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Youngest planet imaged
as it forms

Posted: 20 October 2011

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Using the ten-metre Keck telescopes, astronomers have snapped the first direct image of a super-Jupiter planet in the act of formation.

The evolving protoplanet, known as LkCa 15 b, is surrounded by a swirling mass of gas and dust that is accreting onto the planet. At a youthful two million years in age, it is five times younger than the previous record holder.

Left: The disc around the star LkCa 15 – the hole in the centre indicates an inner gap with radius of about 55 times the distance from the Earth to the Sun. Right: An expanded view of the central part of the cleared region, where the blue light is data at a wavelength of 2.2 microns, representing the forming planet, and the red light is data at a longer wavelength of 3.7 microns, representing the cooler gas and dust accreting onto the planet. Image: Kraus & Ireland 2011.

"The planet looks like it's approximately six times the mass of Jupiter, though this might be an overestimate," Adam Kraus of the University of Hawaii’s Institute for Astronomy tells Astronomy Now. "We're assuming all the light comes from the planet, but much of the light – perhaps all of it – could be produced by material that glows as it falls onto the planet. If so, then the planet itself could be much fainter, and hence much less massive."

Kraus adds that there is quite a bit of material left in the protoplanetary disc, outside of the planet's orbit. "Some [of the material] will fall past the planet and ultimately fall onto the star, some will fall onto the planet and make it larger, and some will be blown away by light from the central star," he explains. "It's feasible that the planet could grow as large as ten Jupiter masses, but this depends on the true mass of the planet right now, and I could see the final number being much smaller."

The discovery of LkCa 15 b began as a survey of 150 young, dusty stars in known star-forming regions, leading to a focused study of a dozen stars. LkCa 15 was the second target and immediately struck Kraus and colleague Michael Ireland (of Macquarie University and the Australian Astronomical Observatory) as interesting. They identified a faint point source near to the star that seemed a likely candidate for a hot-Jupiter.

Artist's impression of the super-Jupiter planet forming in the cleared disc around LkCa 15. Image: Karen L. Teramura, UH IfA.

The follow-up observations relied on the power of Keck’s adaptive optics combined with a technique called aperture mask interferometry, which manipulates the light from the object to cancel out distortions, along with the bright light of the central star, in order to resolve the discs of dust around the star and reveal gaps where protoplanets may be hiding.

"Other observations (not our own) already provided strong evidence that there was a gap in the disc, which was probably tidally opened by a newly-formed planet," says Kraus. "The difficult observation is to directly look for these planets, since we're looking for relatively faint planets next to very bright stars. We realized we had uncovered a super Jupiter-sized gas planet, but that we could also measure the dust and gas surrounding it. We’d found a planet at its very beginning."

Models of planet formation suggest that young, newly-formed exoplanets should be a warm 1500-2000 Kelvin, while gas and dust tends to be cooler. "The best interpretation is that we're seeing a planet at the position with mostly shorter-wavelength light (the blue peak in the image), with circumplanetary gas and dust around the planet that is traced by the longer-wavelength light (the red structure in the image)," explains Kraus. "In this sense, the colours match our intuition for optical light – warmer objects are blue, cooler objects are red."

Directly imaging LkCa 15 b is just the beginning for Kraus and Ireland, who plan to continue observations of this forming planet along with other nearby young stars to learn more about how planets and solar systems take shape.

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