An artist's impression of a young protoplanet orbiting its parent star (Credit: ESO)An international team of astronomers believe that they have captured the very first direct image of a young protoplanet forming around another star.
Hubble Telescope view of HD 100546 and its circumstellar disk (Credit: ESO)The bright, young, blue star HD 100546 is well known to astronomers for the large 'circumstellar' disk of gas and dust that surrounds it. Typical circumstellar disks are around 200 astronomical units (AU) wide (roughly 30 billion kilometres). HD 100546's disk is about three and a half times larger than that, at 700 AUs (105 billion kilometres). This extremely large disk is easily seen by telescopes so it has been studied extensively.
Led by Sascha Quanz of the Institute for Astronomy at the Swiss Federal Institute of Technology (EHT) Zurich in Switzerland, the astronomers used the Very Large Telescope at the European Southern Observatory, in Chile, to observe the HD 100546 and its disk, and they noticed that there was something strange going on there about two years ago.
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This VLT image of the protoplanet shows it much more clearly with the light from the star …Taking further observations, and using a special filter that essentially reduced the 'glare' of the star, so that the planetary disk could be seen more easily, they spotted a bright blob at a distance of about 70 AU from the star (about twice the distance that Pluto orbits the Sun). Using the object's brightness and computer models, they determined that its mass "would be equivalent to roughly twenty times that of Jupiter,” said Quanz.
A planet that big would certainly not have been missed in previous observations, especially when it was already known that a similarly-sized planet orbits much closer to the star (HD 100546 B, at roughly 6.5 AUs). It's also too large for it to have been orbiting out at 70 AUs for any significant amount of time, because a planet that big would create an empty gap in the star's disk as it orbited, which would give away its location, just like a gap much closer to the star gave away the presence of HD 100546 B.
The team did come up with two possible explanations, though.
The first possibility is that this really is a planet 20 times the size of Jupiter, and it formed much closer to the star, and also much closer to another, larger, as-of-yet-undetected planet.
"If so, it could be that the two planets competed with each other, resulting in the smaller planet being ejected," said Michael Meyer, a professor at the Institute of Astronomy.
However, although intriguing, the team doesn't have much confidence in this explanation. If it were true, the planet would have to be moving incredibly fast to get that far away from the star in such a short time. So, according to Meyer, the team "would have discovered a planet the moment it was drifting away from the star. That would be a huge coincidence."
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The second, and more likely possibility is that this is a small proto-gas giant that is glowing extremely brightly as it gathers more matter to it, so it effectively looks like a much bigger planet. A smaller planet would not sweep out as large of a gap in the star's disk, so any gap that it is creating could be missed by telescopes. If this were the case, it would present a unique opportunity to study a planet as it forms.
"So far, planet formation has mostly been a topic tackled by computer simulations," said Quanz. "If our discovery is indeed a forming planet, then for the first time scientists will be able to study the planet formation process and the interaction of a forming planet and its natal environment empirically at a very early stage."
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