If it weren't for Earth's atmosphere, we wouldn't be here. It shelters us from deadly cosmic rays and regulates our climate, and is exactly what seemed to be missing from most Earth-sized planetary candidates beyond our solar system.
However, recent research from the University of Chicago and Stanford University suggests many planets in the universe not only developed atmospheres full of water vapor, but retained them for long periods, according to a recent study published in the journal Astrophysical Journal Letters.
This expands our grasp of how planets form, and could help guide scientists in the ongoing search for Earth-like planets beyond our solar system.
Water vapor might dominate exoplanet atmospheres for a 'long time'
"Our model is saying that these hot, rocky exoplanets should have a water-dominated atmosphere at some stage, and for some planets, it may be quite a long time," said Assistant Professor Edwin Kite of the University of Chicago (UChicago), who is also an expert on the evolution of planetary atmospheres, in a UChicago blog post.
Space- and ground-based telescopes continue to catalog exoplanets, and scientists are trying to discern how they look. Typically, telescopes can show us the physical size of an exoplanet, the distance between it and its host star, and — sometimes — the planet's total mass.
To reach the next level of exoplanet investigation, scientists need to extrapolate from data gathered from Earth and other planets within our solar system. But surprisingly, the most abundant planets in the universe aren't like the nearby ones we can see with the naked eye.
"What we already knew from the Kepler mission is that planets a little smaller than Neptune are really abundant, which was a surprise because there are none in our solar system," said Kite in the blog post. "We don't know for sure what they are made of, but there's strong evidence they are magma balls cloaked in a hydrogen atmosphere."
Liquid magma swallows most water on sub-Neptune exoplanets
Many smaller rocky planets also populate the universe, but without the oppressive coat of a thick hydrogen atmosphere — which is why scientists suspect many planets likely begin like the larger ones with hydrogen cloaks — but then lose their initial atmospheres as the host star comes to life and blasts the hydrogen away.
However, these models remain largely incomplete. Kite and co-author Laura Schaefer from Stanford investigated possible scenarios where a planet is smothered in oceans of molten lava.
"Liquid magma is actually quite runny," said Kite — which means it can recycle layers much like oceans on Earth. Additionally, these magma oceans are probably removing hydrogen from the atmosphere — creating a chemical reaction that forms water, some of which evaporates into the atmosphere, but more of it is swallowed by the hot magma.
Later, after the host star has stripped the exoplanet's hydrogen atmosphere, the water is extracted and becomes water vapor in the atmosphere. On a long enough timeline, such planets gain a water-rich atmosphere.
And, on some planets, this stage could last for billions of years, according to Kite.
A glowing dark side on exoplanets could point to an atmosphere
To put this idea to the test, the James Webb Space Telescope (JWST) could analyze composition measurements of exoplanet atmospheres, looking for the presence of water. But another way involves seeking indirect signs of atmospheres. Since most of these planets are tidally locked (like the moon is to Earth), their surface temperatures are highly polarized — with the star-facing side much warmer than the "dark" side.
This may sound grim, but two UChicago alumni — Laura Kreidberg and Daniel Koll, at the Max Planck Institute for Astronomy and MIT, respectively — suggested an atmosphere could moderate an exoplanet's temperature, dispersing much of the light-side heat around it. By learning how strongly an exoplanet's dark side glows, we might discover an atmosphere actively redistributing heat.
The search for Earth-like planets beyond our solar system is accelerating. And, with the James Webb Space Telescope launching later this year, we may soon find ourselves overwhelmed with a catalog of planetary atmospheres brimming with water.