its first planet within the habitable zone or "Goldilocks region" of its Sun-like star. Kepler-22b is 2.4 times the radius of the earth and occupies an orbit 15% closer to its star than the Earth to the Sun. This is compensated for however, as the star, with a spectral type G5V is slightly dimmer than the Sun (G2V), and thus the surface temperatures would still allow liquid water on its surface.

On 5 December 2011, the Kepler team announced that they had discovered 2,326 planetary candidates, of which 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Compared to the February 2011 figures, the number of Earth-size and super-Earth-size planets increased by 200% and 140% respectively. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars, marking a decrease from the February figure; this was due to the more stringent criteria in use in the December data.

Artist's impression of 55 Cancri e in front of its parent star.

 

On 2011, a density of 55 Cancri e was calculated which turned out to be similar to Earth's. At the size of about 2 Earth radii, it was the largest planet until 2014 which was determined to lack a significant hydrogen atmosphere.

On 20 December 2011, the Kepler team announced the discovery of the first Earth-size exoplanets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20.

Planet Gliese 667 Cb (GJ 667 Cb) was announced by HARPS on 19 October 2009, together with 29 other planets, while Gliese 667 Cc (GJ 667 Cc) was included in a paper published on 21 November 2011. More detailed data on Gliese 667 Cc were published in early February 2012.

2012

In September 2012, the discovery of two planets orbiting Gliese 163 was announced. One of the planets, Gliese 163 c, about 6.9 times the mass of Earth and somewhat hotter, was considered to be within the habitable zone.

2013

On 7 January 2013, astronomers from the Kepler Mission space observatory announced the discovery of Kepler-69c (formerly KOI-172.02), an Earth-like exoplanet candidate (1.5 times the radius of Earth) orbiting a star similar to our Sun in the habitable zone and possibly a "prime candidate to host alien life".

In April 2013, using observations by NASA's Kepler Mission, a team led by William Borucki, of the agency's Ames Research Center, found five planets orbiting in the habitable zone of a Sun-like star, Kepler-62, 1,200 light years from Earth. These new super-Earths have radii of 1.3, 1.4, 1.6, and 1.9 times that of Earth. Theoretical modelling of two of these super-Earths, Kepler-62e and Kepler-62f, suggests both could be solid, either rocky or rocky with frozen water.

On 25 June 2013 Three "super Earth" planets have been found orbiting a nearby star at a distance where life in theory could exist, according to a record-breaking tally announced on Tuesday by the European Southern Observatory. They are part of a cluster of as many as seven planets that circle Gliese 667C, one of three stars located a relatively close 22 light years from Earth in the constellation of Scorpio, it said. The planets orbit Gliese 667C in the so-called Goldilocks Zone — a distance from the star at which the temperature is just right for water to exist in liquid form rather than being stripped away by stellar radiation or locked permanently in ice.

2014

In May 2014, previously discovered Kepler-10c was determined to have the mass comparable to Neptune (17 Earth masses). With the radius of 2.35, it is currently the largest known planet likely to have a predominantly rocky composition. At 17 Earth masses it is well above the 10 Earth mass upper limit that is commonly used for the term 'super-Earth' so the term mega-Earth has been proposed.

2015

On 6 January 2015, NASA announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related stars: two of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky; the third, Kepler-440b, is a super-Earth.

On 30 July 2015, Astronomy & Astrophysics said they found a planetary system with three super-Earths orbiting a bright, dwarf star. The four-planet system, dubbed HD 219134, had been found 21 light years from Earth in the M-shaped northern hemisphere of constellation Cassiopeia, but it is not in the habitable zone of its star. The planet with the shortest orbit is HD 219134 b, and is Earth's closest known rocky, and transiting, exoplanet.

