Astronomers find water vapor on distant, temperate planet

Laura Kreidberg never thought she’d study exoplanets. Sure, when she began graduate school in 2011, the worlds beyond our solar system were all the rage. NASA’s planet-hunting Kepler space telescope had launched two years earlier, and detections were flowing in. But Dr. Kreidberg was lukewarm.

“This is just a flash in the pan,” she recalls thinking. We’ll identify some planets, calculate how big they are and how fast they orbit their stars, and then the excitement will die down. Their other features would remain in the realm of imagination, she thought. After all, how much could you really see from so far away?

So when a professor told her that he was trying to study not just exoplanets, but their atmospheres, she thought, “That sounds ridiculous.” But she was intrigued by the challenge.

Today, Dr. Kreidberg is a research fellow at the Harvard-Smithsonian Center for Astrophysics studying – you guessed it – exoplanet atmospheres. “So, yeah, it’s totally possible,” she says. “And we’ve done a lot already.” 

Just this week, in fact, two teams independently reported a breakthrough: the detection of water vapor in the atmosphere of an exoplanet orbiting a red dwarf star about 111 light-years away from Earth. 

Astronomers have detected water vapor in distant atmospheres before. But K2-18 b is the first exoplanet that also orbits its star in the habitable zone, the region around a star where it’s not too hot and not too cold for liquid water to exist – thought to be a key ingredient for biological life. It’s also one of the smallest planets for which scientists have been able to make such a detection so far.

“It’s not sci-fi anymore,” says Björn Benneke, an exoplanet researcher at the Université de Montréal and lead author on one of the papers published this week. “The really amazing thing about this era is that we’re now actively doing this. We can go and probe each planet by itself and see what’s going on there.”

With breakthroughs like these, scientists have begun to bring some aspects of those distant worlds into focus, but the data they can gather are still limited. Exoplanets largely remain shrouded in mystery, and studying them requires a flexible mind.

“It’s kind of some mental gymnastics that you have to do to imagine the planet as a real place,” says Dr. Kreidberg, who wasn’t involved in either study. “It’s hard sometimes to really grasp the reality of these planets that are so foreign compared to anything that we’ve seen in the solar system, that are so far away that we can’t actually see them.” 

Twinkle, twinkle

Most known exoplanets are too distant to image directly, even by our most powerful space telescopes. So astronomers typically take an indirect approach, known as the transit method.  

When an orbiting planet passes in front of its star, the star’s light dims ever so slightly. Imagine watching a mosquito pass a street lamp from a mile or so away, explains Hannah Wakeford, an exoplanet researcher at the Space Telescope Science Institute in Baltimore, Maryland, who was not part of either study. Starlight dimming at regular intervals suggests that the star hosts an orbiting planet.

But how can you tell anything about the planet – let alone its atmosphere – from such a subtle indication? 

Ironically, it’s the atmosphere’s transparency that enables scientists to detect it. When light passes through a gas, certain wavelengths are absorbed. Scientists can look at the light spectra to determine what’s in the atmosphere.

“The atmosphere is really the only accessible feature that we can observe for exoplanets that will give us any handle on their composition,” says Laura Schaefer, an exoplanet researcher at Stanford University who was not part of either study. And that, in turn, could potentially reveal surface conditions, habitability, or even, perhaps, if the planet is inhabited.

A lot of questions swirl around K2-18 b. The big one that everyone wants to know – could anything live there? – is still unanswerable. Although water vapor in the atmosphere of a planet in the habitable zone of its star is a good sign, researchers don’t know if K2-18 b actually has liquid water or any other characteristics thought necessarily for life as we know it to exist.

One of the research teams, however, did find that in a likely model of K2-18 b, liquid water rain clouds might form in the atmosphere. That scenario opens up the possibility of a sort of water cycle existing, explains Dr. Benneke, the study’s lead author. But, he says, that rain – if it exists – probably wouldn’t reach any kind of surface because the atmosphere is so thick.

K2-18 b sheds light on an exoplanet mystery that scientists have been puzzling over. Many exoplanets discovered so far don’t seem to be much like anything in our solar system, so scientists don’t have a model to use to figure out what they might look like. K2-18 b sits in that same not-so-sweet spot. Nearly twice the size of Earth and nearly nine times as massive, K2-18 b lies in a category somewhere between Earth, our solar system’s biggest rocky planet, and Neptune, our smallest gas giant. But that’s a puzzlingly large gap.

One of the new papers (the one published in Nature Astronomy) calls K2-18 b a “super Earth” while the other (uploaded to arXiv and led by Dr. Benneke) places it more in the “mini Neptune” camp. 

Which is it? Dr. Benneke says maybe it’s actually a sort of hybrid planet, with a rocky core and vast envelope of hydrogen. But outside scientists like Dr. Kreidberg point out that all that hydrogen gas would probably make it more Neptune-like, with the higher pressures deep in its atmosphere possibly creating a liquid hydrogen layer above any kind of rocky surface. 

Still, the new data on K2-18 b’s atmosphere is “definitely a milestone,” says Sara Seager, an astrophysicist and planetary scientist at the Massachusetts Institute of Technology, who isn’t an author on either paper. “It’s a step in the right direction because we’ve got to see the atmospheres in the hopes that the atmosphere will help tell us what these planets are.”

‘Seeing is believing’

It’s not easy to be pushing the boundaries of discovery. Just ask Dr. Seager. Today, she is the deputy science director for NASA’s newest exoplanet-hunting space telescope, the Transiting Exoplanet Survey Satellite, or TESS, which launched in April 2018. But two decades ago, her research was not so readily embraced.

Many scientists were skeptical that exoplanets were even detectable, let alone that someone could study their atmospheres. Instead, they suggested that perhaps the star’s variability caused it to dim, rather than a passing planet.

“It was a little awkward, actually, that even people on my thesis committee questioned the viability of the field,” Dr. Seager says. She was asked why she put clouds in models of atmospheres when exoplanet atmospheres weren’t even detectable. Dr. Seager earned her Ph.D. in 1999, but, she says, “today there are entire Ph.D. theses on clouds in exoplanet atmospheres.”

“What changed is that we got data,” Dr. Seager says. “Seeing is believing.”