Infant planets have been spotted forming in the disk of gas and dust around their stars for the first time.
Four groups of astronomers caught three different stars transforming from lone bachelors with thick disks of material around their middles to proud parents of a growing family of gas giant planets.
The new observations directly show a companion orbiting in the disk around young star T Chamaeleontis (T Cha), the first time a potential planet has been seen midformation. The young stars LkCa15 and AB Auriga also have Saturn-like rings with gaps in the middle, indicating the presence of at least one planet.
"We think we’re seeing the baby photos of a planetary system that is just forming, which in fact may be quite similar to our own solar system at a younger age," said astronomer Christian Thalmann of the Max Planck Institute for Astronomy in Germany, lead author of a paper in Astrophysical Journal Letters describing LkCa 15. "But with a big 'may' there."
An infant star forms from a collapsing cloud of dust and gas and gathers a dense, flat disk of material that rotates with the star like a record album.
The material in the disk will eventually clump up into nascent planets. Theoretical models of planet formation predicted that those protoplanets should suck up more gas and dust with their gravity, clearing a wide gap in the otherwise solid disk.
"If you see this disk with a gap, it’s a pretty clear sign that you probably have a planet forming as you’re watching," said astronomer Adam Kraus of the University of Hawaii, who observed the companion around T Cha, a 7-million-year-old sun-like star about 350 light-years from Earth.
Other snapshots of gaps in disks turned out to have been cleared by binary stars, not planets. Planets have been photographed in dust disks around their stars as well, but those planet systems were much more mature.
"This discovery is the first time that we’ve looked and seen something there causing a cleared region of the disk, but it’s not just another star," Kraus said. "It looks like it’s consistent with maybe being a brown dwarf or a planet."
Earlier observations of T Cha by Johan Olofsson of the Max Planck Institute for Astronomy and colleagues showed that the disk around it was split into two parts, a narrow ring about 12 million miles from the star and a second band starting about 680 million miles from the star. The gap between the two rings could have been formed by a growing planet.
To check, Kraus and an international group of colleagues observed T Cha with a suite of instruments called NaCo at the Very Large Telescope in Chile. They used a special technique called sparse aperture masking that helps clear up the distortions a star's light experiences as it travels through Earth's blurring atmosphere.
"The advantage of this method is that allows you to detect very faint objects very close to a very bright star," said astronomer Nuria Huélamo of the Centro de Astrobiología in Spain, lead author of the paper a paper to appear in Astronomy & Astrophysics reporting the new observations.
When the team looked in near-infrared wavelengths on a clear night in March 2010, they saw a bright object right in the gap. The object is about 620 million miles from the star, or 6.7 times the distance from the Earth to the sun.
Unfortunately, the object didn't show up in follow-up observations on a cloudier night in July. The object could have a disk of gas and dust of its own, perhaps precursors to rings like Saturn's, that smeared it out in the second observing run, the astronomers suggest.
"We still do not know if it is a planet or not," Huélamo said. The object could also be a brown dwarf, a giant ball of gas up to 80 times more massive than Jupiter but still too small to burn like a star.
"We need new observations to understand its nature," Huélamo said.
Thalmann, who was not involved in the T Cha study, thinks the object is probably not a planet.
"But I think the word 'planet' might be a bit overrated in the scientific community," he said. "If it turns out that it’s possible to form such massive objects as a product of planet formation, that could have implications on what exactly is possible. I think that’s pretty neat."
In a different study, published in the August 2010 Astrophysical Journal Letters, Thalmann and colleagues used the Subaru telescope in Hawaii to observe the young star LkCa 15. The star lies about 450 light-years from Earth and is nearly identical to the sun, but about a thousand times younger.
The images did not reveal planets directly, but they did get a good look at the inner edge of the gap in LkCa 15's disk. The gap is wide enough to fit our entire solar system inside.
"Our images are the first that really pierced the glare of the star close enough, at small enough separations and high enough contrasts, to image that sharp edge of the gap," Thalmann said.
The structure of the disk suggests there should be planets there, he said. The planets don't need to be observable to be there. Several infant planets smaller than Jupiter could be hiding in the dust.
"It’s very tempting to think that since the star is also a sun-like star, we’re looking at something that could have been a close relative of the sun at a much younger age," Thalmann said.
A group led by Jun Hashimoto of the National Observatory of Japan also used the Subaru telescope to observe the star AB Auriga, which is even younger than LkCa 15. The team saw nested rings that are tilted with respect to the star's equatorial plane. The disk is clumpy and asymmetric, which also suggests hidden infant planets.
"This is a very exciting discovery to make," Kraus said. "That starts telling you things like where should you start looking for the planets."
The astronomers all agree that they need to take more observations to figure out exactly what is happening in these hole-riddled dust disks. Current observing techniques may not be good enough to actually see the planets around LkCa 15 and AB Auriga.
"But we’re always developing new technology, so this could change next year. It could change next month," Kraus said. "The technical side of the field is progressing very rapidly."
Images: 1) An artist's rendition of the large object in the disk around the star T Cha. (L. Calçada/ESO) 2) The bright edge of the disk around the star LkCa 15. (Christian Thalmann/MPIA/NAOJ)
Citations:
"A companion candidate in the gap of the T Cha transitional disk." N. Huélamo, S. Lacour, P. Tuthill, M. Ireland, A. Kraus and G. Chauvin. Astronomy & Astrophysics, accepted.
“Warm dust resolved in the cold disk around TCha with VLTI/AMBER.” J. Olofsson, M. Benisty, J.-C. Augereau, C. Pinte, F. Ménard, E. Tatulli, J.-P. Berger, F. Malbet, B. Merín, E. F. van Dishoeck, S. Lacour, K. M. Pontoppidan, J.-L. Monin, J. M. Brown and G. A. Blake. Astronomy & Astrophysics, accepted.
"Imaging of a Transitional Disk Gap in Reflected Light: Indications of Planet Formation Around the Young Solar Analog LkCa 15." C. Thalmann et al. Astrophysical Journal Letters, Vol. 718 No. 2, Aug. 1, 2010. DOI: 10.1088/2041-8205/718/2/L87
*"Pre-transitional Disk Nature of the AB Aur Disk." M. Honda et al. Astrophysical Journal Letters, Vol. 718 No. 2, Aug. 1, 2010. DOI: 10.1088/2041-8205/718/2/L199
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