2016

In February 2016, it was announced that NASA's Hubble Space Telescope had detected hydrogen and helium (and suggestions of hydrogen cyanide), but no water vapor, in the atmosphere of 55 Cancri e, the first time the atmosphere of a super-Earth exoplanet was analyzed successfully.
In August 2016, astronomers announce the detection of Proxima b, an Earth-sized exoplanet that is in the habitable zone of the red dwarf star Proxima Centauri, the closest star to the Sun. Due to its closeness to Earth, Proxima b may be a flyby destination for a fleet of interstellar StarChip spacecrafts currently being developed by the Breakthrough Starshot project.

2018

In February 2018, K2-141b, a rocky ultra-short period planet (USP) Super-Earth, with a period of 0.28 days orbiting the host star K2-141 (EPIC 246393474) was reported. Another Super-Earth, K2-155d, is discovered.

Planet Nine

The Solar System contains no known super-Earths, because Earth is the largest terrestrial planet in the Solar System, and all larger planets both have at least 14 times the mass of Earth and thick gaseous atmospheres without well-defined rocky or watery surfaces; that is, they are either gas giants or ice giants, not terrestrial planets. In January 2016, the existence of a hypothetical super-Earth-mass ninth planet in the Solar System, referred to as Planet Nine, was proposed as an explanation for the orbital behavior of six trans-Neptunian objects, but it is speculated to be instead an ice giant like Uranus or Neptune.

Characteristics

Comparison of sizes of planets with different compositions

Density and bulk composition

Due to the larger mass of super-Earths, their physical characteristics may differ from Earth's; theoretical models for super-Earths provide four possible main compositions according to their density: low-density super-Earths are inferred to be composed mainly of hydrogen and helium (mini-Neptunes); super-Earths of intermediate density are inferred to either have water as a major constituent (ocean planets), or have a denser core enshrouded with an extended gaseous envelope (gas dwarf or sub-Neptune). A super-Earth of high density is believed to be rocky and/or metallic, like Earth and the other terrestrial planets of the Solar System. A super-Earth's interior could be undifferentiated, partially differentiated, or completely differentiated into layers of different composition. Researchers at Harvard Astronomy Department have developed user-friendly online tools to characterize the bulk composition of the super-Earths. A study on Gliese 876 d by a team around Diana Valencia revealed that it would be possible to infer from a radius measured by the transit method of detecting planets and the mass of the relevant planet what the structural composition is. For Gliese 876 d, calculations range from 9,200 km (1.4 Earth radii) for a rocky planet and very large iron core to 12,500 km (2.0 Earth radii) for a watery and icy planet. Within this range of radii the super-Earth Gliese 876 d would have a surface gravity between 1.9g and 3.3g (19 and 32 m/s²). However, this planet is not known to transit its host star.

The limit between rocky planets and planets with a thick gaseous envelope is calculated with theoretical models. Calculating the effect of the active XUV saturation phase of G-type stars over the loss of the primitive nebula-captured hydrogen envelopes in extrasolar planets, it's obtained that planets with a core mass of more than 1.5 Earth-mass (1.15 Earth-radius max.), most likely cannot get rid of their nebula captured hydrogen envelopes during their whole lifetime. Other calculations point out that the limit between envelope-free rocky super-Earths and sub-Neptunes is around 1.75 Earth-radii, as 2 Earth-radii would be the upper limit to be rocky (a planet with 2 Earth-radii and 5 Earth-masses with a mean Earth-like core composition would imply that 1/200 of its mass would be in a H/He envelope, with an atmospheric pressure near to 2.0 GPa or 20,000 bar). Whether or not the primitive nebula-captured H/He envelope of a super-Earth is entirely lost after formation also depends on the orbital distance. For example, formation and evolution calculations of the Kepler-11 planetary system show that the two innermost planets Kepler-11b and c, whose calculated mass is ≈2 M_⊕ and between ≈5 and 6 M_⊕ respectively (which are within measurement errors), are extremely vulnerable to envelope loss. In particular, the complete removal of the primordial H/He envelope by energetic stellar photons appears almost inevitable in the case of Kepler-11b, regardless of its formation hypothesis.

