Eccentric Exoplanet Gets Hot Flashes

Victoria Jaggard
National Geographic News
January 28, 2009
A distant Jupiter-like planet on an eccentric orbit swings so close to its parent star that its temperature spikes by about 1,260 degrees Fahrenheit (682 degrees Celsius) in only six hours, a new study reports.

Then as rapidly as it heats up, the extrasolar planet cools back down after zipping past its star, said lead study author Gregory Laughlin, an astronomer at the University of California, Santa Cruz.

The planet's path is unique, Laughlin noted. Most known "hot Jupiters" have tight, roughly circular orbits. They are tidally locked, showing only one face to their stars, just as the moon does to Earth.

(Related: "Half-Hot, Half-Cold Planets Have Supersonic Jet Streams" [October 20, 2008].)

"But this planet has the most eccentric orbit of any discovered," he said. Its elongated elliptical path makes it impossible for the world to be tidally locked, "so it's guaranteed to bring the planet spinning in every 111 days for a harrowing encounter."

The study is exciting, because it offers a glimpse at a very different kind of planet than most known worlds, said Heather Knutson, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), who was not involved in the research.

And more data about this odd world can help scientists calibrate existing models for planetary climates, as well as find out whether they have a firm grasp on the fundamental physics of atmospheres, she said.

Fast and Furious

The exoplanet HD 80606b lies about 190 light-years away, orbiting a star in the constellation Ursa Majoris. It is roughly the same size as Jupiter but has four times the mass.

The planet was discovered in 2001 using a method that looks for the wobble in starlight created by a planet's gravity tugging on its host star.

In addition to showing that a planet is there, this method reveals details of its orbit and gives a minimum possible mass.

Due to its unusual path, HD 80606b spends most of its time about as far from its star as the midway point between Earth and Venus is from the sun. (Explore an interactive solar system.)

But during a stellar encounter that lasts less than a day, the planet swoops in to about ten times closer than Mercury is to the sun.

This means that for a short period the exoplanet is getting 828 times the average amount of heat from its star.

Using the Spitzer Space Telescope, Laughlin and colleagues measured continuous changes in the planet's temperature over a 30-hour period that included a close approach.

The team found that the planet's atmosphere jumps from about 980 degrees Fahrenheit (527 degrees Celsius) to almost 2,240 degrees Fahrenheit (1,227 degrees Celsius).

What's more, the atmosphere reaches these extreme temperatures extremely fast, the scientists report in this week's issue of the journal Nature.

"It's very, very responsive to the dose of heat it's getting from its star," Laughlin said. "This tells us that the planet's atmosphere absorbs starlight very high up where the air is thin.

"If Jupiter was on an orbit like this, it would probably respond in a similar way, but it wouldn't heat up quite so rapidly," he added.

"Earth would have a much more sluggish response, because the sunlight penetrates to a deeper level."

Planet Detectives

According to CfA's Knutson, the study adds to evidence for why some gas giant planets—which are very distant from the sun in our solar system—orbit so close to their parent stars in other systems.

"As we understand the properties of extrasolar planets, we can do detective work to find out where they came from," Knutson said.

The current theory is that hot Jupiters start forming farther away from their stars and slowly migrate inward.

"The fact that this planet has such an eccentric orbit means something odd happened in this system," she said of HD 80606b.

Knutson and Laughlin are both excited that the Spitzer observations also showed the planet passing behind its host star as seen from Earth. This allowed Laughlin's team to calculate a 15 percent chance it will pass in front of, or transit, the star on February 14.

During a transit, the planet blocks some of its host star's light, allowing astronomers to take a more direct measurement of the object's size.

In addition, some of the starlight gets filtered through the planet's atmosphere. Astronomers can then read signals in the light that indicate the types of gases present.

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