Big planet with a small orbit

Artist’s impression of planet Beta Pictoris b

Artist’s impression of how the planet "Beta Pictoris b" may look. The planet is about 9 times as massive as Jupiter.

  • Star is 60 light-years from Earth
  • Planet spotted years apart on both sides of the star
  • Shows that it is in a small orbit

For the first time, astronomers have been able to directly follow the motion of a distant planet as it moves from one side of its host star to the other.

The planet has the smallest orbit so far of all directly imaged exoplanets (ones that orbit stars other than our Sun), lying almost as close to its parent star, Beta Pictoris, as Saturn is to the Sun.

Scientists think that it may have formed in a similar way to the giant planets in the Solar System. And because the star is so young, the discovery shows that “gas giant” planets can form in only a few million years, a short time in cosmic terms.

Only 12 million years old, or less than three-thousandths of the age of the Sun, Beta Pictoris is 75% more massive than our parent star. It is about 60 light-years away and is one of the best-known examples of a star surrounded by a flattened, dusty debris cloud, what astronomers call a “disc”.

Earlier observations showed that there is a “warp” in the disc, plus a second disc at an angle to the first one, and comets falling onto the star.

“Those were indirect, but tell-tale signs that strongly suggested the presence of a massive planet, and our new observations now definitively prove this,” says team leader Anne-Marie Lagrange.

“Because the star is so young, our results prove that giant planets can form in discs in time-spans as short as a few million years.”

Recent observations have shown that disc clouds around young stars disperse within a few million years, and that giant planet formation must occur faster than previously thought.

Composite image showing the observations of the planet obtained in 2003 and late 2009

This composite image shows the observations of the planet obtained in 2003 and late 2009 (with the glare of the star blocked out). The possible orbit of the planet is also indicated.

Smallest orbit for an imaged exoplanet

The team used an instrument mounted on one of the 8.2-metre telescopes of the European Southern Observatory’s (ESO) Very Large Telescope (VLT), to study the immediate surroundings of Beta Pictoris in 2003, 2008 and 2009.

In 2003 a faint dot was seen inside the disc, but it wasn’t possible to exclude the remote possibility that it was a background star. In new images taken in 2008 and early 2009, it had disappeared!

But more recent observations, taken later in 2009, showed the object on the other side of the star after a period of hiding either behind or in front of it (when it would have been hidden in the glare of the star).

This confirmed that the spot was indeed a planet orbiting its host star. It also provided insights into the size of its orbit.

The planet (designated “Beta Pictoris b”) has the smallest orbital distance known so far for any of the ten exoplanets that have had images made of them, between 8 and 15 times that of Earth’s orbit around the Sun.

“The short period of the planet will allow us to record the full orbit within maybe 15-20 years, and further studies of Beta Pictoris b will provide invaluable insights into the physics and chemistry of a young giant planet’s atmosphere,” says student researcher Mickael Bonnefoy.

Adapted from information issued by ESO / A.-M. Lagrange / L. Calçada.

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