Less Water on Exoplanets than Expected
Distant stars are pinpoint specks, too small to resolve. Exoplanets are ten times smaller in diameter and don't emit light of their own. They're vastly fainter than any star; we couldn't even see a single one until 20 years ago.
That's what makes a painstaking new study completed using the Hubble Space Telescope so beautiful. It not only located three exoplanets, but painstakingly measured the water composition of their atmospheres. There's water, but it's less than we expected. How is this possible?
This is a more difficult version of the way we've been investigating stars for a century. We know the chemical makeup of far away suns because of their absorption spectrum: the colors missing from the light they broadcast to us.
The searing hot plasma at the core of a massive star emits light of all colors in the spectrum. The elements in the atmosphere of the star absorb a little bit of that light however, leaving certain colors absent from the light that reaches us.
Due to the quantum nature of energy states in atoms and molecules, they only absorb and emit energy in certain exact amounts, i.e., with very certain electromagnetic frequencies.
Water molecules absorb and emit a certain group of wavelengths due to their quantum transitions too. Upon absorbing an infrared photon of the correct wavelength (1380 nm for example), the atoms of the molecule will be kicked into vibrating back and forth in a certain pattern. This picture shows what such states look like.
An exoplanet atmospheric survey first looks to see how much light the star emits toward us at the colors absorbed by water when nothing else blocks any of the light. When the planet passes between the star and our telescopes during its orbit, we look at each of these wavelengths of light and see how much of it has been blocked. A certain percentage will be blocked solely by the mass of the planet itself.
However, a small part of the star's light will pass through the atmosphere of the planet and escape to the other side. We then look at this light that has passed through the sky of an alien world to see if it is missing a greater amount of those wavelengths that water likes to absorb.
The amount of water present in the measured alien atmosphere was actually something around 100 times less than predicted. This could mean that our models of how elements are distributed and retained in planet formation need tweaking. It could also be due to patterns of cloud or haze in the atmospheres of the planets. Measurements like these are the payoff of incredible recent improvements in astronomical instruments and techniques.
If this trend continues, it may not be long before we begin to look at something even more exciting: the atmospheres of earth-like planets.
(AP photo)
That's what makes a painstaking new study completed using the Hubble Space Telescope so beautiful. It not only located three exoplanets, but painstakingly measured the water composition of their atmospheres. There's water, but it's less than we expected. How is this possible?
Astronomers often find exoplanets by watching the light of many stars. If the brightness of a particular sun has a tiny (usually 1% or smaller) flicker that repeats in a regular pattern, we can calculate whether a planet's continuous orbit crossing in front of the star is the cause. Astronomical techniques have now evolved to the point that we not only look for a flicker in the total light from the star: we can see precisely how much each and every color of the rainbow flickers.
This is a more difficult version of the way we've been investigating stars for a century. We know the chemical makeup of far away suns because of their absorption spectrum: the colors missing from the light they broadcast to us.
The searing hot plasma at the core of a massive star emits light of all colors in the spectrum. The elements in the atmosphere of the star absorb a little bit of that light however, leaving certain colors absent from the light that reaches us.
Due to the quantum nature of energy states in atoms and molecules, they only absorb and emit energy in certain exact amounts, i.e., with very certain electromagnetic frequencies.
Water molecules absorb and emit a certain group of wavelengths due to their quantum transitions too. Upon absorbing an infrared photon of the correct wavelength (1380 nm for example), the atoms of the molecule will be kicked into vibrating back and forth in a certain pattern. This picture shows what such states look like.
An exoplanet atmospheric survey first looks to see how much light the star emits toward us at the colors absorbed by water when nothing else blocks any of the light. When the planet passes between the star and our telescopes during its orbit, we look at each of these wavelengths of light and see how much of it has been blocked. A certain percentage will be blocked solely by the mass of the planet itself.
However, a small part of the star's light will pass through the atmosphere of the planet and escape to the other side. We then look at this light that has passed through the sky of an alien world to see if it is missing a greater amount of those wavelengths that water likes to absorb.
The amount of water present in the measured alien atmosphere was actually something around 100 times less than predicted. This could mean that our models of how elements are distributed and retained in planet formation need tweaking. It could also be due to patterns of cloud or haze in the atmospheres of the planets. Measurements like these are the payoff of incredible recent improvements in astronomical instruments and techniques.
If this trend continues, it may not be long before we begin to look at something even more exciting: the atmospheres of earth-like planets.
(AP photo)
