Sunday, February 21, 2016

Life in Solar Systems with Hot Jupiters

Many of the exo-planets first found were hot jupiters, planets with mass similar to our Jupiter's, but with an orbital radius more like Mercury, with some even closer in. These were found by the velocity method, where the spectral lines of the star were observed with very precise spectroscope, and periodic fluctuations in the wavelengths were found. These fluctuations were correlated with what there would be if a large planet were orbiting close to the star, causing it to be moving around the common center of mass.

The prevalence of hot jupiters in the initial tallies of exo-planets was attributed to the selection effect. Spectroscopes were just good enough to detect the motion of the star when a large planet was causing it to move, as there are various noise effects in the data, plus some instrumental accuracy to worry about. There is no doubt that the selection effect was a fact, and as spectroscopes became better and better, and at least local noise was tamped down, many more planets of smaller size and larger orbits, mostly both, were discovered, thus validating the suspicion of the selection effect. However, there are solar systems out there with hot jupiters in them, and the question is, are there planets there, and if so, could they support life?

Where do they come from? Humor aside, the common theory of origin of these planets is that they were formed out further in their solar systems, and then they migrated in. They migrated in because they lost angular momentum, but it is not so clear why they would, in a system where everything is orbiting around the same central mass, with the same velocity vs. radius curve. They would have to be born in close, as a hot jupiter, to have the star's tidal pull grab it and drag it in closer. So perhaps an alternative explanation is that they were born in closer, about where they were observed.

When a cloud of gas starts condensing to form a solar system, it has some angular momentum. It keeps that angular momentum all through the condensation process, with the star and the planets sharing it. Perhaps some gets lost when the solar wind picks up enough power to push out some of the remaining gas, but by and large, the planets and star inherit it. Angular momentum is just a measure of how fast the gas cloud was rotating. Some clouds should be rotating faster than others, in other words, there is a distribution of angular momentum, or better put of the angular momentum divided by the total mass of the cloud. At the upper end of the distribution, things are merrily whirling around, and the gas cloud starts forming planetesimals out far from the star. But at the lower end of the distribution, there isn't much whirling at all, and the gas keeps falling directly into the star. If there was virtually no angular momentum, all the gas would fall into the star, and there would be no planets. Do we know how many stars have no planets – not yet, so this avenue does not provide clues. But assuming the distribution doesn't have some minimum to it, there will be gas clouds condensing with little angular momentum. These can form hot jupiters, as all the gas gets in pretty close before the little angular momentum it has provides enough centrifugal force to counteract the centripetal force of the central star.

If this concept of formation is correct, it would mean that there wouldn't be much mass left out far beyond the hot jupiter to form other planets. It's all heading inward. But there might be enough for some other planets to form close in near the star. These would likely be smaller planets. At this point in the data collection efforts on exo-planets, it is not clear if many stars with hot jupiters have other planets. At least one is known, but not a lot.

The stars that form hot jupiters seem to be preferentially F and G stars, which is reasonable because of the mass involved. K stars are smaller, and red dwarfs, M stars, much smaller, and if the planetary mass is proportional to the total mass, there isn't much around for a jupiter-sized planet.

F and especially G stars are ones which are expected to have origin planets, provided a lot of conditions are met. One of the easy to check ones is the orbital radius. Is the planet in the LWZ, the liquid water zone? Recall that it is not enough that a planet is sitting in the LWZ, but that longevity of that location is important. Life doesn't form overnight, and a planet has to sit for a long time in the LWZ in order to let life get started.

If a hot jupiter is in the same solar system, this means that there is a source of angular momentum near the star that is much greater than tidal effects. What this source means is that a coupling of angular momentum out from the hot jupiter to the outer planet, over a billion years or so, might move it either into the LWZ or out of it, or even through it. This means the planet is not a candidate for life to evolve there, or if it did, that it would be snuffed out by the radial migration of the planet.

The coupling of angular momentum to an outer planet happens by an analogous mechanism to tidal effects, which means it is a second-order effect. Second-order effects are weak, but sensitive to the mass of the hot jupiter and the outer planet. It is easy to understand. Suppose you are on the outer planet and are watching the hot jupiter orbit around the star, your sun, in close. When the hot jupiter is on the approaching side of its orbit, it is trying to slow your planet down, assuming you are both orbiting in the same direction and roughly the same orbital plane. When the hot jupiter is on the receding side of its orbit, it is trying to speed your planet up. The two effects are almost identical, and so each orbit of the hot jupiter has an averaging effect.

Your planet is trying to speed up the hot jupiter during its approaching phase and slow it down during its receding phase, and this means that, ever so slightly, it is going to be spending less time on the approaching phase than the receding phase, so the net effect of the approaching phase will be just a bit stronger. Your planet is going to go in toward the star. With luck, the effect will not be enough to move your planet out of the LWZ or perhaps, with a lot of luck, the star might be losing a bit of its energy as it gets older and the hot jupiter might just be keeping you in the LWZ.

It is hard to say whether solar systems with hot jupiters should be taken off the list for doing the very precise and very expensive work of looking for some clues for life there. What can be done is some good simulations of the effects to see just which ones of them have planets moving out of the zone during a period of interest, or whether they were just pulled into it. Simulations of planetary motion is so well figured out that it can be done easily – wasn't it Galileo or one of his successors that first did it?

One thing that is fortunate is the selection effect. Solar systems with hot jupiters are hard to miss, unless the orbit of the jupiter is almost perpendicular to the line of sight from here to there, and that is not very likely, given that the dominant source of all the angular momentum comes from galactic rotation. That means looking perpendicular to the galactic disk might be problematic, but most stars aren't in that direction anyway.

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