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Astronomers spot the birth of a planet

Posted on behalf of Ron Cowen.

Astronomers have for the first time imaged a planet so young that it’s still packing on the pounds from its birth site, a disk of gas and dust surrounding a star 450 light-years from Earth. The hatchling’s age, estimated mass and distance from its parent star support one of two competing models for planet formation.

“This may be the first time we have been privileged to see such a young protoplanet — it is like being able to look at the babies in the maternity ward of the local hospital, instead of looking at the rest of the patients, middle-aged and beyond,” says theorist Alan Boss of the Carnegie Institution for Science in Washington DC, who was not part of the study.

Adam Kraus of the University of Hawaii at Manoa and his colleague Michael Ireland of the Australian Astronomical Observatory in Epping, homed in on the youthful, sunlike star LkCa 15 because the planet-forming disk of gas and dust swaddling it has a special property: the disk has a large gap in the middle of it, which could be a sign that the gravity of one or more orbiting planets has cleared away material.


Left: The disk of gas and dust around the young star LkCa 15 contains a large gap, an indication that a planet has cleared the region. Right: An expanded infrared view of the central part of the cleared region shows a composite of a newborn planet (blue), surrounded by a swath of cool dust (red) and gas that is falling onto the planet and heating the orb’s outer layers. The location of the parent star is marked by the five-pointed star.

Credit: A. Kraus & M. Ireland 2011

Kraus and Ireland, who reported their findings on 19 October on the preprint server and at a conference on signposts of planets at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, went searching for a planet in the gap using the large Keck-II telescope atop Hawaii’s Maua Kea. Young exoplanets had never been imaged because telescopes typically lack the resolution to pick out faint planets that lie near the blazing light of their youthful parent stars, and because of the distortion caused by Earth’s turbulent atmosphere.

To overcome these problems, the astronomers inserted a mask with holes in it into the telescope’s optics, which allows a small but carefully chosen fraction of the light from LkCa 15 to enter the observatory. The distance between any pair of holes in the mask is never the same, which enables observers to determine which light paths are affected by a particular atmospheric distortion and to cancel the distorting effect.

Using the method, Kraus and Ireland recorded a compact object in the gap with an estimated mass between that of Saturn to several times that of Jupiter (pictured). But the body has a much bluer colour than predicted — an indication of a higher than expected temperature. Observations also revealed the presence of dust in the gap.

Kraus and Ireland say the observations indicate that they have not only captured the first portrait of a young, two-million year-old giant planet, but that surrounding dust and gas are falling onto the body, heating the orb up as they does so.

The young planet lies roughly as far from LkCa 15 as Uranus does from the Sun — a relatively remote region where material in the planet-forming disk would be expected to be sparse even before it was cleared out. In fact, the density of gas and dust would be so thin where the giant planet formed that its core could not have been built up by the gradual accumulation of rocky material, a process known as core accretion. Instead, says Kraus, the planet’s existence favors the gravitational instability model, proposed by Boss, in which a large mass of gas and dust in the outer part of the disk suddenly forms into clumps as big as a modest-size planet, drawing in neighbouring gas and dust to form a giant planet in just a few hundred years.

“State-of-the-art core accretion simulations fail to explain the existence of a massive gas giant out at [Uranus’ distance] from a two-million-year-old star,” says Boss. Planets might still be made by core accretion at smaller distances from stars, Kraus says.

Kraus had previously presented his team’s findings at several other meetings, including the American Astronomical Society’s winter meeting in Seattle last January. A report on the newly imaged planet that Kraus gave at a meeting of Hubble and Spitzer Space Telescope fellows in Pasadena in March has even been posted on the web. But Kraus says he hadn’t been interested in publicizing the result until now.

“My approach to this has been that since this detection is the first of its kind I wanted to get feedback from a professional audience about how to interpret the unexpected features in this discovery,” he says. “If this was a normal [middle-aged] planet, we would have kept it secret, timing the press release with the [first] talk.” He says he wasn’t worried about getting scooped because the technique used is so specialized that it would be very difficult for anyone to duplicate — even if they knew where to look for the planet.


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