If a super-Earth is detectable by both the radial-velocity and the transit methods, then both its mass and its radius can be determined; thus its average bulk density can be calculated. The actual empirical observations are giving similar results as theoretical models, as it's found that planets larger than approximately 1.6 Earth-radius (more massive than approximately 6 Earth-masses) contain significant fractions of volatiles or H/He gas (such planets appear to have a diversity of compositions that is not well-explained by a single mass-radius relation as that found in rocky planets). After measuring 65 super-Earths smaller than 4 Earth-radii, the empirical data points out that Gas Dwarves would be the most usual composition: there is a trend where planets with radii up to 1.5 Earth-radii increase in density with increasing radius, but above 1.5 radii the average planet density rapidly decreases with increasing radius, indicating that these planets have a large fraction of volatiles by volume overlying a rocky core. Similar results are confirmed by other studies. Another discovery about exoplanets' composition is that about the gap or rarity observed for planets between 1.5–2.0 Earth-radii, which is explained by a bimodal formation of planets (rocky Super-Earths below 1.75 and sub-Neptunes with thick gas envelopes being above such radii).

Additional studies, conducted with lasers at the Lawrence Livermore National Laboratory and at the OMEGA laboratory at the University of Rochester show that the magnesium-silicate internal regions of the planet would undergo phase changes under the immense pressures and temperatures of a super-Earth planet, and that the different phases of this liquid magnesium silicate would separate into layers.

Geologic activity

Further theoretical work by Valencia and others suggests that super-Earths would be more geologically active than Earth, with more vigorous plate tectonics due to thinner plates under more stress. In fact, their models suggested that Earth was itself a "borderline" case, just barely large enough to sustain plate tectonics. However, other studies determine that strong convection currents in the mantle acting on strong gravity would make the crust stronger and thus inhibit plate tectonics. The planet's surface would be too strong for the forces of magma to break the crust into plates.

Evolution

The new research suggests that the rocky centres of super-Earths are unlikely to evolve into terrestrial rocky planets like the inner planets of the Solar System because they appear to hold on to their large atmospheres. Rather than evolving to a planet composed mainly of rock with a thin atmosphere, the small rocky core remains engulfed by its large hydrogen-rich envelope.

Theoretical models show that Hot Jupiters and Hot Neptunes can evolve by hydrodynamic loss of their atmospheres to Mini-Neptunes (as it could be the Super-Earth GJ 1214 b), or even to rocky planets known as chthonian planets (after migrating towards the proximity of their parent star). The amount of the outermost layers that is lost depends on the size and the material of the planet and the distance from the star. In a typical system a gas giant orbiting 0.02 AU around its parent star loses 5–7% of its mass during its lifetime, but orbiting closer than 0.015 AU can mean evaporation of the whole planet except for its core.

The low densities inferred from observations imply that a fraction of the super-Earth population has substantial H/He envelopes, which may have been even more massive soon after formation. Therefore, contrary to the terrestrial planets of the solar system, these super-Earths must have formed during the gas-phase of their progenitor protoplanetary disk.

Temperatures

Since the atmospheres, albedo and greenhouse effects of super-Earths are unknown, the surface temperatures are unknown and generally only an equilibrium temperature is given. For example, the black-body temperature of the Earth is 255.3 K (−18 °C or 0 °F ). It is the greenhouse gases that keep the Earth warmer. Venus has a black-body temperature of only 184.2 K (−89 °C or −128 °F ) even though Venus has a true temperature of 737 K (464 °C or 867 °F ). Though the atmosphere of Venus traps more heat than Earth's, NASA lists the black-body temperature of Venus based on the fact that Venus has an extremely high albedo (Bond albedo 0.90, Visual geometric albedo 0.67), giving it a lower black body temperature than the more absorbent (lower albedo) Earth.

Magnetic field

Earth's magnetic field results from its flowing liquid metallic core, but in super-Earths the mass can produce high pressures with large viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Magnesium oxide, which is rocky on Earth, can be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths. That said, super-Earth magnetic fields are yet to be detected observationally.

