An Exomoon in the Making

Astronomers have detected a disk around a planet outside our Solar System for the first time.

Orange ring against black universe with blow up of bright orange spot suspended inside donut.

Wide and close-up views of a moonforming disc as seen with ALMA.

Alma Large Millimeter/submillimeter Array

The recent discovery of a disk of dust around a distant planet—one of over 4,000 extrasolar worlds confirmed in recent decades—shines new light on the formation of solar systems, including our own. The planet in question is one of two Jupiter-sized objects, designated PDS 70b and PDS 70c, that orbit a newly forming star, PDS 70, located nearly 400 light years away in the constellation Centaurus. Astronomers had previously detected a disk of gas and dust surrounding PDS 70, presumably the remnants of material condensing to form the young star. PDS 70b and 70c are embedded in this cloud, orbiting twenty-two and thirty-four times farther from their star, respectively, than the Earth is from the Sun (that’s a little more than the distances of Uranus and Neptune from the Sun). Earlier images from large optical telescopes revealed that the two giant planets had swept their surroundings clear of material as they orbited, carving out two concentric circular cavities in the circumstellar disk.

Now, new observations at higher resolution—made with the Atacama Large Millimeter/submillimeter Array (ALMA), an ensemble of sixty-six radio telescopes located in the high desert of Chile—reveal that PDS 70c is surrounded by its own “circumplanetary” disk of gas and dust, a miniature version of the larger cloud in which it is embedded. The disk is about the size of Earth’s orbit and contains three times as much material as our Moon. “Our work presents a clear detection of a disk in which satellites could be forming,” remarked lead author Myriam Benisty of the University of Grenoble, France, and the University of Chile.

Of the many planetary systems studied so far, the PDS 70 system is the only one still in the process of formation, so these observations indicate how planets grow as they accumulate material from their birthclouds. Unlike PDS 70c, the closer planet PDS 70b is not accompanied by a detectable circumplanetary disk. Its surroundings have already been swept nearly clean of moon-forming material, perhaps by the gravitational influence of its more distant companion or by the pull of the central star.

As new giant telescopes come on line in the near future, further observations of the PDS 70 system at even higher resolution should illuminate details of how complex interactions within star-forming clouds determine the size, spacing, and composition of their planets and satellites. (Astrophysical Journal Letters)

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