ALMA spots possible formation site of icy giant planet

September 14, 2016, National Astronomical Observatory of Japan
ALMA image of the disk around the young star TW Hydrae. Several gaps are clearly depicted. Researchers found that the size of the dust particles in the inner 22 au gap is smaller than in the other bright regions and guess that a planet similar to Neptune is located in this gap. Credit: ALMA (ESO/NAOJ/NRAO), Tsukagoshi et al.

Astronomers found signs of a growing planet around TW Hydra, a nearby young star, using the Atacama Large Millimeter/submillimeter Array (ALMA). Based on the distance from the central star and the distribution of tiny dust grains, the baby planet is thought to be an icy giant, similar to Uranus and Neptune in our Solar System. This result is another step towards understanding the origins of various types of planets.

A number of extrasolar planets have been found in the past two decades and now researchers agree that planets can have a wide variety of characteristics. However, it is still unclear how this diversity emerges. Especially, there is still debate about how the icy giant , such as Uranus and Neptune, form.

To take a close look at the planet formation site, a research team led by Takashi Tsukagoshi at Ibaraki University, Japan, observed the young star TW Hydrae. This star, estimated to be 10 million years old, is one of the closest young stars to the Earth. Thanks to the proximity and the fact that its axis of rotation points roughly in the Earth's direction, giving us a face-on-view of the developing planetary system, TW Hydrae is one of the most favorable targets for investigating planet formation.

Past observations have shown that TW Hydrae is surrounded by a disk made of . This disk is the site of planet formation. Recent ALMA observations revealed multiple gaps in the disk. Some theoretical studies suggest that the gaps are evidence of planet formation.

The team observed the disk around TW Hydrae with ALMA in two radio frequencies. Since the ratio of the radio intensities in different frequencies depends on the size of the dust grains, researchers can estimate the size of dust grains. The ratio indicates that smaller, micrometer-sized, dust particles dominate and larger dust particles are absent in the most prominent gap with a radius of 22 astronomical units.

Why are smaller dust particles selectively located in the gap in the disk? Theoretical studies have predicted that a gap in the disk is created by a massive planet, and that gravitational interaction and friction between gas and push the larger dust out from the gap, while the smaller particles remain in the gap. The current observation results match these theoretical predictions.

Artist's impression of the dust disk and a forming planet around TW Hydrae. Credit: NAOJ

Researchers calculated the mass of the unseen planet based on the width and depth of the 22 au gap and found that the planet is probably a little more massive than the Neptune. "Combined with the orbit size and the brightness of TW Hydrae, the planet would be an icy giant planet like Neptune," said Tsukagoshi.

Following this result, the team is planning further observations to better understand . One of their plans is to observe the polarization of the radio waves. Recent have shown that the size of can be estimated more precisely with polarization observations. The other plan is to measure the amount of gas in the disk. Since gas is the major component of the disk, the researchers hope to attain a better estimation of the mass of the forming planet.

These observation results were accepted for a publication as Tsukagoshi et al. "A Gap with a Deficit of Large Grains in the Protoplanetary Disk around TW Hya" by the Astrophysical Journal Letters.

Explore further: Footprints of baby planets in a gas disk

More information: Tsukagoshi et al. "A Gap with a Deficit of Large Grains in the Protoplanetary Disk around TW Hya" accepted, Astrophysical Journal Letters, 2016. DOI: 10.3847/2041-8205/829/2/L35

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Sep 14, 2016
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4.3 / 5 (12) Sep 14, 2016

What we know for sure is that they will continue to interpret everything about this object through the lens of the textbook theories, even though there are alternative explanations (we could simply be looking down the barrel of a cylinder, for instance) and even though the exoplanet observations have thoroughly undermined the textbook theories for planetary formation.

Are you serious? "....the barrel of a cylinder......"! Is this a belated April fool?
1.5 / 5 (8) Sep 14, 2016
It's not a random guess. It's the shape of electricity flowing over plasma -- more technically known as a Birkeland current.
1.6 / 5 (7) Sep 14, 2016
Let's not forget that Herschel has observed stars forming like beads along a string. Why do you suppose their formation would in any manner be colinear? What geometry does that imply?

Astrophysicists would have us believe that such things are simply accidents, but these are observational patterns, and suggestions of accidents is dismissive of any meaning in the observation.
Sep 15, 2016
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3.7 / 5 (3) Sep 16, 2016
Chris, you made the same assertion in a different article where it became quite evident that you had little understanding of geometric arguments. You shouldn't be surprised now when no one wants to waste more time trying to discuss geometry with you.

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