Habitability

According to one hypothesis, super-Earths of about two Earth masses may be conducive to life. The higher surface gravity would lead to a thicker atmosphere, increased surface erosion and hence a flatter topography. The end result could be an "archipelago planet" of shallow oceans dotted with island chains ideally suited for biodiversity. A more massive planet of two Earth masses would also retain more heat within its interior from its initial formation much longer, sustaining plate tectonics (which is vital for regulating the carbon cycle and hence the climate) for longer. The thicker atmosphere and stronger magnetic field would also shield life on the surface against harmful cosmic rays.

Habitability of red dwarf systems

From Wikipedia, the free encyclopedia

 

An artist's impression of a planet in orbit around a red dwarf

 

This artist's concept illustrates a young red dwarf surrounded by three planets.

The habitability of red dwarf systems is determined by a large number of factors from a variety of sources. Although the low stellar flux, high probability of tidal locking, small circumstellar habitable zones, and high stellar variation experienced by planets of red dwarf stars are impediments to their planetary habitability, the ubiquity and longevity of red dwarfs are positive factors. Determining how the interactions between these factors affect habitability may help to reveal the frequency of extraterrestrial life and intelligence.

Intense tidal heating caused by the proximity of planets to their host red dwarfs is a major impediment to life developing in these systems. Other tidal effects, such as the extreme temperature differences created by one side of habitable-zone planets permanently facing the star and the other perpetually turned away and lack of planetary axial tilts, reduce the probability of life around red dwarfs. Non-tidal factors, such as extreme stellar variation, spectral energy distributions shifted to the infrared relative to the Sun, and small circumstellar habitable zones due to low light output, further reduce the prospects for life in red-dwarf systems.

There are, however, several effects that increase the likelihood of life on red dwarf planets. Intense cloud formation on the star-facing side of a tidally locked planet may reduce overall thermal flux and drastically reduce equilibrium temperature differences between the two sides of the planet. In addition, the sheer number of red dwarfs, which account for about 85% of at least 100 billion stars in the Milky Way, statistically increases the probability that there might exist habitable planets orbiting some of them. As of 2013, there are expected to be tens of billions of super-Earth planets in the habitable zones of red dwarf stars in the Milky Way.

Red dwarf characteristics

Red dwarf stars are the smallest, coolest, and most common type of star. Estimates of their abundance range from 70% of stars in spiral galaxies to more than 90% of all stars in elliptical galaxies, an often quoted median figure being 73% of the stars in the Milky Way (known since the 1990s from radio telescopic observation to be a barred spiral). Red dwarfs are either late K or M spectral type. Given their low energy output, red dwarfs are never visible by the unaided eye from Earth; neither the closest red dwarf to the Sun when viewed individually, Proxima Centauri (which is also the closest star to the Sun), nor the closest solitary red dwarf, Barnard's star, is anywhere near visual magnitude.

Research

Luminosity and spectral composition

Relative star sizes and photospheric temperatures. Any planet around a red dwarf, such as the one shown here (Gliese 229A), would have to huddle close to achieve Earth-like temperatures, probably inducing tidal lock. See Aurelia. Credit: MPIA/V. Joergens.

For years, astronomers ruled out red dwarfs, with masses ranging from roughly 0.08 to 0.45 solar masses (M_☉), as potential abodes for life. The low masses of the stars cause the nuclear fusion reactions at their cores to proceed exceedingly slowly, giving them luminosities ranging from a maximum of roughly 3 percent that of the Sun to a minimum of just 0.01 percent. Consequently, any planet orbiting a red dwarf would have to have a low semimajor axis in order to maintain Earth-like surface temperature, from 0.268 astronomical units (AU) for a relatively luminous red dwarf like Lacaille 8760 to 0.032 AU for a smaller star like Proxima Centauri, the nearest star to the Solar System. Such a world would have a year lasting just six days.

