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Posted: Feb 28, 2013
Young protoplanet discovered?
(Nanowerk News) It is something of a celebrity among astronomers: star HD100546. It is around 337 light years away from Earth, visible in the southern sky in the constellation Musca and only a few million years old. What makes it so special, however, is its circumstellar disk. Gas and dust accumulate in these disks, which form around all young stars. As far as we know, they are the birthplace of planets. Typically, circumstellar disks are 200 AU in diameter (one AU is roughly equivalent to the mean distance between Earth and the Sun – namely around 150 million kilometres). With a size of approximately 700 AU, the disk of HD100546 is enormous and thus easy to observe with telescopes. Now, for the first time, researchers from the Institute of Astronomy at ETH Zurich may well have discovered a so-called protoplanet – in other words, a planet in the making. For their observations, they used the Very Large Telescope (VLT) of the European Southern Observatory (ESO), the results of which they have now published in the journal The Astrophysical Journal Letters ("A Young Protoplanet Candidate Embedded in the Circumstellar disc of HD 100546"; pdf).
This image from Hubble shows a visible light view of the outer dust around the young star HD100546. The position of the newl discovered protoplanet is marked with an orange spot. The inner part of this picture is dominated by artifacts from the brilliant central star, which has been digitally subtracted and the black blobs are not real. (Image: ESO)
Calculated mass too big
Although the disk around HD100546 is one of the most well-known and observed, the ETH-Zurich scientists noticed that the disk displayed unusual asymmetries only about two years ago. The researchers subsequently used a special piece of optics installed in the high-resolution camera at the VLT called the Apodising Phase Plate (APP), which they developed to minimize the distribution of starlight, to study the immediate surrounding and disk of HD10546 more closely. In doing so, they were able to locate a point source at around 70 AU from the central star. Based on the brightness of this source and using models, they calculated the object's mass. “Using this approach the mass of this source would be equivalent to roughly twenty times that of Jupiter,” says Sascha Quanz, a postdoc at the Institute of Astronomy and first author on the paper. The problem: if this planet were actually that big, the circumstellar disk of HD100546 would have to have large gaps because the planet will have had to incorporate a considerable amount of material in the course of time. However, these gaps do not exist, which is why the researchers discuss two hypotheses in their publication: either the planet has a much lower mass and is in the process of formation, which would make it a protoplanet, or it developed further inside the disk and was ejected.
Hypothesis 1: a sensation
The younger the planet, the brighter and hotter it is. This is because it is still actively absorbing and compressing mass. Astronomers call this process accretion and the brightness of the planet discovered suggests that it is still accreting. In other words, although the planet is shining like a large, high-mass object, because it cannot be that massive, the chances are that it is young. If this theory that it is a protoplanet proves true, it must only be a few 100,000 years old. “There are models that predict how the brightness of a protoplanet develops. Our observations are very much in line with these models, which is a key indication that this hypothesis might be correct,” explains Quanz. The discovery of a young protoplanet would be a sensation. It would be the first time astronomers have been able to observe a forming planet embedded in its host star's disk, which would enable many longstanding theories and models to be verified based on a concrete object.
Hypothesis 2: competitor planet invisible for the moment
According to the second hypothesis, the planet could come from the inner section of the disk. The scientists suspect that, in the case of star HD100546, a second planet might exist within the first ten AU. “If so, it could be that the two planets competed with each other, resulting in the smaller planet being ejected,” explains Michael Meyer, a professor at the Institute of Astronomy. The planet just discovered would then have had to move very quickly: in only about twenty years, it would have to have reached its current position and would soon leave the disk altogether. The authors of the paper thus deem this hypothesis rather improbable. “We’d have discovered a planet the moment it was drifting away from the star. That would be a huge coincidence,” says Meyer. Another problem: as yet, no one has seen the inner planet. It would have to be so close to the star that it is simply impossible to be detected with today’s telescopes.
Conclusive results soon
Nonetheless, there is an amazingly simple way to confirm or disprove one of these hypotheses: with more observation. If the point source moves away from the star radially in the next decade, this would support the ejection theory. If it is a protoplanet, it would have to move in an orbit around HD100546. In one year, that would mean roughly a one-degree deviation from its present location. “If we observe the object regularly, we’ll soon be able to prove which hypothesis is correct,” says Quanz. Consequently, the researchers will already be studying the disk and the planet at different wavelengths in April this year.
Cooperation with the cosmologists
With all the researchers’ astonishing observations and hypotheses, however, the publication can still offer a special methodical feature: the way the data were analysed. All exoplanet researchers always struggle with the same difficulty: they have to try to eliminate the bright signatures of the star and the disk to be able to reveal the relatively faint signal of the planet. This requires are very good characterization of the brightness distribution that the star creates on the detector of the camera.
Scientists working in cosmology who have nothing to do with exoplanets but have to tackle similar problems. They too have to know the precise brightness distribution of a star to be able distinguish astrophysical effects in their data from telescope-related effects. Adam Amara, also from the Institute of Astronomy at ETH Zurich and second author on the paper, is a cosmologist and works on so-called lensing problems, where these effects are especially relevant. Together with his colleagues at the institute, he succeeded in applying an proven method from cosmology to exoplanets. “Researching exoplanets is one of the most exciting new areas in astronomy. And if an interdisciplinary exchange of ideas between our groups can now lead to such successes, that’s an extra boost, of course,” says the ETH-Zurich cosmologist.