Frontiers and Controversies in Astrophysics: Lecture 6 Transcript
February 1, 2007
Professor Charles Bailyn:
You'll recall that where we got to last time is that there are lots of Hot Jupiters;
lots of Hot Jupiters. And the alternative theories that were presented to explain the evidence for Hot
Jupiters--the evidence for Hot Jupiters comes in this form of these velocity curves, where you plot the radial
velocity versus time of the star. And the star goes back and forth, and you infer from that that there must be a
planet going around it, which is pulling the star back and forth. And some alternative explanations were
proposed to explain these kinds of data. Those alternative explanations didn't seem to work very well, and so
you have to kind of take the Hot Jupiters seriously.
Before I go on, let me mention that, by now, Hot Jupiters are not the only kind of planets that have been seen
this way. There are also planets in--now known to have much longer orbits, up to a few years. These are
harder to see for two reasons. First of all, it takes you a few years to see them. And second of all, when the
orbits get longer, the velocities that they induce go down, because things in long orbits go slowly. But,
nevertheless, now we've been able to see a bunch of things.
Oddly, in many cases, they turn out to have highly elliptical orbits, in some cases. Not the Hot Jupiters, not
the ones that are close in. Those all are more or less circular. But some of these longer ones have highly
elliptical orbits. That's also very weird in terms of our theories of planetary formation because one of the
things that our theories were designed to explain was the fact that all the major planets in our own Solar
System have orbits that are close to circular. It does a very good job of explaining that and then, naturally
enough, has trouble explaining the ones we see that are in highly elliptical orbits--highly elliptical--yes, I
didn't--highly elliptical orbits, in some cases. And just so you know, the way you recognize a highly elliptical
orbit is it's not sinusoidal anymore. It's got some other shape. So, these come in: periodic but non-sinusoidal
So, all this is very amusing and you pile up all these very strange kinds of planets, or what you think are
planets. But there's still sort of the nagging question of whether these radial velocity curves might be
explainable in some other way. And it would be awfully nice to have evidence of some other kind for the
existence of these planets.
This is, you know, what happens in science. You find some kind of fairly strong evidence for something, but
it would be way stronger if you found two different kinds of evidence. Different--collected in different ways
or reflecting different aspects of what you're observing that both point in the same direction.