Much of the low luminosity of a red dwarf falls in the infrared part of the electromagnetic spectrum, with lower energy than the visible light in which the Sun peaks. As a result, photosynthesis on a red dwarf planet would require additional photons to achieve excitation potentials comparable to those needed in Earth photosynthesis for electron transfers, due to the lower average energy level of near-infrared photons compared to visible. Having to adapt to a far wider spectrum to gain the maximum amount of energy, foliage on a habitable red dwarf planet would probably appear black if viewed in visible light.

In addition, because water strongly absorbs red and infrared light, less energy would be available for aquatic life on red dwarf planets. However, a similar effect of preferential absorption by water ice would increase its temperature relative to an equivalent amount of radiation from a Sun-like star, thereby extending the habitable zone of red dwarfs outward.

Another fact that would inhibit habitability is the evolution of the red dwarf stars; as such stars have an extended pre-main sequence phase, their eventual habitable zones would be for around 1 billion years a zone where water wasn't liquid but in its gaseous state. Thus, terrestrial planets in the actual habitable zones, if provided with abundant surface water in their formation, would have been subject to a runaway greenhouse effect for several hundred million years. During such an early runaway phase, photolysis of water vapor would allow hydrogen escape to space and the loss of several Earth oceans of water, leaving a thick abiotic oxygen atmosphere.

Tidal effects

At the close orbital distances planets around red dwarf stars would have to maintain for liquid water to exist at their surfaces, tidal locking to the host star is likely, causing the planet to rotate around its axis once for every revolution around the star; as a result, one side of the planet would eternally face the star and another side would perpetually face away, creating great extremes of temperature. For many years, it was believed that life on such planets would be limited to a ring-like region known as the terminator, where the star would always appear on the horizon.

It was also believed that efficient heat transfer between the sides of the planet necessitates atmospheric circulation of an atmosphere so thick as to disallow photosynthesis. Due to differential heating, it was argued, a tidally locked planet would experience fierce winds with permanent torrential rain at the point directly facing the local star, the subsolar point. In the opinion of one author this makes complex life improbable. Plant life would have to adapt to the constant gale, for example by anchoring securely into the soil and sprouting long flexible leaves that do not snap. Animals would rely on infrared vision, as signaling by calls or scents would be difficult over the din of the planet-wide gale. Underwater life would, however, be protected from fierce winds and flares, and vast blooms of black photosynthetic plankton and algae could support the sea life.

In contrast to the previously bleak picture for life, 1997 studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases CO₂ and H₂O) need only be 100 millibar, or 10% of Earth's atmosphere, for the star's heat to be effectively carried to the night side, a figure well within the bounds of photosynthesis. Research two years later by Martin Heath of Greenwich Community College has shown that seawater, too, could effectively circulate without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Additionally, a 2010 study concluded that Earth-like water worlds tidally locked to their stars would still have temperatures above 240 K (−33 °C) on the night side. Climate models constructed in 2013 indicate that cloud formation on tidally locked planets would minimize the temperature difference between the day and the night side, greatly improving habitability prospects for red dwarf planets. Further research, including a consideration of the amount of photosynthetically active radiation, has suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.

The existence of a permanent day side and night side is not the only potential setback for life around red dwarfs. Tidal heating experienced by planets in the habitable zone of red dwarfs less than 30% of the mass of the Sun may cause them to be "baked out" and become "tidal Venuses." Combined with the other impediments to red dwarf habitability, this may make the probability of many red dwarfs hosting life as we know it very low compared to other star types. There may not even be enough water for habitable planets around many red dwarfs; what little water found on these planets, in particular Earth-sized ones, may be located on the cold night side of the planet. In contrast to the predictions of earlier studies on tidal Venuses, though, this "trapped water" may help to stave off runaway greenhouse effects and improve the habitability of red dwarf systems.

