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Starts With A Bang

Come See Astronomy’s Most Important Moon-Forming Image Ever

This image, taken with the Atacama Large Millimeter/submillimeter Array (ALMA), shows a close-up view on the moon-forming, circumplanetary disc surrounding PDS 70c. The exoplanet PDS 70c is a young, Jupiter-like gas giant located nearly 400 light-years away. It shows this planet and its disc are located inside a larger circumstellar ring-like disc on the right, and the circumplanetary disk contains enough mass to form three worlds the same size as Earth’s Moon. (ALMA (ESO/NAOJ/NRAO)/BENISTY ET AL.)

We just observed the first ‘lunar formation’ in an exoplanetary system.


This one image, above, is the first to show moons actively forming around a planet.

This colourful image shows the sky around the faint orange dwarf star PDS 70, circled. This faint star is well below the threshold of what’s visible to the naked eye. Only the bright blue star at the upper right, Chi Centauri, at approximately +5 magnitude, can be seen by the unaided human eye. (ESO/DIGITIZED SKY SURVEY 2; ACKNOWLEDGEMENT: DAVIDE DE MARTIN; ANNOTATION: E. SIEGEL)

The system’s central star, PDS 70, lies ~400 light-years away on the edge of the constellation Centaurus.

The SPHERE instrument on ESO’s Very Large Telescope reveals a planet caught in the very act of formation around the young dwarf star PDS 70. This was the first planet found in the act of forming, revealed in 2018. The planet stands clearly out, visible as a bright point to the right of the centre of the image, which is blacked out by the coronagraph mask used to block the blinding light of the central star. There is a second planet, PDS 70c, farther out.(ESO/A. MÜLLER ET AL.)

Two planets have been found: PDS 70b and 70c.

Composite image of PDS 70. Comparing new ALMA data to earlier VLT observations, astronomers determined that the young planet designated PDS 70 c has a circumplanetary disk, a feature that is strongly theorized to be the birthplace of moons. Recent observations have confirmed this at much higher resolutions. (ALMA (ESO/NOAJ/NRAO) A. ISELLA; ESO)

The latter contains a circumplanetary disk, newly revealed for the first time ever.

Wide-field (left) and close-up (right) views of the moon-forming disc surrounding PDS 70c. Two planets have been found in the system, PDS 70c and PDS 70b, the latter not being visible in this image. They have carved a cavity in the circumstellar disc as they gobbled up material from the disc itself, growing in size. In this process, PDS 70c acquired its own circumplanetary disc, which contributes to the growth of the planet and where moons can form. (ALMA (ESO/NAOJ/NRAO)/BENISTY ET AL.)

In theory, planets only acquire moons through three main processes.

The entirety of Saturn’s main rings, from the inner D ring to the outer F ring, may be much newer than the rest of the Solar System. It’s plausible that a few hundred million years ago, before the rise of the dinosaurs, these rings may not have existed at all. In another 300 million years ago, they likely will have disappeared entirely. (NASA/JPL)

1.) The giant impact scenario: leading to debris clouds around rocky bodies.

An illustration of what a synestia might look like: a puffed-up ring that surrounds a planet subsequent to a high-energy, large angular momentum impact. It is now thought that our Moon was formed by an early collision with Earth that created such a phenomenon. (SARAH STEWART/UC DAVIS/NASA)

That debris then coalesces into satellites: like Earth’s, Mars’s, and Pluto’s moons.

Rather than the two Moons we see today, a collision followed by a circumplanetary disk may have given rise to three moons of Mars, where only two survive today. This hypothetical transient moon of Mars, proposed in a 2016 paper, is now the leading idea in the formation of Mars’s moons. (LABEX UNIVEARTHS / UNIVERSITÉ PARIS DIDEROT)

2.) Gravitational capture: explaining Saturn’s Phoebe and Neptune’s Triton.

