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Planet formation in Earth-like orbit around a young star

The disks of dust and gas that surround young stars are the formation sites of planets. New images from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal never-before-seen details in the planet-forming disk around a nearby Sun-like star, including a tantalizing gap at the same distance from the star as the Earth is from the Sun.

Planet formation in Earth-like orbit around a young star
ALMA's best image of a protoplanetary disc to date. This picture of the nearby young star 
TW Hydrae reveals the classic rings and gaps that signify planets are in formation
 in this system [Credit: S. Andrews (Harvard-Smithsonian CfA); 
This structure may mean that an infant version of our home planet, or possibly a more massive "super-Earth," is beginning to form there.

The star, TW Hydrae, is a popular target of study for astronomers because of its proximity to Earth (approximately 175 light-years away) and its status as a veritable newborn (about 10 million years old). It also has a face-on orientation as seen from Earth. This affords astronomers a rare, undistorted view of the complete disk.

"Previous studies with optical and radio telescopes confirm that this star hosts a prominent disk with features that strongly suggest planets are beginning to coalesce," said Sean Andrews with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and lead author on a paper published in the Astrophysical Journal Letters. "The new ALMA images show the disk in unprecedented detail, revealing a series of concentric dusty bright rings and dark gaps, including intriguing features that suggest a planet with an Earth-like orbit is forming there."

Planet formation in Earth-like orbit around a young star
ALMA image of the planet-forming disc around the young, Sun-like star TW Hydrae. 
The inset image (upper right) zooms in on the gap nearest to the star, which is at the
 same distance as the Earth is from the Sun, suggesting an infant version of our home
 planet could be emerging from the dust and gas. The additional concentric light
 and dark features represent other planet-forming regions farther out in the disc 
[Credit: S. Andrews (Harvard-Smithsonian CfA), ALMA (ESO/NAOJ/NRAO)]
Other pronounced gap features are located 3 billion and 6 billion kilometers from the central star, similar to the distances from the Sun to Uranus and Pluto in our own Solar System. They too are likely the result of particles that came together to form planets, which then swept their orbits clear of dust and gas and shepherded the remaining material into well-defined bands.

For the new TW Hydrae observations, astronomers imaged the faint radio emission from millimeter-size dust grains in the disk, revealing details on the order of one astronomical unit (about 150 million kilometers, or the distance between the Earth and the Sun). These detailed observations were made possible with ALMA's high-resolution, long-baseline configuration. When ALMA's dishes are at their maximum separation, up to nearly 15 kilometers apart, the telescope is able to resolve finer details. "This is the highest spatial resolution image ever of a protoplanetary disk from ALMA, and that won't be easily beaten going forward," said Andrews.

"TW Hydrae is quite special. It is the nearest known protoplanetary disk to Earth and it may closely resemble our Solar System when it was only 10 million years old," said co-author David Wilner, also with the Harvard-Smithsonian Center for Astrophysics.

Planet formation in Earth-like orbit around a young star
This is the inner region of the TW Hydrae protoplanetary disk as imaged by ALMA. 
The image has a resolution of 1 AU (Astronomical Unit, the distance from the Earth
 to the Sun in our own Solar System). This new ALMA image reveals a gap in the disk
 at 1 AU, suggesting that a planet with the same orbit as Earth is forming there 
[Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); 
Earlier ALMA observations of another system, HL Tau, show that even younger protoplanetary disks -- a mere one million years old -- can display similar signatures of planet formation. By studying the older TW Hydrae disk, astronomers hope to better understand the evolution of our own planet and the prospects for similar systems throughout the Galaxy.

The astronomers' next phase of research is to investigate how common these kinds of features are in disks around other young stars and how they might change with time or environment.

Source: National Radio Astronomy Observatory [April 01, 2016]

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