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Iconic ALMA image backs presence of baby planets

Amy Tyndall, News reporter
May 7, 2015, 19:56 UTC

Sen—The idea that the dusty disc surrounding the young star HL Tauri has planets forming in its midst has gained further support in a new study.

The image, one of the most iconic taken so far by the Chile-based Atacama Large Millimeter/submillimeter Array (ALMA), was released to the public back in November 2014. It revealed the most detail ever seen before in a protoplanetary disc—the disc of dust and gas that surrounds a recently formed star in the constellation of Taurus.

Dark concentric rings seen interspersed in the disc were thought to be gaps carved out by the gravitational processes that form planets from the surrounding material. The new study supports that theory by analysing the mass of the hypothetical planets.

Dr Amaury Triaud of the University of Toronto, co-author of the new research paper, was amazed by the press release image of HL Tauri. "We were surprised at how much information we could extract from a simple image," he told Sen.

Rather than providing direct evidence for baby planets carving out the ring pattern seen in the image, Triaud and his collaborators worked out the maximum mass a planet would need to be in order to keep the pattern observed stable.

"For instance, for certain masses, the planets would become unstable and no longer follow near circular, closed orbits," explained Triaud. "We know the time it takes for the planet in the outermost gap to go around the star and know roughly how long it takes for a gap to be established. We also know the age of the system. In that timeframe we can say that planets cannot have deviated much from the gaps we see, otherwise we would not see such a regular and obvious set of gaps."

By then running simulations that could also account for some effects on the planetary orbits caused by the disc itself, the maximum allowed mass was determined. The hard limit of this value was shown to be slightly above the mass of Saturn (95.16 times that of Earth), with likely masses around twice that of Neptune's mass (17.15 times that of Earth) or lower.

"In that way we find that the gaps' spacings are compatible with them containing planets and that the masses are of the right order of magnitude to have created similar gaps," Triaud said.

"Had we found a maximum mass of one Earth's mass, then it would have shown that the gaps would likely not be created by planets—this was an early criticism, that the gaps were too close to each other for massive planets to have formed them, but we showed that, actually, they could!"

The idea that planets forming in a disc would sculpt such gaps by accreting dust is something that astronomers have been considering for decades. The results, published in a paper led by Dr Daniel Tamayo also from the University of Toronto, show something that is coherent with this concept.

"Several observations had claimed evidence for such structure, but they were marginal and certainly did not have the quality of the HL Tauri image," explained Triaud. "This system has several gaps, clearly defined, and clearly thin (as opposed to large cavities in previous papers). Furthermore the pattern looks very regular and is striking for its aesthetic beauty."

It has previously been conjectured that the HL Tauri system may be representative of our how our own Solar System formed. However, Triaud thinks it is hard to come to that conclusion at the moment as we simply do not know what happened in order to make a direct comparison.

"The aim of studying such discs is actually to find out," he said. "However, this disc is much wider than the Solar System, and the three outer gaps are further than any known planet in the Solar System. If those are planets [in the dark rings] and those planets do not subsequently move much, then that particular system will become very different than our own."

"This target was chosen [for observation] very probably because the disc is particularly massive and extended. That fact alone would imply that this system is not within the norm; this implies that it represents conditions that are special, and therefore likely to be very different from the birth of the Solar System."

The analysis carried out by the team has created a base from which other scientists could analyse their own data. "We provide a framework to estimate the masses in protoplanetary discs containing gaps sculpted by planets," explained Triaud. "This means any subsequent observer can take our paper, follow our step-by-step procedure, and get to know how massive the planets in their system are.

"We also provide guidelines as to which disc systems are preferable to use in order to get meaningful constraints on the masses. For instance, discs as massive as HL Tauri's actually make it hard to provide a firm statement on the mass of any putative planet, and we recommend to focus observations on less-massive discs. Those may be harder to observe, but more accurate planetary masses can be extracted."

The results have been accepted for publication in the Astrophysical Journal. A draft version of the paper can be downloaded from the ArXiv.