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Triple planet system spotted again

Image of three planets orbiting the star HR 8799

This image shows the infrared light (ie. heat) from three planets orbiting a star 120 light-years away. The planets' star, called HR8799, is located at the spot marked with an "X." The three planets, called HR8799b, c and d, are thought to be gas giants like Jupiter, but more massive.

Astronomers have snapped a picture of three planets orbiting a star beyond our own, using just a modest-sized telescope on the ground.

The surprising feat was accomplished by a team at NASA’s Jet Propulsion Laboratory in Pasadena, California, using a small portion of the Palomar Observatory’s huge Hale Telescope, north of San Diego.

The planets had been imaged previously by two of the world’s biggest ground-based telescopes — one of the two 10-metre-wide telescopes of W.M. Keck Observatory and the 8.0-metre Gemini North Observatory, both on Mauna Kea in Hawaii.

Image of the three planets orbiting HR 8799, made in 2008 by the Keck Observatory

Image of the three planets, made in 2008 by the Keck Observatory. The multi-coloured blob in the middle is residual light from the star, most of which has been blocked.

The planets, which orbit a star called HR 8799, were among the very first to be directly imaged, a discovery announced in November 2008.

The new image of the planets, taken in infrared light as before, was captured using just a 1.5-metre-wide portion of the Hale telescope’s mirror.

The team took painstaking efforts to push current technology to the point where such a small mirror could be used. They combined two techniques — adaptive optics and a coronagraph — to minimise the glare from the star and reveal the dim glow of the much fainter planets.

“Our technique could be used on larger ground-based telescopes to image planets that are much closer to their stars, or it could be used on small space telescopes to find possible Earth-like worlds near bright stars,” said Gene Serabyn, an astrophysicist at JPL and visiting associate in physics at the California Institute of Technology in Pasadena.

Planets of a young star

The three planets, called HR8799b, c and d, are thought to be “gas giants” similar to Jupiter, but more massive.

They orbit their host star at roughly 24, 38 and 68 times the distance between the Earth and our Sun, respectively (Jupiter orbits five times further out from the Sun than the Earth does).

It’s possible that rocky worlds like Earth circle closer to the planets’ star, but with current technology, they would be impossible to see under the star’s glare.

The star HR 8799 is a bit more massive than our Sun, and much younger, at about 60 million years, compared to our Sun’s approximately 4.6 billion years. It is 120 light-years away in the constellation Pegasus.

Palomar Observatory's Hale Telescope

Astronomers used a small portion of the Palomar Observatory's Hale Telescope, north of San Diego, to image the three planets.

This star’s planetary system is still active, with bodies crashing together and kicking up dust, as recently detected by NASA’s Spitzer Space Telescope. Like fresh-baked bread out of the oven, the planets are still warm from their formation and emit enough infrared radiation for telescopes to see.

Seeing faint planets

To take a picture of HR 8799’s planets, Serabyn and his colleagues first used a method called adaptive optics to reduce the amount of atmospheric blurring, or to take away the “twinkle” of the star.

Once the twinkle was removed, the light from the star itself was blocked using the team’s coronograph, an instrument that selectively masks out the star. The final result was an image showing the light of three planets.

“The trick is to suppress the starlight without suppressing the planet light,” said Serabyn.

Keeping telescopes small is critical for space missions. “This is the kind of technology that could let us image other Earths,” said Wesley Traub, the chief scientist for NASA’s Exoplanet Exploration Program at JPL.

“We are on our way toward getting a picture of another pale blue dot in space.”

Adapted from information issued by Caltech / NASA / JPL-Caltech / Palomar Observatory.