Friday, October 26, 2012

Speculation - Hot Jupiters as Expended Batteries

Hot Jupiters are large planets – about the size of Jupiter or larger - that orbit very close to their star.  They comprise the upper left corner of this plot.   A year on one of these planets is only a few days, or even less. These planets are relatively easy to discover because of their large and short-period effect on their star's motion and a much higher probability of transit, so the statistics we have on extrasolar planets are skewed a bit toward this type of planet.  Nevertheless, they appear to be fairly common, though are no longer in the majority of the 843 2030 exoplanets catalogued to date.

Exoplanet Orbital period vs. age of host star
Hot Jupiters were a surprise to astronomers when first discovered, although Otto Struve had speculated on their existence in 1952, arguing that they were the only planets detectable by ground based telescopes. Hot Jupiters were not a real paradigm-buster, but were unexpected, because they can't form that close to the star -  it's like trying to get steam to condense on a hot soldering iron. Hot Jupiters have to form much further out from their star, and then migrate inward. There are plausible models for how this could happen fairly quickly as a result of complex tidal interaction with a disk of dust still surrounding the star shortly after it forms.  Such a natural migration may take only a few million years, but it has to happen early or there won't be enough of a disk to interact with.  What astronomers would really like to do is catch one of these planets in the act of migrating, but that will require a bit of luck and probably better telescopes.

However, what if not all the Hot Jupiters result from natural processes?  After all, it didn't happen with any of the four gas giants in our solar system.  They migrated over time, but nowhere near that far in.  Once we have more statistics on exoplanets  we should be able to see if older stars are acquiring hot Jupiters, which would be weird and raise questions, which is what we like here.

A Hot Jupiter's orbit has very low energy compared to the energy of a cool Jupiter. We are talking really massive amounts of energy.  The amount of energy a planet in a circular orbit has in its orbit  increases as its distance from it's star decreases, so you have to take energy out of the orbit to move it inward.  That energy has to go somewhere.

If you took our own Jupiter (a middlin' mass as Hot Jupiters go), and sucked energy out of its orbit until it was one tenth as far from the sun as the Earth (that would be a rough time for the Earth on the way in, so let’s don’t), the amount of energy you would have to remove would be a bit more than 40 trillion trillion kiloWatt hours. If you took a million years to do it (say, until the next time the Cowboys win a Superbowl), this would be require a rate of energy loss a million times the power output of the Earth's electrical generating capacity (approximately 2 Terawatts at present). That is roughly 10 times all the solar energy falling on the Earth. Some of the hot Jupiters out there could have given up quite a bit more energy than that, depending on how far out from their star they began their orbit, their mass, and the mass of the star.

Freeman Dyson
The best part of this is that presumably the energy would all be generated in fairly local area near the planet, an area perhaps a few million kilometers in radius, instead of tens or hundreds of millions as with a Dyson Sphere, which is something like a huge spherical solar panel. A Dyson Sphere could in principle generate orders of magnitude more energy, but is very hard to build and maintain and the energy harvested is distributed over the sphere.  All this building and distribution is a big, long term investment in the star in question.  In a future blog post, I hope to address the question of whether aliens would have economic issues like Return on Investment to deal with.

With that kind of local energy generation capacity, what could you do? My first thought: antimatter. Antimatter (say, positrons) is inefficient to make, but is a great way to store energy compactly for interstellar flight. Even more exotic things might be possible - small wormholes, or even warp drives.  No doubt, it would be something I haven't thought of.  Once again, the question is more interesting than answer.

Do I believe this has happened?  No, but I don't know why it's impossible, and as we search for large scale evidence of advanced civilizations, Hot Jupiters are low hanging fruit.  The test is to see if there are any older solar systems in the process of acquiring a Hot Jupiter - a big planet with an unexplainable decreasing trend in its orbital period.  We probably haven't been observing these planets long enough to measure that, but I hope we'll have observations with smaller error bars over  longer period of time before I am too old to appreciate it.

BTW, the exoplanet data is nicely curated over at .  Go over and have a look.

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