Smaller "Siblings" of Jupiter, Saturn Discovered

Anne Casselman
for National Geographic News

February 14, 2008

Two newfound extrasolar planets orbiting a star about 5,000 light years away resemble smaller versions of our own Jupiter and Saturn, an international team of astronomers reports.

These planetary "siblings" are about 80 percent as big as our gas giants and orbit a star that's about half the size of the sun.

What's more, the smaller planet lies about twice as far from its star as the larger one, just as Saturn is twice as far from the sun as Jupiter.

"The interesting thing about this system is that it looks very similar to our own solar system, but scaled down," said team leader Scott Gaudi, an astronomer at Ohio State University. (Explore an interactive solar system.)

Gaudi and colleagues from 11 ground-based observatories describe the work in tomorrow's issue of the journal Science.

The discovery is the first time that an alien planetary system with both Saturn and Jupiter analogues has been spotted, said Sara Seager, a planetary scientist at Massachusetts Institute of Technology, who was not involved in the study.

It's also only the third time that a Jupiter-mass planet has been found using a technique called gravitational microlensing.

This method has been used to study dark matter and has only recently been applied to the search for so-called exoplanets.

"Until now microlensing has been kind of like a poor cousin in exoplanet sighting," Seager said.

"So I think it's a big discovery, because the true power of this technique is just becoming clear."

Wobble vs. Lensing

The most common planet-hunting technique is known as the Doppler method.

Stars and their planets orbit around a common center of mass, so a star with planets will move differently than a star without them.

Looking at how a star wobbles allows scientists to deduce how many and what types of planetary bodies are in orbit.

The wobble method has been especially successful at finding "hot Jupiters"—large Jupiter-mass planets that circle tightly around their stars.

(Related news: "First Proof of Wet 'Hot Jupiter' Outside Solar System" [July 11, 2007].)

But with current technology, astronomers using this method can study only systems that are tens to hundreds of light-years away.

By contrast, gravitational microlensing can study stars that are thousands of light-years away.

That's because the method uses the gravity of a nearby star and its planets to magnify the light from a much more distant background star.

"We wait for a system to pass in front of background star, and by monitoring the peaks and brightness we see we can figure out a lot about the planetary system," Gaudi said.

The better the alignment between the two stars, the better the magnification.

In the case of the two new siblings, the alignment between March and April of 2006 was so good that the background star was magnified by a factor of 500.

"This is the first time that we've found a Jupiter-size planet where the magnification was high enough that we could have detected a second, lower-mass system, and we found it the first time," Gaudi said.

"So that tells you either we're incredibly lucky or these things are actually quite common."

One-Time View

"The one thing about microlensing … is you can't study the system in detail ever again," MIT's Seager said.

That's because the "lens" star and its planets will probably never pass in front of another background star with such good alignment again, she said.

But microlensing techniques do give scientists a way of taking a "population census" of the planets that are out there, study leader Gaudi said.

Jaymie Matthews, an astronomer and mission scientist for the MOST Space Telescope Project at the University of British Columbia in Vancouver, was not involved in the new study.

"The larger the sample that you can survey, the more reliable the results and the more likely that you will find the exceptional thing as well as the common ones," Matthews said.

"The next real milestone [for microlensing] that will truly excite the public and scientists will be the discovery of Earth-like planets in terms of Earth size and Earth mass."

Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).