The planet (circled) is eight times the size of Jupiter and orbits a star about 500 light years from Earth, say astronomers at U of T. (Gemini Observatory)

Planet-hunting astronomers at the University of Toronto say they have taken a picture of a rare sight — a planet orbiting a star outside our solar system.

Astronomers David Lafreniere, Ray Jayawardhana and Marten van Kerkwijk made their finding after doing a survey of 80 stars taken using the Gemini North telescope on the summit of Mauna Kea in Hawaii. Their findings, which have been submitted for publication in Astrophysical Journal Letters, have been posted online on Cornell University's arXiv website.

In the near-infrared images taken of one young star about 500 light years from Earth, they discovered a companion to the star which they say is a giant planet about eight times the size of Jupiter.

The unique discovery raises the possibility that followup study will be able to take a more detailed spectra of the planet and learn about its chemical composition, said Jayawardhana. (Listen to a Quirks and Quarks interview with Jayawardhana.)

"Astronomers have discovered over 300 extrasolar planets since we first started looking for them some 20 years ago, but most times we are inferring the existence of a planet based on indirect methods of detection," he said. "This is one of the rare cases where we can actually see the planet in the near-infrared and take a picture."

Most extrasolar planets are discovered using indirect means. Many are inferred by observing slight alterations in the orbit of stars due to the gravitational influence of the planets, while some are noticed when they pass in front of — or transit — our view of the star, slightly altering the light we see.

Actual imaging of a planet on the other hand, is extremely rare, since it is so difficult to spot relatively dim planets next to bright stars.

Star similar to our sun

Near-infrared images have been taken of planets orbiting dim brown dwarf stars, but this is the first image of a planet orbiting a star that is similar in size to our sun, they said.

The planetary-mass companion to the star is photogenic in part because not only is it huge, it is also hot, with an estimated temperature of about 1,800 kelvin (about 1,500 C). In that sense, the discovered object is closer to a brown dwarf star than to a planet like Earth, but is still small enough to be considered a planet.

It also appears to orbit the star at a distance of roughly 330 times the distance from the Earth to the sun. By comparison, the astronomers say the distance from the sun to the most distant planet in our solar system — Neptune — is only 30 times the Earth-sun distance.

While the planet's size and youth make it shine bright in the sky, its distance from the star allows it to stand out on its own, says Jayawardhana.

For planets so far distant from their parent star, seeing them directly may actually be easier than inferring their existence through other means, says Jayawardhana, since at that distance the planet's gravitational influence on a star is likely to be minimal.

Van Kerkwijk said the discovery of the planet raises a host of new questions about planetary formation. Mainly, how did such a large-massed planet get so far away from its star?

It's a problem the first planet hunters faced when they discovered Jupiter-sized planets in close orbits to their stars. These hot Jupiters at first confounded astronomers until they began to discover they were relatively common. Van Kerkwijk said the same theories that suggest hot Jupiters form at greater distances and then move closer to their parent stars could also work in reverse, with the gas giants moving farther away over time.

He also suggests a more mundane solution would be that the planet and its star formed in a manner similar to the way double or binary stars form in tandem.

Van Kerkwijk said a very distant Jupiter-like planet like the one discovered is a relatively rare find, and would probably only be found in one in 100 stars. But he said its discovery opens up yet another potential kind of planet astronomers could be looking for.

"This is the start of what I expect will be many more discoveries," said van Kerkwijk.