Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found signs of a growing Neptune-like planet in the protoplanetary disk around TW Hydrae, a 10-million-year-old star approximately 175 light-years away from Earth.
Artist’s impression of the dust disk and a forming Neptune-like planet around TW Hydrae. Image credit: NAOJ.
A number of exoplanets have been found in the past two decades and now astronomers 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 planets, such as Uranus and Neptune, form.
To take a close look at the planet formation site, a team of Japanese scientists led by Ibaraki University astronomer Takashi Tsukagoshi observed the young star TW Hydrae, also known as HIP 53911 and IRAS 10594-3426.
This star is a popular target of study for astronomers because of its proximity to Earth and its status as a veritable newborn. It also has a face-on orientation as seen from Earth.
Past observations have shown that TW Hydrae is surrounded by a protoplanetary disk, which is the disk of gas and dust from which planets form.
Recent ALMA observations revealed multiple gaps in the disk. Some theoretical studies suggest that the gaps are evidence of planet formation.
ALMA image of the disk around TW Hydrae. Several gaps are clearly depicted. Astronomers 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 Neptune-like planet is located in this gap. Image credit: ALMA / ESO / NAOJ / NRAO / Takashi Tsukagoshi et al.
Dr. Tsukagoshi and co-authors 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, astronomers 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 AU (astronomical units).
“Combined with the orbit size and the brightness of TW Hydrae, the planet would be an icy giant planet like Neptune,” Dr. Tsukagoshi said.
The team’s findings have been accepted for publication in the Astrophysical Journal Letters (arXiv.org preprint).
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Takashi Tsukagoshi et al. 2016. A Gap with a Deficit of Large Grains in the protoplanetary disk around TW Hya. ApJL, accepted for publication; arXiv: 1605.00289