The orbit of Triton (red) has a 157° tilt in comparison to moons that co-rotate with Neptune’s rotation (green), and a tilt of 130° to objects that co-rotate with the ecliptic plane. Triton’s orientation is the strongest evidence that it is a captured body. (WIKIMEDIA COMMONS USER ZYJACKLIN; NASA / JPL / USGS)

3.) A circumplanetary disk: likely explaining most of the Universe’s moons.

According to simulations of protoplanetary disk formation, asymmetric clumps of matter contract all the way down in one dimension first, where they then start to spin. That “plane” is where the planets form, with that process repeating itself on smaller scales around giant planets: forming circumplanetary disks that lead to a lunar system. (STSCL OPO — C BURROWS AND J. KRIST (STSCL), K. STABELFELDT (JPL) AND NASA)

Stars and planets form from a cloud of collapsing gas, leading first to a proto-star.

30 protoplanetary disks, or proplyds, as imaged by Hubble in the Orion Nebula. Hubble is a brilliant resource for identifying these disk signatures in the optical, but has little power to probe the internal features of these disks, even from its location in space. Many of these young stars have only recently left the proto-star phase. Star-forming regions like this will frequently give rise to thousands upon thousands of new stars all at once. (NASA/ESA AND L. RICCI (ESO))

A disk of planet-forming material rapidly coalesces around it: a protoplanetary disk.

Artist’s impression of a young star surrounded by a protoplanetary disk. When nuclear fusion first ignited in our Sun’s central core, our Solar System may have looked very similar to this. The protoplanetary disks we image are most frequently not yet found around mature stars, but rather protostars that still have not begun to fuse hydrogen into helium in their cores. (ESO/L. CALÇADA)

Infrared and radio observatories have revealed these gap-rich disks in detail.

20 new protoplanetary disks, as imaged by the Disk Substructures at High Angular Resolution Project (DSHARP) collaboration, showcasing what newly-forming planetary systems look like. The gaps in the disk are likely the locations of newly-forming planets, with the largest gaps likely corresponding to the most massive proto-planets. (S. M. ANDREWS ET AL. AND THE DSHARP COLLABORATION, ARXIV:1812.04040)

Every gap contains planets, which capture/clear the surrounding material.

A composite radio/visible image of the protoplanetary disk and jet around HD 163296. The protoplanetary disk and features are revealed by ALMA in the radio, while the blue optical features are revealed by the MUSE instrument aboard the ESO’s Very Large Telescope. They are at perpendicular, 90 degree angles to one another. (VISIBLE: VLT/MUSE (ESO); RADIO: ALMA (ESO/NAOJ/NRAO))

However, the largest planets accrete their own disks: forming moons.

The young, Sun-like star TW Hydrae contains a face-on protoplanetary disk with substantial gaps in it, corresponding to newly forming planets. In the innermost region, however, there is a gap nearest to the star at approximately the Earth-Sun distance. There may be material falling onto this protoplanet, suggestive of, but not quite evidence for, the potential of generating a circumplanetary disk. (S. ANDREWS (HARVARD-SMITHSONIAN CFA), ALMA (ESO/NAOJ/NRAO))

This connection explains why Jovian moons resemble low-mass exoplanetary systems.

The planets we’ve found around red dwarf systems, like TRAPPIST-1, more closely resemble the lunar system around a planet like Jupiter than they do the planets around our own Sun. The dominant thought for a long time was that Jovian moons arise from an initial circumplanetary disk, a picture newly confirmed by observations of the proto-stellar system PDS 70. (NASA / JPL-CALTECH)

Only PDS 70c has a circumplanetary disk; the next step will measure internal gas motions.

Jupiter and its rings, bands and other heat-sensitive features in the infrared. Note how everything we observe, Jupiter’s bands, rings, and moons, all orbit in the same plane. This is a strong indication that they all formed at the same time: from the initial circumplanetary disk around Jupiter that dates to the formation of the Solar System. (TROCCHE100 AT THE ITALIAN WIKIPEDIA)

Perhaps, soon, we’ll understand the specifics of the emerging lunar system.


Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


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