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A Close Look at Newborn Planets Reveals Hints of Infant Moons
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    [Image: Join David Kaplan on a virtual-reality tour showing how the sun, the Earth and the other planets came to be.]  
    Video: Join David Kaplan on a virtual-reality tour showing how the sun, the Earth and the other planets came to be.

    Quanta Magazine and Chorus Films

    Most of these discoveries come from the Very Large Telescope (VLT), which is actually a set of four enormous telescopes perched on a mountaintop in Chile’s Atacama Desert. To find the second planet and provide even further confirmation for the first one, a team led by Sebastiaan Haffert at Leiden University in the Netherlands used MUSE, a new instrument on the VLT, to search for emission from hydrogen. They got their data on just one clear night in the summer of 2018, when MUSE was still being tested. “Given how robustly they detected both planets, it’s really exciting for the future,” Follette said.

    So far, the two planets seem to match predictions by previously untested theories of planet formation, said Zhaohuan Zhu, an astrophysicist at the University of Nevada, Las Vegas. Back in 2012 — eons ago in this fast-moving field — astronomers first saw an empty, dust-free band in the disk of PDS 70. It starts from a position equivalent to where Uranus orbits in our solar system and extends to about three times that distance.

    The first planet, dubbed PDS 70b, prowls right outside the inner edge of this gap in the disk. But one planet alone couldn’t explain why the gap was so wide. Two planets, however, would in theory open up a much wider gap. That’s exactly what the new data show. With a second planet found near the outer edge of the gap, circling the star once for every two complete orbits by the inner planet, “everything seems to start to connect,” Zhu said.

    Because it has bright planets far from their star, PDS 70 is perfectly tuned to give up its secrets. “PDS 70 is really becoming a benchmark system,” said Julien Girard, an astronomer at the Space Telescope Science Institute who collaborated on the discoveries of both the second planet and the possible moon-forming disk.

    The more tentative detection of the moon-hatching disk also came from the VLT, through another instrument called Sinfoni. Once the initial discovery of PDS 70b was made last year, Valentin Christiaens at Monash University in Australia looked for the same point of light in his own separate measurements of the system. “When we eventually saw the blob, I got very excited,” he said.

    [Image: IMAGE TRIPTYCH: a figure that explains very simply how ESO inferred the presence of the circumplanetary disc (not resolved from the planet signal in the first image). Possible caption:  Infrared image (at 2.4 µm wavelength) of the newborn planet PDS 70 b (left), and predictions for a planet + circumplanetary disc (middle) and a planet alone (right).]  
    An infrared image of PDS 70b (left) appears more like what astronomers would expect if the planet had a circumplanetary disk (center) than the predictions for a planet alone (right).

    ESO/V. Christiaens et al.

    These measurements showed that the light from PDS 70b included more red light than would be expected from just a planet. Christaens and his team argue that the data reveal a disk of dusty material around the planet that absorbs heat, then rereleases it in infrared wavelengths.

    In models of how solar systems form, smaller disks like these coalesce into satellites such as the four Jovian moons discovered by Galileo. If the initial observations are confirmed by other observatories, they’ll help to validate these ideas. “It’s nice confirmation that you could have a circumplanetary disk around giant planets, that Jupiter is not that special,” Zhu said.

    He also points out that in our own solar system, two of the most promising targets in the search for life, Europa and Enceladus, orbit giant planets. If infant gas giants often host disks that can make moons, Zhu said, “maybe Europa or Enceladus could be common in our galaxy.”

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