Launch Pad, Day Five Catch-up, Kevin Grazier on Extrasolar Planets

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When we talked about planetary formation, we said Jovial planets were far out.

But when we started looking around the universe, the first planets we found were Jovial planets near stars. They’re called hot Jupiters. They’re puzzling. We don’t exactly know why they happen.

Currently our detection techniques are wildly biased toward big planets, and planets close to stars. As we improve our detection criteria, we are able to look for smaller, further planets.

Many M stars aren’t bigger than Jupiter. Jupiter is as big as a planet gets… sort of. In astronomy, factors of ten don’t matter much, so really to be a star, Jupiter would need to be about 84x more massive. But as you added mass, it would compress, and by the time it became a star, it would be about the size of Jupiter again.

So if you have two stars of equal mass, they will orbit around the center of mass. If you want to balance these, you’d balance them in the middle.

In the earth/moon system, the barry center is 3/4 of the way between the earth and the moon. The moon takes 4 weeks to orbit, so after a week, the moon moves 1/4 of its orbit, and earth wobbles slightly around its barry center.

Bud: So it’s the barry center that is the slightly elliptical orbit.

Kevin: Yes. So, a week later, the moon moves again. The moon’s orbit isn’t completely circular, but it’s close. The earth wobbles again. The moon orbits the barry center, the earth wobbles around the barry center.

The doppler shift occurs when you have a moving source of a wave–sound or light wave, e.g. a horn wave going past you. Coming toward you, the sound wave is compressed, moving away, it’s extended. The doppler shift also works with light. If something emits light and comes toward you, the light shifts toward the blue, with a shorter wavelength. As it recedes, it shifts red.

So we take stars of known spectral type and see how much they’re redshifted.

So when we have two objects orbiting around their barry center, if we are observing, we see tihs alternatingly approaching and receding (shifting blue and red), and that was one of the first techniques we used to find planets. We could watch the spectral lines shift back and forth, indicating it was in orbit with something we couldn’t see.

It also explains why the things we detect are big and close, because those things throw the star around more.

We’ve improved as computation has. As computation improves, our detection has shot through the roof, and that’s why things are changing so fast.

If the orbit is exactly in the plane in which you’re looking, you can get an eclipse.

So if we have two stars, with very different sizes, in mutual orbit, and you point an instrument at the central star, and then wait as the smaller star orbits, what you see is that if you count the photons, you will find a plunge in the starlight during the eclipse.

In a planet, which is much less hot, the plunge will be even more pronounced.

This is the eclipse method of looking for planets, the Keppler method, looking for dips in the light curve.

There’s an effect called resonances which leads to an interesting phenomenon. Look at the distribution of asteroids in the asteroid belt. Unlike the depiction in SF, asteroids in our belt are quite dispersed. It would be unlikely to see one asteroid from another. So there are gaps where there are no asteroids–Kirkwood gaps.

Similarly, the main rangs of Saturn, the B and C rings are separated by a gap called the Cassini division which is created by the orbital resonance effect adhering to the Death Star moon, Midas. The moon does not orbit in the gap. It orbits well outside the gap. But there’s an effect called resonance that creates the gap.

Io is volcanic in part because of a resonance.

(Side story. We teach the scientific method as an ideal, but in science you quickly realize that’s not always how science is done But there’s a story about Io which worked just like that. There were three scientists who looked at Io in the 70s and asked what it would be like to be on Io. It’s close to Jupiter so it experiences strong tidal stress. It gets stretched out of round by Jupiter’s gravity. But Jupiter’s evidence is also a little eccentric, so Io will stretch and relax, stretch and relax, stretch and relax, and eventually it snaps, and the tip will be both sharp and warm. So they predicted it would be volcanic, which it was.

Left to its own devices, if Io were alone, its orbit would eventually circularize, and it would no longer be volcanic. But it turns out that Io resonates with two other moons of Jupiter. So we see by Keppler’s law that Io, which you would expect to go faster, but it’s in a 2-1 resonance with Europa, so every other orbit, Io gets tugged by Europa. It stays out of round, and volcanic.

The belt–the gaps are caused by orbital resonance with Jupiter.

Going back to Hot Jupiters, if you have a big planet, and a lot of mass that hasn’t accreted yet, a forming Jupiter could interact with some of that mass, and get pulled in. That’s the best answer we have right now for how hot jupiters work, though there is also a theory about gas drag. We think the gas giants didn’t form that close in, but actually migrated.

Extrasolar earths–we hear that Keppler is designed to detect extrasolar earths.

Earth is habitable because of a catastrophic event. Catastrophic, colloquially, means something bad. Catastrophic, scientifically, means something happens suddenly, as opposed to uniformly. The impact that gave Earth the moon means that we, humans, are here; without it, we would not be.

The big four ices: water, ammonia, carbon dioxide, and methane. These make comets. Comets impact in the early solar system, impact earth. Rock floats on top of the metals, and the lighter fluid floats on rock, and lighter gases above that. Earth was acreted, and gases boiled off, and we had vulcanism.

Carbon dioxide is heavy. It’s a heavy molecule for a gas. It’s less buoyant. So, when the gases boil off, carbon dioxide remains in our first atmosphere. If we still had that now, we’d be radiating 600-800 degrees. That’s incompatible with life as we know it.

