Friday, June 7, 2013

The Halting of the Hot Jupiter

We haven’t talked about exoplanets for a while, and we should ‘cause they are pretty badass. Through various podcasts and the like, I've been hearing some really cool things about NASA’s Kepler Mission and all of neat astronomical bodies it’s been finding. So I decided to browse around the NASA and JPL websites to see what new coolness has been discovered recently.

NASA’s Kepler Mission was launched in 2009. It was built to detect potentially life-supporting planets around other stars. This satellite has a 0.95-meter diameter telescope that continuously and simultaneously monitors the brightness of 100,000 stars brighter than 14th magnitude in the constellations Cygnus and Lyrae. Kepler uses the transit method of planet finding, looking for the drop in the brightness of a star as a planet crosses in front of it at repeated, regular intervals. This dip in brightness not only tells us of the existence of a planet but also its size and orbit, from which we can calculate temperature. So far, the discovered extrasolar planets (or exoplanets) have been giant, mostly the size of Jupiter and bigger. A “hot Jupiter” is one of these large planets that orbits very close to its star. We’re talking less than 1 astronomical unit (AU), with orbital times of only 1-3 days! As such, they are really really hot. A new paper published in The Astrophysical Journal takes a look at one of the biggest questions in exoplanet research: how did these really big planets get so close to their stars? Or, perhaps more importantly, since they are so close, why weren't they pulled into their stars?

It is currently accepted that hot Jupiters formed further out from their host stars, likely beyond the snow line (or frost line), and then migrated in to their current, closer orbits. By observing how an exoplanet moves and how its parent star rotates (the “Rossiter–McLaughlin effect” for you space nerds), we also know that some exoplanets are misaligned and others are not. The misaligned exoplanets are likely directed inward due to interactions with other bodies in the system (gravitational scattering, the “Kozai mechanism”). Essentially, big things that have lots of gravity affect other big things that have lots of graving and they all push each other around. Aligned planets probably ended up where they are through migration in their primordial disk. As the conditions (density, temperature, magnetic fields, etc.) within this disk and the forming planet’s mass and density change over time the planet moves its orbit. Type-I migration assumes the density structure of the disk is affected by turbulence rather than by planets, and as such, is applicable to small mass planets. With Type-II migration, a gap between the planet and disk forms as the result of tidal torques from the planet becoming stronger than the viscous torques of the disk, and as such, is applicable to larger mass planets. The Type-II migration model is good at telling us how gas giants form beyond the snow line and move inwards but not so good at telling us how this migration stops once it is started.

The new study looks at the large ensemble of close-in exoplanets covering a wide range of host star (or stellar) masses in order to discern which mechanism halts exoplanet migration. First, the researchers collected exoplanet data and subdivided by mass the confirmed exoplanets located less than 1 AU from their star. Then they further subdivided Kepler candidates by estimated planet radius into three groups that approximated the terrestrial, super Earth/Neptune, and Jovian planet masses. Next, they attached stellar masses to the system, constraining their study to masses between 0.1 and 1.5 mass of the Sun.

The researchers then took this exoplanet data and put it into several “migration halting mechanism models” to see which model best explained the observations. Their goal was to “generate a reasonably simple prediction for the density of exoplanets as a function of stellar mass and semi-major axis within 0.1 AU.” Now, I’m not a modeler. I’m not even going to pretend I know how to put one together or even really describe it to you without confusing myself and you. Suffice it to say that the authors ran a bunch of models and did a bunch of Bayesian evaluations (my brain rebels at all things Bayesian too).

The astronomers found their tidal circularization model to provide the best mechanism for halting planet migration. Planet-planet scattering, secular chaos, and the Kozai cycle are all mechanisms that migrate planets inward and invoke tidal (or gravitational) forces on it. These forces lower the semi-major axis (the longest radii of an elliptical orbit) and eccentricity until the orbit of the planet becomes more circular. Put simply, when the gas giant gets close to its star, tidal forces cause the exoplanet’s elliptical orbit to become more circular. This circularization stabilizes the planet’s orbit, halting the inward migration and preventing the planet from getting eaten by its star. This result does not rule out the Type-II migration model, but instead, it says that it isn't the role of the primordial disk (or getting to the edge of it) to halt the migration.

If you will remember, they also took a look at stellar mass. They wanted to see if the mass of the star had an effect on the planets’ distance from it. Their models showed that halting distance depends on the stellar mass. This result actually provides further support to the favored tidal forces model. The tidal forces model predicts that hot Jupiters of more massive stars should, on average, orbit further out. Their results also show a halting distance-stellar mass dependence that was stronger than predicted, suggesting that future theoretical work may be needed to reproduce the observed exoplanet distributions.

This begs the question of why did our own Jupiter not migrate inwards? Or if it did, why did it stop so far away? We should definitely be glad that it did (or didn’t?) or our puny little Earth would have been eaten up or thrown out. Thank you Jupiter for ending up where you are.

ResearchBlogging.orgPlavchan, P., & Bilinski, C. (2013). Stars Do Not Eat Their Young Migrating Planets: Empirical Constraints on Planet Migration Halting Mechanisms The Astrophysical Journal, 769 (2) DOI: 10.1088/0004-637X/769/2/86

Read JPL's story about this study: "Stars Don't Obliterate Their Planets (Very Often)"

Learn more about NASA's Kepler Mission:
And I always find good stuff on the NASA and JPL websites!

(image via Cosmos, credit: ESA)

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