Astronomers discover distant building site of giant planetary system
Sen—A team of Japanese astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found evidence of a planetary system being formed a long way from its star -- about five times further out than Neptune is from the Sun.
The star, labelled HD142527, lives in the constellation of Lupus, the Wolf. The research team, led by astronomers at Osaka University and Ibaraki University, used ALMA to observe the ring of cosmic dust -- the building blocks of planets -- swarming around the star. After measuring the densest part of the dust ring the researchers concluded it was highly possible that planets are now being formed.
Of particular interest to students of planet formation was the location of the planet forming region which is much further away from the star than previously discovered protoplanetary disks. Though exoplanets have been found orbiting their stars at greater distances than Neptune, the planet formation process is yet to be fully understood.
Planet formation takes place when dust particles in a disk collide and coalesce together to form protoplanets, which are the cores of planets. Some continue to clump together with more dust to form rocky worlds, whilst other protoplanets capture gas particles to form gas giants. Prior to the teams's findings around HD142527 it was generally assumed that such planet formations occurred much closer to their parent star.
The submillimeter emission from the dust ring showed a non-uniform distribution, with part of the ring much denser than the rest. Misato Fukagawa, leader of the team and an assistant professor at Osaka University, said: "The brightest part in submillimeter wave is located far from the central star, and the distance is comparable to five times the distance between the Sun and the Neptune. I have never seen such a bright knot in such a distant position. This strong submillimeter emission can be interpreted as an indication that large amount of material is accumulated in this position. When a sufficient amount of material is accumulated, planets or comets can be formed here. To investigate this possibility, we measured the amount of material."
Dust and gas disk around HD142527. The dust and gas distributions observed by ALMA are shown in red and green, respectively. Near-infrared image taken by the NAOJ Subaru Telescope is shown in blue. The circle in the image shows the position of the dust concentration, in which planets are thought to be formed. Credit: ALMA (ESO/NAOJ/NRAO), NAOJ, Fukagawa et al.
The team's research led them to consider that it was highly possible planets are being formed in the dense part of the disk.
"Seeing the site of planet formation directly is one of the most important goals for ALMA" explained Munetake Momose, a team member and a professor at Ibaraki University. "Our observations successfully located a unique candidate in an unexpectedly distant place from the central star. I believe that ALMA will bring us more surprising results."
The researchers are planning further observations with ALMA to measure the amount of gas in the disc in order to determine whether the planets forming are rocky worlds or gas giants. If the dense region has the same amount of gas as is typically found in the Universe (a mass ratio of dust to gas being 1 to 100) then there would be enough gas to form giant gaseous planets several times more massive than Jupiter. If, however, the dense region is a dust trap, an area in the disc of dust where the dust concentration is exceptionally higher than in other parts of the disk, then another possibility would be the formation of rocky worlds like Earth and Mars.
Misato Fukagawa concluded: "Our final goal is to reveal the major physical process which controls the formation of planets. To achieve this goal, it is important to obtain a comprehensive view of the planet formation through observations of many protoplanetary disks."
ALMA is a powerful telescope array located 5,000 meters above sea level on the remote Chajnantor Plateau in the Atacama desert in Chile. The observatory is operated by the European Southern Observatory and other international partners including the US National Science Foundation (NSF), the National Research Council of Canada (NRC) and the National Institute of Natural Sciences (NINS) in Japan.