ALMA Spots Possible Formation Site of Icy Giant Planet

12 September, 2016

Scientific Paper

Astronomers found a sign 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 distribution of tiny dust grains, the baby planet is estimated to be an icy giant, similar to Uranus and Neptune in our Solar System. This result is another step for understanding the origins of various types of planets.

A number of extrasolar planets have been found in the past two decades and now researchers have a common view that planets take a wide variety of characteristics. However, it is still unclear how this diversity emerges. Especially, there is still debate about how the icy giant planets, 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 surround by a disk of tiny dust particles. This disk is the site of planet formation. Recent ALMA observations revealed multiple gaps in the disk1. Some theoretical studies suggest that the gaps are the evidence of planet formation.


Figure 1. 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 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. | Download image

The team observed the disk around TW Hydrae with ALMA in two radio frequencies. Since the ratio between the intensities in different radio frequencies depends on the size of the dust grains, researchers can estimate it. This ratio indicates that the smaller, micrometer-sized dusts dominates and larger dusts are missing in the most prominent gap with the radius of 22 au 2.

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 dust particles push the larger dust out from the gap, while the smaller particles remain in the gap. The current observation results match the theoretical prediction.


Figure 2. Artist’s impression of the dust disk and a forming planet around TW Hydrae. Credit: NAOJ | Download image

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 a little more massive than Neptune. “Combined with the orbit size and the brightness of TW Hydrae, the planet would be an icy giant planet,” said Tsukagoshi.

Following this result, the team is planning further observations to better understand planet formation. One of their plans is to observe polarization of the radio waves. Recent theoretical studies have shown that the size of dust grains 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.

Additional information

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.

The research team members are:

Takashi Tsukagoshi (Ibaraki University), Hideko Nomura (Tokyo Institute of Technology), Takayuki Muto (Kogakuin University), Ryohei Kawabe (National Astronomical Observatory of Japan, National Institutes for National Sciences), Daiki Ishimoto (Tokyo Institute of Technology/Kyoto University), Kazuhiro D. Kanagawa (University of Szczecin), Satoshi Okuzumi (Tokyo Institute of Technology), Shigeru Ida (Tokyo Institute of Technology), Catherine Walsh (Leiden University), T. J. Millar (Queen’s University Belfast).

This research was supported by the Japan Society for the Promotion of Science through Grants-in-Aid for Scientific Research No. 24103504, 23103005, 25400229, 26800106, 15H02074, 16K17661, and Polish National Science Centre MAESTRO grant DEC- 2012/06/A/ST9/00276.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

  1. See the press release “ALMA’s Best Image of a Protoplanetary Disk” issued on March 30, 2016 for more detail. Astronomers observed radio waves from TW Hydrae in only one frequency in the past observations and could not estimate the size of the dust particles.
  2. 1 au = 1 astronomical unit. One astronomical unit corresponds to the distance between the Sun and the Earth, about 150 million kilometers.