Published online 13 November 2008 | Nature | doi:10.1038/news.2008.1224

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Astronomers claim first snaps of planets beyond the Solar System

Images of potential planets orbiting distant stars after decade-long search.

The HR 8799 planetary systemInfrared image of the HR 8799 planetary system. The planets (red dots) b,c and d are 70, 40 and 25 times further away from the star than the Earth is from the Sun.National Research Council Canada

Two teams of astronomers are independently claiming to have the first ever images of planets in orbit around a star other than the Sun — with pictures from one team showing three planet-like bodies orbiting a distant star.

Using the Keck and Gemini telescopes in Hawaii, one team took infrared images of three objects, each 5-13 times the mass of Jupiter, in orbit around HR 8799, a star 130 light years from Earth in the constellation Pegasus1.

The other team used the Hubble Space Telescope to take photographs of a potential planet that's no bigger than three Jupiters2. It circles Fomalhaut, a star 25 light years away from Earth in the constellation of Piscis Austrinus, completing one orbit every 872 years. The object is roughly 119 times further away from its star than Earth is from the Sun, and is located at the inner edge of a debris disk that it appears to have sculpted into a sharp smooth ring by pulling in stray dust as it orbits.

Astronomers have been hunting for direct evidence of planets orbiting a star outside the Solar System for nearly a decade. More than 300 extrasolar planets, or exoplanets, have been discovered so far but these have been found using indirect methods — for example, by detecting the way a star wobbles as planets orbit them rather than using images.

If the objects observed by the two teams are confirmed to be planets, not only would they be the first images of planets orbiting a star other than the Sun, they would also be the first direct observations of a multiplanetary system and of a planet in orbit on the edge of a debris disk.

These claims of direct imaging are "very exciting", but are "no more definitive than all past claims", says David Charbonneau, an exoplanet expert at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. The claims are no more definitive because the planets' masses are calculated indirectly, he explains.

Size uncertainty

In the case of the triad of prospective planets, the team used the brightness and calculated age of the objects to infer their masses. HR 8799 and its constituents are thought to be about 60 million years old. So, based on a model of how planets cool as they form, they are between 5 and 13 Jupiter masses, the team reports in Science.

"If you believe the model and estimated ages, then these are planets," says Drake Deming, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Maryland. But he adds, "The principal uncertainty here is in the model. We don't have enough information about early planet formation to test it, and if you use a different model these could be brown dwarfs." Brown dwarfs are failed stars that are just over 13 Jupiter masses.

The three objects observed around HR 8799 are all orbiting in the same direction. Whereas planets tend to travel around a star in the same direction because they're formed from a single rotating disk of dust, stars going around another star may not. In addition, astronomers have never observed three brown dwarfs orbiting a single star before.

"That's why we're certain these are planets," says Christian Marois, lead author of the study and an astronomer at the National Research Council's Herzberg Institute of Astrophysics in British Columbia, Canada. It's also likely that these objects are planets because the star hosts a massive debris disk from which they could have formed, he adds.

Based on those two arguments, the objects "sure do look like planets," agrees Deming. Capturing pictures of planets near stars' debris disks is also what makes the Fomalhaut findings so compelling, he adds.

Disk discovery

Optical image of planet orbiting FomalhautOptical image of the putative planet orbiting Fomalhaut (left) and a close up (right).P. Kalas, University of California in Berkeley

The team that discovered the object circling Fomalhaut used the gravitational interaction between the debris disk and the prospective planet, called Fomalhaut b, to calculate that it was between a half and three Jupiter masses. "If it were any bigger or any smaller, the debris disk near Fomalhaut b wouldn't have such a sharply defined inner edge," says Eugene Chiang, an astronomer at the University of California in Berkeley, and co-author on a report about the object that also appears in Science.

That is a strong argument, Charbonneau says, adding that both teams make excellent cases for their discoveries. But he and Deming agree it's too early to declare these images the first, historic images of worlds beyond our Solar System. 

  • References

    1. Marois, C. et al. Science doi: 10.1126/science.1166585 (2008).
    2. Kalas, P. et al. Science doi: 10.1126/science.1166609 (2008).
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