Moons of gas giants within a habitable zone could overcome this problem since they would become tidally locked to their primary and not their star, and thus would experience a day-night cycle. The same principle would apply to double planets, which would likely be tidally locked to each other.
Note however that how quickly tidal locking occurs can depend upon a planet's oceans and even atmosphere, and may mean that tidal locking fails to happen even after many gigayears. Additionally, tidal locking is not the only possible end state of tidal dampening. Mercury, for example, has had sufficient time to tidally lock, but is in a 3:2 spin orbit resonance.

Variability

Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in starspots that can dim their emitted light by up to 40% for months at a time. On Earth life has adapted in many ways to the similarly reduced temperatures of the winter. Life may survive by hibernating and/or by diving into deep water where temperatures could be more constant. Oceans would potentially freeze over during extreme cold periods. If so, once the dim period ends, the planet’s albedo would be higher than it was prior to the dimming. This means more light from the red dwarf would be reflected, which would impede temperatures from recovering, or possibly further reduce planetary temperatures.

At other times, red dwarfs emit gigantic flares that can double their brightness in a matter of minutes. Indeed, as more and more red dwarfs have been scrutinized for variability, more of them have been classified as flare stars to some degree or other. Such variation in brightness could be very damaging for life. Flares might also produce torrents of charged particles that could strip off sizable portions of the planet's atmosphere. Scientists who subscribe to the Rare Earth hypothesis doubt that red dwarfs could support life amid strong flaring. Tidal-locking would probably result in a relatively low planetary magnetic moment. Active red dwarfs that emit coronal mass ejections would bow back the magnetosphere until it contacted the planetary atmosphere. As a result, the atmosphere would undergo strong erosion, possibly leaving the planet uninhabitable. Otherwise, it is suggested that if the planet had a magnetic field, it would deflect the particles from the atmosphere (even the slow rotation of a tidally locked M-dwarf planet—it spins once for every time it orbits its star—would be enough to generate a magnetic field as long as part of the planet's interior remained molten). But actual mathematical models conclude that, even under the highest attainable dynamo-generated magnetic field strengths, exoplanets with masses like that of Earth lose a significant fraction of their atmospheres by the erosion of the exobase's atmosphere by CME bursts and XUV emissions (even those Earth-like planets closer than 0.8 AU—affecting also GK stars— probably lose their atmospheres). Atmospheric erosion even could trigger the depletion of water oceans. Planets shrouded by a thick haze of hydrocarbons like the one on primordial Earth or Saturn's moon Titan might still survive the flares as floating droplets of hydrocarbon are really efficient at absorbing ultraviolet radiation.

Another way that life could initially protect itself from radiation, would be remaining underwater until the star had passed through its early flare stage, assuming the planet could retain enough of an atmosphere to sustain liquid oceans. The scientists who wrote Aurelia believed that life could survive on land despite a red dwarf flaring. Once life reached onto land, the low amount of UV produced by a quiescent red dwarf means that life could thrive without an ozone layer, and thus never need to produce oxygen.

It is worth noting that the violent flaring period of a red dwarf's life cycle is estimated to only last roughly the first 1.2 billion years of its existence. If a planet forms far away from a red dwarf so as to avoid tidelock, and then migrates into the star's habitable zone after this turbulent initial period, it is possible for life to have a chance to develop.

Abundance

The major advantage that red dwarfs have over other stars as abodes for life: they produce light energy for a very long time. It took 4.5 billion years before humans appeared on Earth, and life as we know it will see suitable conditions for as little as half a billion years more. Red dwarfs, by contrast, could exist for trillions of years, because their nuclear reactions are far slower than those of larger stars, meaning that life both would have longer to evolve and longer to survive. Furthermore, although the odds of finding a planet in the habitable zone around any specific red dwarf are unknown, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around Sun-like stars given their ubiquity. The first super-Earth with a mass of a 3 to 4 times that of Earth's found in the potentially habitable zone of its star is Gliese 581g, and its star, Gliese 581, is indeed a red dwarf. Although tidally locked, it is thought possible that at its terminator liquid water may well exist. The planet is thought to have existed for approximately 7 billion years and has a large enough mass to support an atmosphere.

