Exoplanet Water – Common or Surprising?

We are reaching the point in our study of exoplanets, planets orbiting other stars, where the atmospheres of distant worlds are within the limits of our technology.  Once we could barely see the wobble of a star, the telltale sign of an exoplanet, and now we can see reflected starlight and study a distant atmosphere.  Now we can probe deeper questions, are atmospheres of exoplanets similar to solar system planets? What are they made of? Do other solar systems have the same raw materials as ours? Do they have what we believe to be the raw materials for life?

A recent study has looked at a problem in hot Jupiters, the largest exoplanets that orbit close to their parent stars.  The problem is that there appears to be a lot less water than we expect.  If this is the case, then maybe these planets formed in an environment devoid of water.  This would cause us to rethink our entire theory of how planets form.  It’s likely the water is there, but why don’t we see it?

This image shows an artist’s impression of the ten hot Jupiter exoplanets studied by David Sing and his colleagues. From top left to to lower left these planets are WASP-12b, WASP-6b, WASP-31b, WASP-39b, HD 189733b, HAT-P-12b, WASP-17b, WASP-19b, HAT-P-1b and HD 209458b.

To try and answer this question, an international team of astronomers led by Dr. David Sing of the University of Exeter has used the Hubble and Spitzer space telescopes to study the atmospheres of a sample of ten hot Jupiter exoplanets, the largest sample size to date.  Previously, only three had been studied across multiple wavelengths, but with the complete study of ten planets across a wide infrared band of the electromagnetic spectrum, we can look for some of the key similarities and differences in the sample.  “I’m really excited to finally ‘see’ this wide group of planets together, as this is the first time we’ve had sufficient wavelength coverage to be able to compare multiple features from one planet to another,” says Sing. “We found the planetary atmospheres to be much more diverse than we expected.”

You may be thinking that just ten planets is a small sample, but considering most of the 2000+ confirmed exoplanets are too small or close to their star to be seen, ten is pretty good.  It’s not easy to study the atmospheres of distant worlds either, because planets do not give off their own light.  Instead, astronomers have to look for the light from the parent star that passes through the atmosphere of the exoplanet, and look for the fingerprint of certain molecules in order to determine what is present.  “The atmosphere leaves its unique fingerprint on the starlight, which we can study when the light reaches us,” explains co-author Hannah Wakeford, now at NASA Goddard Space Flight Center, USA.

The study revealed that the exoplanets that had the expected abundance of water did not have any clouds, while the ones that were apparently lacking water contained clouds and haze, two things that can obscure water from view. This means that the exoplanets likely have the expected amount of water, and so we don’t need to rethink our major theories of planet formation.

It’s always nice when things agree with the most comprehensive theory.  As we continue to study the atmospheres of exoplanets in finer detail, and increase the sample size, the trends will become more obvious.  As we push our technology further, we can put our theories to the test, and see if things end up the way we expect, or if the universe ends up being stranger than we think.




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