Have a look at the spiral of pinwheeling dust that can be seen around the young star Elias 2-27. We’re looking at gravitational perturbations in a protoplanetary disk that, as this National Radio Astronomy Observatory news release says, mimic the vast arms we expect in a spiral galaxy. But here we’re looking at a process with implications for planet formation, one that draws on data from the Atacama Large Millimeter/submillimeter Array (ALMA). This is the first time a spiral density wave has been detected in a protoplanetary disk’s planet formation areas.
Image: ALMA peered into the Ophiuchus star-forming region to study the protoplanetary disk around the young star Elias 2-27. Astronomers discovered a striking spiral pattern in the disk. This feature is the product of density waves – gravitational perturbations in the disk. Credit: L. Pérez (MPIfR), B. Saxton (NRAO/AUI/NSF), ALMA (ESO/NAOJ/NRAO), NASA/JPL Caltech/WISE Team.
Some 450 light years from Earth in the Ophiuchus star-forming region, Elias 2-27 is about half the mass of the Sun, though its protoplanetary disk is massive. Although the young star (about a million years old, according to current estimates) is shrouded by the molecular cloud from which it grew, ALMA was able to peer into the mid-plane of the disk to identify the spiral density waves. The spiral arms extend as much as 10 billion kilometers away from the host star.
All this catches the eye because while we can account for star formation from the collapse of gas and dust under the influence of gravity, we need a mechanism to keep enough material from falling into the protostar to ensure that it doesn’t spin up enough to shred itself. The protostellar disk projects angular momentum outward, and is where we can expect planets to form. But standard core accretion models have problems explaining the formation of planets 20 to 30 AU out, where the disk may not be dense enough to allow the process to be efficient.
Gravitational instabilities in the outer disk, however, can produce the kind of dense spiral arms we see here, with new material being pushed out into regions far from the star and collapsing under its own gravity to begin planet formation. Andrea Isella (Rice University) explains:
“We don’t completely understand how planets form, but we suspect there are two ways: Either small particles stick together until they form something like the Earth or Mars, or accreting gas forms a planet like Saturn or Jupiter. But this process works only very close to the star, within a few astronomical units (roughly the distance from the Sun to the Earth), because that’s where all the material is, and it has to have enough density… If a disk is massive enough to be gravitationally unstable, a spiral will form naturally.”
Thus the spiral arms of Elias 2-27 may be the manifestation of an instability that gives birth to a particular kind of exoplanet. Near to the star, the ALMA observations found a flattened dust disk that extends out beyond 30 AU, followed by a narrow band of sharply diminished dust that may indicate a planet in formation. The spiral arms extend outward from the edge of this gap in the disk. Lead author Laura Pérez (Max Planck Institute for Radio Astronomy) notes that an upcoming program will use ALMA data to home in on similar protoplanetary disks as we try to find out whether Elias 2-27’s spiral density waves actually do reveal planet(s) in formation.
The paper is Pérez et al., “Spiral density waves in a young protoplanetary disk,” Science Vol. 353, Issue 6307 (30 September 2016), pp. 1519-1521 (abstract). A Rice University news release is also available.
“A young star” – this phrase led me to a bit of daydreaming about the moment of a star’s birth.
Materials coalesce, density growing, and then- what? A sudden flash as fusion begins where pressures allow? Or, is there a ‘lumpy and gradual’ start, assuming a heterogenous interior with pockets of deep pressure and the resultant fusion that find one another with time?
I suppose the smart people know the answer; but if there’s an image of an incipient star, or one not millions of years but, say, hours old, even minutes, I haven’t seen it?
We hear constantly about ‘star forming regions’ of gas
Star formation has no particular moment. It takes up to millions of years. Material condensing to form the star would build up huge speeds due to gravity which means (1) it needs to lose energy and angular momentum before it’s capable of falling into the star (it will just orbit in a rotating cloud/disk otherwise); (2) it does eventually do that but largely by bits of material colliding with each other/rubbing together in order, so lots of heat even before nuclear fusion starts. Stars take time to reach a stable internal structure. Nuclear fusion rates will ramp up rapidly as pressure & temperature at the star’s core increase but it’s not an on/off thing. Stars are also balls of plasma which is is not transparent to radiation so that heat takes a long time to reach the surface. All of these things are working against their being a definite starting time for ‘being a star’.
AUs light years, parsecs millions of km make comparison difficult.
What about Light years, Light hours, Light minutes, , Light seconds,
That was an intriguing article on planet formation! Do these latest observations provide some of the first observational evidence for Alan Boss’s ‘gravitational instability’ model of gas giant planet formation? Nature is diverse and so I would not be surprised if gravitational instability ends up being responsible for at least a fraction of the Universe’s gas giants; however, I suspect that core accretion is the predominant mode by which most planets form. Astronomers have said that increased metallicity favors core accretion and helps explain why giant planets are more likely to form around stars with more metals than the Sun. I remember reading that gravitational instability is not as dependent on metallicity, as the gas would collapse into planets without the reliance on solid materials and these giants would have NO core…does this ring a bell to anyone? Speaking of cores, will the JUNO mission help resolve whether or not Jupiter has a core? If Jupiter has no core, then this would provide us with strong evidence that gravitational instability occurred billions of years ago in our own solar system. It will also be interesting to understand how Planet Nine (IF it really exists) was made.
BTW, I love how much the ALMA image shown above resembles a spiral galaxy. It fascinates me that we can find similar geometric patterns on a variety of scales related to a variety of phenomena in the Universe!
Binary system boasts 3 planet-forming disks
A pair of young stars in the binary system IRS 43 is surrounded by three planet-forming disks, something researchers have never seen before, according to findings published online in Astrophysical Journal Letters. Each star has its own disk plus a larger shared disk that lies across the other two.
http://www.space.com/34347-three-planet-forming-disks-binary-star.html