Astronomers have detected evidence of water on a potentially habitable planet outside our solar system for the first time.
The findings show that planet K2-18b, which orbits a red dwarf star 110 light-years from Earth and was already thought to be potentially habitable, has water vapor within its atmosphere.
It’s the first time such a discovery, reported in a pair of papers published in Nature Astronomy and uploaded to arXiv, has been made for a planet that lies in the habitable zone—the orbital range around a star where liquid water could theoretically exist on a planet’s surface.
“This is the only planet right now that we know of outside the solar system that has the correct temperature to support water, has an atmosphere, and has water in it—making this planet the best candidate for habitability that we know right now,” says Angelos Tsiaras, an exoplanet scientist from University College London and the lead author of the Nature Astronomy study.
While the two papers share the general conclusion that K2-18b has water content, there is quite a bit of daylight between their approaches, and their results have different implications for the overall structure of the planet itself.
Where the two groups agree is that K2-18b is not another Earth. The exoplanet has eight times the mass of Earth and is twice its size. It’s quite close to its host star (with an orbital period of 33 days), but this star is half the size and temperature of the sun. K2-18b’s existence was confirmed in 2015, and subsequent studies suggested it was rocky and possibly in possession of a “significant gaseous envelope or an ocean.”
Both teams looked at Hubble data to observe the stellar transits of K2-18b as it orbited its host star. When stellar light moves through a planet’s atmosphere, it becomes scattered by the presence of different atmospheric elements and compounds. Scientists can analyze this scattering to determine what a planet’s atmosphere is made of. The team led by Björn Benneke, an exoplanet researcher from the Université de Montréal and lead author on the arXiv paper, also had access to data from the Kepler and Spitzer space telescopes.
The result is that the Nature Astronomy paper, which is peer-reviewed and more conservative in its findings, concludes there’s a significant concentration of water within K2-18b’s atmosphere, though the researchers speculate it could make up as little as 0.01% of the atmosphere or as much as 50%.
Tsiaras and his team think the planet is likely a rocky “super-Earth” in possession of an atmosphere that’s either very water dominant, heavily mixed with a transparent gas like nitrogen, or features significant cloud formation. That atmospheric water could be a sign of liquid water on the surface (which could perhaps be covered completely by an ocean), but that’s unclear for now.
Meanwhile, Benneke and his team come down hard on the conclusion that K2-18b has clouds made of water—lots of them. They also propose a much more exotic version of the planet. Their model, which incorporates the Kepler and Spitzer data, suggests that K2-18b is a “mini-Neptune” consisting of a small icy or rocky core wrapped in a gaseous envelope. Within certain layers of the atmosphere, the water vapor is capable of condensing into liquid droplets. “From an astrobiology point of view, that’s much more important,” says Benneke. “It’s the presence of liquid water—not vapor—that allows the biochemical processes of life to take place.” K2-18b might be home to microscopic life floating through the gas layer and living off condensed water droplets in the clouds.
Both papers are affected by the limitations of current technology. Indeed, Hubble is something of an antique these days. Its cameras weren’t really meant for these kinds of studies, and there’s only so much software can do in post-observation analysis to model an atmosphere 110 light-years away.
Kepler and Spitzer provide a broader range of observations (hence the arXiv paper’s cloud formation models), but they’re less precise than Hubble. We still have no idea about temperatures on the planet (Tsiaras’s team pitches a range between -100 and 116 °F, or -73 and 47 °C), where precisely the water vapor is located in the atmosphere, what other compounds could be found in the atmosphere, whether it’s a tidally locked planet, or any other traits that could paint a sharper picture of how habitable K2-18b could be.
Ground-based telescopes might be able to make some useful observations, but it’s actually very difficult to observe an atmosphere with water through an atmosphere with water. Some telescopes, like the Extremely Large Telescope in northern Chile and the Thirty Meter Telescope in Hawaii, might have some luck studying K2-18b, but neither will be operational until the latter half of the next decade.
So instead, our best bet is to do follow-up observations with two successors to Hubble: the James Webb Space Telescope, which is due to launch into orbit in 2021 but has been repeatedly delayed, and the European Space Agency’s ARIEL space telescope, scheduled to launch in 2029. Both will be capable of making observations more precise and more broadly across the electromagnetic spectrum, identifying more types of molecules in the atmosphere, like carbon dioxide, methane, or ammonia. Those telescopes will also help us achieve a much more concrete sense of the water content and structure of K2-18b—whether the planet turns out to be a super-Earth or a mini-Neptune.
“The fact that we can still use Hubble to do innovative science like this is absolutely astounding,” says Ryan Cloutier at the Harvard & Smithsonian Center for Astrophysics, who was not involved with either study. Yet an instrument like the JWST “will provide an order-of-magnitude improvement in measurement precision over Hubble,” he says, making it much easier to do detailed studies of any of the 55 habitable-zone exoplanets.
It’s important to note that the K2-18b signals are far from airtight. “These types of studies are always tricky,” says Cloutier. “In the past some signals have essentially ‘disappeared’ after further analysis or after an independent analysis by another research team.”
Still, it’s rare to glimpse evidence of water in another planet’s atmosphere, and that alone should help spark enough interest in keeping these types of studies going when the next decade’s technologies go online.
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