Water, also known as H2O (remember its molecular from chemistry class?), is crucial to sustain life as we know it. For that reason, our search for life on other planets has focused heavily on looking for water in the first place: if there's some of it, there's hope.
Which is why scientists were so excited when they discovered that a far-away planet, called K2-18b, has an atmosphere full of H2O—leading many to consider it one of the best candidates to host life outside our solar system.
An alien world
K2-18b is a pretty big planet compared to earth. About twice the size of our homely rock and with eight times its mass, it's estimated to have between 0.01% and 50% of water in its atmosphere—with clouds and everything.
And it's pretty far away too, so don't expect humans flying there any time soon. Nestled deep within the constellation of Leo, 110 light years separate Earth form K2-18b. And the conditions on its surface are likely so different from ours, it couldn't actually host human life anyway. But it might host other, more resilient lifeforms.
For starters, K2-18b's sun is a red dwarf less than half the size of the sun. Red dwarfs are relatively small stars that produce far less heat than many other types, which means planets would have to orbit closer to it in order to sustain a hospitable atmosphere. K2-18b is lucky enough to rotate 14 million miles away from its star, which allows its surface to reach a temperature of about 50ºF (10ºC).
That alone makes it pretty different from our own home. It also doesn't help that, given their distance from each other, K2-18b would take a mere 33 days to orbit its star—meaning a whole year there would be equivalent to just one month on Earth. But hey, at least it lies within its star's "Goldilocks zone": that perfect area where the temperature is neither too high nor too low for there to be liquid water.
K2-18b was first spotted in 2015 by Nasa, using its Kepler space telescope. And though we cannot yet take anything remotely resembling an HD picture of such distant objects, scientists have a pretty good idea of how to determine what they're like based on information about how their star's light reaches us.
For example, if our current space telescopes can glimpse how a star looks when one of its planets passes right in front of it, we can start taking measurements. Pretty much all elements and chemical compounds, H2O included, absorb light in different ways, so when we observe that a particular planet absorbed a particular wavelength of light from its star before it reached us, then we'd know what kind of elements its atmosphere might host.
And that's exactly how scientists determined that there is water on K2-18b's atmosphere. After a team from the University College London used Nasa's Hubble Telescope to monitor the planet for over two years, they could see that the exact light wavelength that water absorbs was missing from the measurements, indicating the presence of the life-sustaining substance. Exciting, isn't it?
It's still not clear whether there's any water on K2-18b's surface, but this discovery still makes it the best candidate to host life outside the solar system. And that's pretty huge.