UC Berkeley researchers publish study on orbits of exoplanets

This artist's illustration depicts the extrasolar planet HD 189733b with its parent star peeking above its top edge.
NASA, ESA, and G. Bacon/Courtesy
This artist's illustration depicts the extrasolar planet HD 189733b with its parent star peeking above its top edge.

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On Friday, UC Berkeley researchers published a study in the journal Science that helps solve one of the mysteries of the orbital features of certain exoplanets, or planets that orbit stars in other solar systems, but the discovery itself poses new problems.

Warm Jupiters, massive planets composed primarily of gasses, are a galactic oddity because they orbit much closer to their stars than their counterparts in the Earth’s solar system. The researchers have been able to model a process by which these giants might come to reside so close to their stars.

The researchers presented data showing observed warm Jupiters with giant companion planets orbiting the star on separate, skewed planes from one another. Both exoplanets exhibit eccentric orbits, meaning they are not always equidistant from the sun as they travel around it but rather alternate in proximity through each rotation.

“Eccentric warm Jupiters are close to their star, about the distance of Mercury,” said Rebekah Dawson, the study’s co-author and a campus postdoctorate Miller Research Fellow. “It is challenging to explain how they got so close in and why they’re on an elliptical orbit.”

A problem arises in explaining how a planet’s orbit would become tilted from the inclination of the original disk.

“We call them Jupiters but one of the objects is often 10 times the size of Jupiter. It can really kick its neighbor, maybe one Jupiter mass, so it seems natural,” said Eugene Chiang, a campus professor of astronomy and earth and planetary science.

In the solar system, planets that are the size and composition of warm Jupiters reside in an orbit much further from the sun. Hot and warm Jupiters are so named because their proximity to their star means much more radiation is transmitted to their atmospheres, causing surface temperature to rise.

Similar to the way in which the moon is slowly moving away from the Earth due to the process of tidal dissipation, Dawson postulates that the warm Jupiters may be doing the same.

“These systems are still tidally evolving, still evolving in a slow gentle way,” she said.

While the existence of this class of planets has been known for years, the mechanisms causing their orbital orientation had not been well established. The team was able to analyze data regarding the gravitational pull of the planets on their host star by observing the Doppler effect, the shift in wave frequency as the star is tugged back and forth by the planets.

“For me what’s most exciting is just the finding that … not every solar systems looks as flat as our own. That to me has rocked my world,” Chiang said. “How it got there remains a mystery. So this idea of planet-planet scattering, this post formation violent encounter, is something I’m inclined to further explore.”

Contact Isaac Smith at [email protected] and follow him on Twitter @IsaacGSmith.

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