So: the moon impacts earth, and that blows off our first atmosphere.

Later comet impacts bring back a different mix of gases, which combine with outgasing from vulcanism. Life–breathing carbon dioxides–exhales oxygen, changing our atmosphere to what it is now.

The atmosphere on Mars is 95% CO2. It can’t hang on to lighter gases.

What we have is the result of a cosmic error.

So therefore Kevin claims that when we look for extrasolar earths, we are more likely to find extrasolar Venuses. Venus is close to the size of Earth, but it has a CO2 atmosphere. We may find earth-sized planets but find they act similarly to Venus.

Carrie–Have they found any terrestrial planets?

Kevin–Yes, and even at some fairly nearby stars. 40 Aradani might have a planet, which is where Vulcans came from in TOS.

Bud–Are planets proportional to stars?

Kevin–Yeah, it’s all interwoven.

Bud–So what’s the detectable range for other G2s?

Kevin–No idea. It’s not a function of size or proximity, it’s torque. I don’t know what torque they’d need.

Dave–The number of exoplanets we’re finding, is it what we’d have expected?

Kevin–I don’t know what they expected, but we have about 450 now, and it’s increasingly rapidly. I suspect that our expectations have gone up and down a lot.

Dave–I just wonder about early stabs at filling in the Drake equation, are they wildly out of date?

Kevin–I don’t know.

Cecilia–The main reason we didn’t find them before was we didn’t have instruments that could detect that shift?

Kevin–Yeah, we could detect stars that had companions with this method for along time, but the instruments are better now.

Ian–points out that another selection effect is that we’re more likely to find stars around less massive stars that are more affected by planetary influences.

Carrie–it’s interesting that everyone used to assume that there were lots of planets, but it was all theoretical. It’s cool that we know now.

Kevin–it could be that planets are the rule, and not the exception.

Monte–Weren’t there people who thought the solar system was unique?

Kevin–Yes, and people are always trying to add more variables to the drake equation to make a smaller and smaller result. Got tricked into doing a religious movie, The Privileged Planet, that was trying to prove the earth was unique.

Me–Cepheid variables could host life?

Kevin–They could… our star is expected to go through the first red giant and then go down and expand back out. On the instability strip, stars get variable. No reason why they couldn’t have planets, but usually their fairly big, though. An error on BSG–they used a helium flash as a warning when a star was about to expand, but actually it wouldn’t happen then, but rather when the star was about to contract.

Ian points out that yesterday Mike said he didn’t think helium flashes would be visible because the photons would have too far to travel.

Kevin says he thinks the flash takes place in the shell and would be visible.

One of the arguments about creationism that frustrates Kevin is that a lot of the dinos in the cretaceous are bent backwards, and the creationist view of that is that the animals are trying to keep their heads above water, when this has to do with the sun dessicating tendons and bending the creatures back.

A planet might be affected by what kind of galaxy it’s in because they might not do well in elliptical galaxies–check out Cosmic Perspective–you would expect you’d find more planets in spiral than elliptical or irregular galaxies.

Would planets mind clusters? On the side, they might not mind–in the middle, they might get too much radiation from the millions of stars in the clusters.

Ian, but how far apart are the clustered stars?

Kevin, well, it depends, and they can be close-enough packed to collide and things.

Kevin adds that stars are more separated from each other than galaxies are, relatively. Which is why when you have galactic collisions, there are rarely stellar collisions.

Me: Do they still think that the meteors hit the earth on a regular basis because of the sun’s position in relation to the galaxy? No, they don’t anymore, the meteor hits seem random.

Bud: Will the Andromeda collide with the Milky Way glancingly or directly? No one knows, even in the simulations. Doing a correct simulation is still a few years off.

Kelly: Does the moon get an inch further away every year forever? Kevin: To a point. Kelly: What point? And does that mean we’ll get smaller tides? Kevin: Yes, we’ll get smaller tides. So, we have the moon and the earth, with tides being the difference between the leading and trailing edge of an object, so tides earth literally gets bowed by the moon. The moon pulls the earth out of round. So the tidal bulge on earth causes the moon to recede, and as the moon recedes, that tidal bulge will be less. Kelly: Will it recede out of orbit? Kevin: Probably not. Kelly: So what’s the limit? Kevin: I don’t know.

Walter: If the moon is made out of debris from this collision can it be knocked apart again easily? Kevin: No, it’s gravitationally bound. Walter: But there are no geological processes.

Ian: Why does the bowing of the earth lead the moon? Kevin: That’s long and counter-intuitive, but…Imagine I have a rubber ball. And the rubber ball is like a kick ball, and it actually deforms, and you have the moon near it. You have anchors on the rubber ball, and a big strong guy in the middle, and chain or something connecting the guy to the anchors so he can pull the ball. So he takes the chains and flexes, pulling in both sides so the ball becomes spherical. Now we spin the ball, and tell Arnold to let go when he reaches the same orientation he was in to start with. So, he does. Earth is not infinitely elastic. It takes a while for it to go from spherical to bowed. And in that time ,the earth will have rotated slightly–about 3 degrees, which is why it leads the moon that much.


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