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Hot, giant planet seen again

Artist's impression of Beta Pictoris b

Artist's impression of the hot, giant planet that orbits the star Beta Pictoris. Also visible is the cloud of gas and dust that encircles the star.

  • Beta Pictoris is a nearby star (63.4 light-years away) surrounded by gas and dust
  • Large, suspected planet was spotted in images taken in 2003 and 2009
  • New images have confirmed it is definitely a planet orbiting the star

IN 2009, ASTRONOMERS ANNOUNCED they had taken images of a suspected planet orbiting a nearby star.

Ordinarily, the presence of such exoplanets can be determined using various methods, but almost all of them are too small and too far away to be directly seen and imaged.

This particular planet, though, orbits a relatively nearby star called Beta Pictoris. It also happens that its orbit around the star is at right angles to our line-of-sight, making it much easier to spot.

Today, astronomers announced (in a paper published in the journal Astronomy & Astrophysics) that they have taken new images the planet, and have confirmed that its position has changed, consistent with it orbiting its star.

In the new observations, the planet—called ‘Beta Pictoris b’—was seen with the NaCo instrument on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile.

The observations have also enabled the astronomers to measure its mass and the effective temperature.

Huge and hot

Located 63.4 light-years away, Beta Pic is a very young star—only about 12 million years old, compared to our Sun’s age of 4.5 billion years. It is also 75% more massive than our Sun.

Beta Pic is well known for being encircled by a large cloud of gas and dust…what astronomers call a ‘circumstellar disc’. It was actually the first star to have its disc directly imaged more than 25 years ago.

Movement of exoplanet Beta Pictoris b

Moving pictures. The exoplanet Beta Pictoris b was imaged in 2003 (left) and again in October 2009 (middle) when it had moved to the other side of the star. The new observations (right), made in March 2010, show the planet has moved yet again. (The glare of the star has been blocked out.)

In 2009, the giant planet was spotted orbiting within the disc. With an orbital distance of 8 to 15 astronomical units, Beta Pictoris b is the closest exoplanet to its star that has ever been imaged. (An astronomical unit, or AU, is a standard measurement in astronomy, being the distance between the Earth and the Sun.)

Analysing the new observations, the team have estimated the planet’s mass—around 7 to 11 times the mass of Jupiter (the largest planet in our Solar System). They’ve also estimated its temperature—between 1,100 and 1,700 degrees Celsius.

Just a youngster

The planet offers a new opportunity for astronomers to study planetary formation processes, and in particular the way planets and their stars’ circumstellar discs interact.

In fact, the new data has already told the astronomers something important about the formation of the planet (especially because the system is very young)—that the planet is still warm implies that it has retained most of the primordial heat acquired during its formation.

If it had formed in a similar way to the giant planets of our Solar System, its mass and temperature could not be explained by some models that suggest a total release of that energy.

More observations with NaCo and also with the next generation VLT instrument, SPHERE, should soon provide more details about the planet’s atmosphere and orbital properties, and about the way it influences the gas and dust cloud surrounding the star.

The team of astronomers includes M. Bonnefoy, A.-M. Lagrange, G. Chauvin, D. Ehrenreich, D. Mouillet (IPAG, Grenoble, France), A. Boccaletti, D. Rouan, D. Gratadour (LESIA-Observatoire de Paris, Meudon, France), D. Apai (Space Telescope Institute, Baltimore, USA), F. Allard (CRAL-ENS, Lyon, France), J.H.V Girard (ESO, Santiago, Chile), M. Kasper (ESO, Garching, Germany).

Adapted from information issued by Astronomy & Astrophysics. Illustration courtesy ESO / L. Calçada.

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Big stars aren’t special

Artist's impression of the dusty cloud surrounding the star IRAS 13481-6124

Artist's impression of the dusty cloud surrounding the star IRAS 13481-6124, and the jets of particles spat out by the star. Observations suggest big stars such as this one—20 times the mass of the Sun—are born in the same way as smaller stars.

  • Dusty cloud discovered encircling a young star
  • Star is 20 times as massive as the Sun
  • Big stars form the same way as small ones

Astronomers have obtained the first image of a dusty disc-shaped cloud closely encircling a massive baby star, providing direct evidence that massive stars form in the same way as their smaller brethren.

The discovery, made thanks to a combination of the European Southern Observatory’s (ESO) telescopes, is described in an article in this week’s issue of Nature.

“Our observations show a disc surrounding an embryonic young, massive star, which is now fully formed,” says Stefan Kraus, who led the study. “One can say that the baby is about to hatch!

The team of astronomers looked at an object known by the cryptic name of IRAS 13481-6124. About twenty times the mass of our Sun and five times its radius, the young central star, which is still surrounded by its pre-natal cocoon, is located about 10,000 light-years away.

From archival images obtained by the NASA Spitzer Space Telescope as well as from observations done with the APEX 12-metre submillimetre telescope, astronomers discovered the presence of a jet, or focused stream of particles.

“Such jets are commonly observed around young low-mass stars and generally indicate the presence of a disc,” says Kraus.

Circumstellar discs are an essential ingredient in the formation process of low-mass stars such as our Sun. However, it is not known whether such discs are also present during the formation of stars of more than about 10 times the mass of the Sun, where the strong light emitted might prevent mass falling inwards towards the star.

For instance, it has been proposed that massive stars might form when smaller stars merge.

A view of the dust cloud surrounding IRAS 13481-6124

A real view of the dust cloud surrounding the star. Combining the light from multiple telescopes enabled a more detailed view to be made of it.

Telescope team effort

In order to discover and understand the properties of this disc, astronomers employed ESO’s Very Large Telescope Interferometer (VLTI). By combining light from three of the VLTI’s 1.8-metre Auxiliary Telescopes with the AMBER instrument, the facility enables astronomers to see details equivalent to those a telescope with a mirror of 85 metres in diameter would see.

The resulting resolution is about 2.4 milliarcseconds, which is equivalent to spotting the head of a screw on the International Space Station from the ground, or more than 10 times the resolution possible with current visible-light telescopes in space.

With this unique capability, complemented by observations done with another of ESO’s telescopes, the 3.58-metre New Technology Telescope at La Silla, Kraus and colleagues were able to detect a disc around IRAS 13481-6124.

“This is the first time we could image the inner regions of the disc around a massive young star”, says Kraus. “Our observations show that formation works the same for all stars, regardless of mass.”

The astronomers conclude that the system is about 60,000 years old, and that the star has reached its final mass. Because of the intense light of the star—30,000 times more luminous than our Sun—the disc will soon start to evaporate.

The flared disc extends to about 130 times the Earth–Sun distance—or 130 astronomical units (AU)—and has a total mass similar to that of the star, roughly 20 times the Sun.

Adapted from information issued by ESO / Spitzer / NASA / JPL / S. Kraus.

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