As mentioned in the previous post, Pääbo’s was not the only interesting talk I went to last week.
On thursday, I saw John Asher Johnson talk about the search for exoplanets. You may know John Johnson from this PhD-comics video made by Jorge Cham about what you can learn by looking at stars. I highly recommend it to all non-astronomers (even if the real life John Johnson has somewhat less hair than the cartoon version).
What Johnson and his group specifically learn by looking at stars is whether they have planets, and what kind of planets they do have. They do this in one of two main ways. The first is by looking at the velocity of the star, and how it changes. If there is a planet orbiting around the star, it will pull the star slightly in its direction. Since that direction is changing as the planet orbits its star, we can see a periodic change in the star’s velocity. The velocity of the star can be determined because of the Doppler effect (light from a source coming at us looks bluer, while light from a source moving away looks redder). The second is taking a time sequence of images of one star, and looking whether there are periodic changes in brightness. The way this works is if we are aligned with both the planet and the star, there will be “eclipse”-like events when the planet comes in front of its star and blocks a part of the star light. A planet is much smaller than a normal star, so it would only block a small part of the light. Even though it is fine-tuned to look for minute variations in star brightness, this technique is not enough to notice small earth-size planets. To do that, they look at whether there is some change in the periodic nature of this eclipse cycle. If sometimes it comes slightly earlier and sometimes slightly later than expected, that’s a sign that there’s another object around exerting a gravitational effect. And then, from the shape of the “difference from perfect recurrence” you can obtain another periodic signal which you can use to figure out the properties of other bodies in that planetary system.
One thing Johnson emphasized is that to learn anything about the planets, you need to understand the stars well – you learn the period “for free” essentially, but to figure out the planet’s mass and distance from the star – and whether it’s in the “habitable zone” – you need to convert from ratios using known properties of the star. And it’s easy to screw up. One problem is that since a lot of the signatures are gravitational, you have to think of other potential things that can interact with other things via gravity, which is, literally, everything. But that’s not the end of it. Johnson had an example of finding an “eclipse” event which kept starting and ending way more abruptly than expected for a planet that has to traverse the less bright “outside” area of a star for some time. They were initially puzzled but then realized what they were actually seeing was a neutron star and normal star orbiting each other. (A neutron star is a star which is very tiny (about the size of earth) but massive (over a solar mass)). The “eclipse” was actually the normal star eclipsing the smaller amount of light coming from the neutron star. This was confirmed by looking at the velocity of the normal star and seeing that it was changing way, way more than a planet would cause. So while they didn’t find a planet in that case, they still found an interesting stellar system to study.
One question is, why continue looking for more and more planets in the same fashion? If all science is either physics or stamp collecting, this certainly looks more like the latter. Johnson embraced this analogy in some ways, but explained that we need new planets not just to put more names into a catalogue or continue looking until we see one with life on it. One reason is that planets which we find often have properties which make us rethink our notions of planetary science. When we find something exceedingly strange (e.g. a planet moving “backwards” compared to its star, or several planets arranged in 1:2:3 distance ratio from each other – both things that have been found), it forces us to figure out what could have caused this situation. Another reason is that having never seen the formation of a planetary system, our explanation of what happened to form the solar system was in some sense a “just so” story. We have little idea if that’s how things actually happened. Seeing different planetary systems in various stages of evolution is actually changing our understanding of how planets form. Finally, by having enough planets to get some sort of statistical information, we can learn about the “normal” conditions in our world. One interesting outcome of that last exercise, for example is the finding that by far the majority of planets should be orbiting red dwarfs. So in a way our solar system is quite special in that we’re orbiting a main sequence star. Since this violates the Copernican Principle (that you are unlikely to be living at any special time or in any special place, astronomically speaking) maybe this means something interesting? New planets are being discovered every day (the record for smallest known extrasolar planet, previously held by Johnson’s group, was broken the day before his talk) so we are likely to learn the answers to these puzzles in the near future.