My name is Heather Knutson, and I'm a professor in the planetary science department here at
Caltech. I study the properties of extrasolar planets, which are planets that orbit stars
other than the sun, so mostly these are our closest exoplanetary neighbors. We're not
talking about planets in other galaxies, we're mostly talking about planets which are in
kind of the same part of our own little corner of our galaxy. So these are around some of the closest stars
to the sun.
What is a hot jupiter?
The planets that I've found the most surprising, out of all of the ones I've discovered so
far, I guess the sort of classic example, is that we've discovered this sort of giant
planets which are very similar to Jupiter, but orbit very much closer in than Mercury
is to our sun, so these planets orbit their stars every two or three days and are slowly
getting roasted. We know that they couldn't have formed there - they had to have formed
farther out and migrated in, so one thing we're still trying to understand are is that what are the
forces that caused them to migrate in, whereas Jupiter seems to have migrated a little bit
but otherwise stayed put in our own solar system.
What do hot jupiters mean for our understanding our own Solar System?
The implications of these "hot jupiters" as we call them are actually huge for our own
solar system, because if you want to know how many potentially habitable earthlike planets
are out there, having one of these giant planets just rampage their way though the inner part
of the planetary system, and it could toss out your habitable earth and put it into either
a much closer orbit or a much further orbit. So knowing how things have moved around will
tell you a lot about where you might find interesting planets.
What is their atmosphere like?
So, the atmospheres of hot jupiters are very exotic, by solar system standards. They typically
have temperatures of a thousand to several thousand Kelvin, so at these temperatures
these planets could have clouds of molten rock, for example. They have atmospheric compositions
that would seem very exotic to us - they're actually more similar to the compositions
of relatively cool stars, so we actually adapt to describe these planets - we actually use
stellar models to describe their atmospheres. We think that they're also probably tidally
locked, which is very interesting because it means that one side of the planet is getting
all of the heat and the other side is sort of in permanent night. And one thing we do
is to try and understand the effect that has on the weather that planet has, so you have
winds that are pretty good at carrying that energy around the night side and mixing everything
up, or do these planets have these just extreme temperature gradients between the day side
and the night side.
How'd they get there?
So, we have a couple of theories for how a hot jupiter may have ended up in their present
day orbits. One theory is that, after they formed, that
they were still embedded in the gas disc where they formed, and maybe they interacted with
the disc as such that it kind of torqued and pulled them and so that's kind of an early
migration theory. There's also a late migration theory version where when after the disc had
gone away, these planets had interacted with a third body in the system, so maybe you had
another distant massive planet or maybe you had a planet that was part of a binary star
system, and those three body interactions excited a large orbital eccentricity in the
innermost planet, and once it starts coming in closer to the star, the tides start to
damp out the eccentricities, so what you end up with is something which is a gas giant
planet in a very short period circular orbit.
So that's kind of a more complicated story, but there are some clues in the data that
might be true for at least a subset of the hot jupiters that we study.
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