Another possibility could come in the far future, when according to computer simulations a red dwarf becomes a blue dwarf as it is exhausting its hydrogen supply. As this kind of star is more luminous than the previous red dwarf, planets orbiting it that were frozen during the former stage could be thawed during the several billions of years this evolutionary stage lasts (5 billion years, for example, for a 0.16 M_☉ star), giving life an opportunity to appear and evolve.

Water retention

Planets can retain significant amounts of water in the habitable zone of ultracool dwarfs, with a sweet spot in the 0.04-0.06 M_⊙ range, despite FUV-photolysis of water and the XUV -driven escape of hydrogen.

Water worlds exoplanets orbiting M-dwarfs, could have their oceans depleted over the Gyr timescale due to the more intense particle and radiation environments that exoplanets experience in close-in habitable zones. If the atmosphere were to be depleted over the timescale less than Gyr, this could prove to be problematic for the origin of life (abiogenesis) on the planet.

Methane habitable zone

If methane-based life is possible (similar to the hypothetical life on Titan), there would be a second habitable zone further out from the star corresponding to the region where methane is liquid. Titan's atmosphere is transparent to red and infrared light, so more of the light from red dwarfs would be expected to reach the surface of a Titan-like planet. 

Frequency of Earth-sized worlds around ultra-cool dwarfs

TRAPPIST-1 planetary system (artist's impression)

 

A study of archival Spitzer data gives the first idea and estimate of how frequent Earth-sized worlds are around ultra-cool dwarf stars: 30-45%. A computer simulation finds that planets that form around stars with similar mass to TRAPPIST-1 (c. 0.08 M_⊙), most likely have sizes similar to the Earth.

In fiction

The following examples of fictional "aliens" existing within Red Dwarf star systems exist:

• Ark: In Stephen Baxter's Ark, after planet Earth is completely submerged by the oceans a small group of humans embark on an interstellar journey eventually making it to a planet named Earth III. The planet is cold, tidally locked and the plant life is black (in order to better absorb the light from the red dwarf).
• Draco Tavern: In Larry Niven's "Draco Tavern" stories, the highly advanced Chirpsithra aliens evolved on a tide-locked oxygen world around a red dwarf. However, no detail is given beyond that it was about 1 terrestrial mass, a little colder, and used red dwarf sunlight.
• Nemesis: Isaac Asimov avoids the tidal effect issues of the red dwarf Nemesis by making the habitable "planet" a satellite of a gas giant which is tidally locked to the star.
• Star Maker: In Olaf Stapledon's 1937 science fiction novel Star Maker, one of the many alien civilizations in the Milky Way he describes is located in the terminator zone of a tidally locked planet of a red dwarf system. This planet is inhabited by intelligent plants that look like carrots with arms, legs, and a head, which "sleep" part of the time by inserting themselves in soil on plots of land and absorbing sunlight through photosynthesis, and which are awake part of the time, emerging from their plots of soil as locomoting beings who participate in all the complex activities of a modern industrial civilization. Stapledon also describes how life evolved on this planet.
• Superman: Superman's home, Krypton, was in orbit around a red star called Rao which in some stories is described as being a red dwarf, although it is more often referred to as a red giant.
• The propulsion family: In the children's show Ready Jet Go!, (Carrot, celery and Jet) are a family of aliens known as Bortronians who come from Bortron 7, a planet of the fictional Red dwarf Ignatz 118 (also called Bortron). Apparently they discovered Earth and the Sun when they picked up a "primitive" radio signal. (Episode: "How We Found Your Sun"). They also gave a description of the planets in the Bortronian solar system in a song in the movie "Ready Jet Go!: Back to Bortron 7".
• Aurelia This planet, seen in the speculative documentary Extraterrestrial (also known as Alien Worlds), details what scientist theorise alien life could be like on a planet orbiting a red dwarf star