January 4th, 2013 by Michael Ricciardi
At just 2 million years old, the 450-light-years-distant star HD 142527 is considered to be quite young by cosmological standards. The star — about twice that mass of our sun — is still growing. Such stars grow by “feeding” off of the large, surrounding disc of gas and dust that is left over following its initial nucleosynthesis.
These discs of gas and matter are also the source of planetesimals, or ‘baby planets’, that slowly accrue more and more gas and dust and, over long time scales, become more dense and can form the cores of planets. It is this process that is theorized to cause the gaps observed in other star discs. But this earlier stage of planet formation has never before been observed.
Recently, an international team of astronomers working with the (partially completed) Atacama Large Millimeter/submillimeter Array (ALMA) have identified two “thin” (but vast) filaments, or “bridges”, of gas flowing from the outer ring of the star’s accretion disc to its inner ring, with the gap between inner and outer having been cleared, apparently, by two young, gaseous planets.
Gravitational forces from these young planets (termed “gas giants”) are actually channeling the gaseous material in the outer disc across the huge gap to the inner portion, and thereby helping the young start to continue growing.
Earlier computer simulations predicted that such bridges would arise in the gaps between sections of the gaseous disc, but, until now, this has never been observed directly.
Note: a “hot protoplanet” was observed forming in the accretion disc of LkCa 15, in October of 2011. This most recent observation is of a later stage in planetary evolution, where sufficient mass has accreted to begin forming the filaments
The Give and Take of a Solar System’s ‘Ecology’
What is most interesting here is that such a gap or break in the star’s accretion disc — this gap is 14 times bigger that that of Saturn from our sun — could endanger the star’s survival. Lead scientist Simon Casassus of the University of Chile, and colleagues, had previously calculated that star HD 142527 would most likely run out of fuel within a year.
According to the team’s calculations, without some mechanism to redirect gas and dust to the inner portion of the disc — a bridge — the young star could not sustain its growth, and would eventually “die”.
Fortunately, the gravitational force of the two planets is sufficiently strong that when gas and dust are drawn into orbit around the two gas giants, some of this over-shoots the planets and the momentum carries it to the inner portion of the disc, to be absorbed, ultimately, by the growing star.
We are accepting of the fact that star formation frequently leads to the formation of planets (of various kinds, rocky to gassy). This is why we typically refer to these as parent stars…but here we have direct evidence of how planets, in their earliest formative stages, actually help stars to grow, in a sort of cosmo-ecological mutualism
The child schools the parent, if you will.
This process of inward flow (via the bridges) is not endless, however. As the gas giants continue to grow, their ‘radius of influence’ increases and this will slow the flow of gas and dust to the inner disc.
In a statement to Space.com, Casassus noted:
“Eventually, the proto-gaseous giants will exhaust the material within their radius of influence. How much material will have infallen, in what timescales, and how this impacts the planet location and eventual migration are all open questions in the field of planet formation. Our observations are a step forward.”
Measurements from the ALMA instrument also revealed that the gaps were not empty but contained carbon monoxide (CO) gas (in trace amounts).
It should be noted that the dense gas in the observed discs prevents direct observation of the (inferred) planets themselves, but astronomers are confident that once the high-resolution ALMA instrument is completed, it will reveal the tell-tale “knots” in these filaments that confirm the presence of large planets. ALMA should also enable estimation of the mass of the baby gas giants.
The discovery/research was published online, Jan. 2, in the journal Nature.
Top image: (artist rendering) Photo By ALMA (ESO/NAOJ/NRAO)/M. Kornmesser (ESO) Wed, Jan 2, 2013
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