ALMA reveals exoplanet formation in transitional disks around young stars

December 20, 2015

Using images from Atacama Large Millimeter/submillimeter Array (ALMA), astronomers discovered young planets being formed in transitional disks around four stars. These Jupiter-like exoplanets were indirectly discovered by observing the lack of gas and dust clouds in their respective disks. Gaps in gas were three times narrower than gaps in dust, indicating that planets are orbiting the star and taking necessary material.

Artist's impression of a transitional disk around a young star. Credit: ALMA (ESO/NAOJ/NRAO)/M. Kornmesser

Detecting exoplanets and understanding planets formation

For over two decades we’ve known that our solar system is not unique and that there are planets around other stars. Over 2000 exoplanets have been detected so far. Considering we now presume there’s at least one planet around most stars, the number mentioned above is miniscule. Yet, we consider the figure a success and expect it to grow at ever higher rate as methods and technology used in detection of exoplanets improves.

Despite having a good idea how planetary systems are formed, we are missing a lot of details. Since we assume most stars have planetary systems, it is important to find out as many of these details as possible. For example, we know planets are formed from the rotating disk around a young star. This disk is made of dust and gas particles.

A particular type of planetary disk, called transitional disk, lacks the dust in the central region, around a star. Two theories offer a possible answer:

  1. Strong stellar winds and radiation from the star have blown away the dust needed for planet formation
  2. Planets are actually forming in that region, taking the surrounding dust for themselves


ALMA’s ability makes discovery possible

This is where technological advancement comes into play. By using ALMA, astronomers mapped the distribution of dust and gas in four star systems with young stars at their centres and transitional disks around them. The resulting images were so sharp that they enabled the team to notice that gaps in dust were actually mostly filled with gas. However, even the gas possessed the gap, about three times smaller than the gap in the dust.

This is indication that of two theories mentioned above, only the second one makes sense. If there is a gas giant planet, a few times more massive than Jupiter, orbiting near the star, it would sweep both the dust and gas on its path. However, such planet’s gravitational pull would also attract dust particles that are farther away than gas particles, since dust particles are heavier.

“Previous observations already hinted at the presence of gas inside the dust gaps,” explains Nienke van der Marel, Institute for Astronomy at the University of Hawaii in Honolulu, USA. “But as ALMA can image the material in the entire disk in much greater detail than other facilities, we could rule out the alternative scenario. The deep gap points clearly to the presence of planets with several times the mass of Jupiter, creating these caverns as they sweep through the disk.”


What will future bring?

Another bit of good news is that ALMA used only 10% of its resolving power for these images. This was before the array of antennas was fully built. With fully operational ALMA and future generations of telescopes, we can hope that by detecting transitional disks and the gaps in gas and dust, we will have better images of planetary systems while they form. That would be great as it would deepen our knowledge on formation of planetary systems.

“All the transitional disks studied so far that have large dust cavities also have gas cavities. So, with ALMA, we can now find out where and when giant planets are being born in these disks, and compare these results with planet formation models,” says Ewine van Dishoeck, of Leiden University in Leiden, Netherlands. “Direct planetary detection is just within reach of current instruments, and the next generation telescopes currently under construction, such as the European Extremely Large Telescope, will be able to go much further. ALMA is pointing out where they will need to look.”

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