Thread by @aussiastronomer

GUYS. Two independent announcements today of the same thing: We have found water vapor in the atmosphere of a planet called K2-18 b!

BUT WAIT?

WHAT DOES IT MEAN?

K2-18 b is not a rocky planet. So that's a bummer. But it IS in the 'habitable' zone of its star, meaning under certain assumptions the temperature is right for liquid water on the surface on the planet.

BUT.

K2-18 b is massive enough (about 8 times the mass of the Earth) that it has a thick, deep, gaseous atmosphere of hydrogen and helium. Very, very unlike our atmosphere.

So what did the teams find? Using the Hubble Space Telescope, they found a signature of water vapor in the atmosphere. Here are the detections side by side. Reassuringly similar.

BUT JESSIE, ISN'T WATER EXCITING?

The most exciting part of these announcements is that, as we are able to probe ever smaller and ever cooler planets, we are finding water. Lots of water. SO MANY PLANET ATMOSPHERES HAVE WATER IN THEM.

That is really very promising, if we ascribe to the habitability criterion of liquid water. All life on Earth needs water. [What about other life? We don't know.] That means that the water budget in planetary disks and ending up on planets themselves is looking good.

K2-18b is not exciting because it's habitable. It's not habitable by our current definitions. It's exciting because it continues a trend for smaller and cooler planets (if you get small enough and cool enough, you get to Earth!) to continue to have water. Water water everywhere.

Here are the two studies I have been referring to:

Benneke+2019 (submitted to Astronomical Journal): arxiv.org/abs/1909.04642

Tsiaras+2019 (published in @NatureAstronomy): nature.com/articles/s4155…

@NatureAstronomy If I had one #scicomm wish it would be to ban the phrase "the most habitable" from the lexicon (scientists and journalists both). It's misleading and confusing and frankly undefinable.

Pulling this out of a DM thread because it’s another important point:

We’re pushing the instruments really hard to get a result we really want.

Everyone needs to remember both those things.