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Starts With A Bang

Shocking exoplanet image reveals a new way to make Jupiters

The story of how Jupiter, Saturn, Uranus and Neptune were made isn’t a universal one. Some gas giants were built different.
Image of the star AB Aurigae taken by the Subaru Telescope showing the spiral arms in the disk and the newly-discovered protoplanet AB Aur b. The bright central star has been masked, and its location is indicated by the star mark (☆). The size of Neptune’s orbit in the Solar System is shown to provide scale.
(Credit: T. Currie/Subaru Telescope)
Key Takeaways
  • When new stars and planetary systems form, the planets always arise from a protoplanetary disk, encircling the young star and growing from planetesimals.
  • All of our Solar System’s planets grew in the same way: by a small imperfection gravitationally growing, forming a solid core, and then gathering the remaining gas onto these newly massive bodies.
  • But out beyond distant Neptune, that mechanism wouldn’t work anymore. For the first time, we’ve discovered a giant planet forming at a great distance from its parent star, suggesting an entirely new way to make giant planets.

How are planets made? The story just got more complicated.

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. Radio telescopes like ALMA, as well as infrared observatories like the VLT and JWST, are far superior at that aspect of measuring these details. Planets largely arise from protoplanetary disks, but different mechanisms might be responsible for different planetary formation scenarios at various distances from the parent star.
Credit: NASA/ESA and L. Ricci (ESO)

For stellar systems like our own, one story has been overwhelmingly compelling.

how many planets
Here in our own Solar System, a single star anchors the system, where inner, rocky planets, an intermediate-distance asteroid belt, and then more distant gas giant planets eventually give way to the Kuiper belt and Oort cloud. This configuration is not universal among stellar and planetary systems, which means our Solar System possesses many properties that are not necessarily common among exoplanetary systems.
Credit: NASA/Dana Berry

Gas clouds collapse, forming protostars surrounded by protoplanetary disks.

In a system dominated by a single protostar, there will be major regions defined by multiple lines, including the soot line and the frost line for each specific molecular species. Although imperfections in the disk that grow, accruing a large gas envelope beyond a certain mass threshold, may well-describe the planets formed in our Solar System and many others, they do not account for giant planets found well beyond the Sun-Neptune distance.
Credit: NASA/JPL-Caltech/Invader Xan

Within each disk, gravitational imperfections arise and grow.

This artist’s illustration shows a proto-star surrounded by a protoplanetary disk, with young protoplanetesimals inside. The largest protoplanets are found in the regions where the density of the disk is lowest, and the first “gaps” in the disk will correspond to the earliest, most massive planets that arise.
(Credit: ESO/L. Calçada)

These protoplanetary cores vacuum up surrounding material.

A sample of 20 protoplanetary disks around young, infant stars, as measured by the Disk Substructures at High Angular Resolution Project: DSHARP. Observations such as these taught us that protoplanetary disks form primarily in a single plane and tend to support the core accretion scenario of planet formation. The disk structures are seen in both infrared and millimeter/sub-millimeter wavelengths. We have recently learned that gaps begin to form in protoplanetary disks after ~0.5-2 million years, with younger disks displaying no such substructure. These disks tend to disappear and give way to debris disk systems after around ~10 million years.
Credit: S.M. Andrews et al., ApJL, 2018

The largest cores accrete into planets, even developing their own circumplanetary disks.

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.
(Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.)

Eventually, giant planets with large lunar systems and thick atmospheres arise.

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. Superficially, these objects appear similar to some spiral galaxies.
(Credit: STScl OPO — C. Burrows and J. Krist (STScl), K. Stabelfeldt (JPL) and NASA)

This “core accretion” picture explains our own gas giants.

Of the eight planets in our Solar System, the four gas giant worlds are the least dense, with less than half the density of the least dense rocky planet (Mars), and with Saturn being even less dense than water.
Credit: NASA/Lunar and Planetary Institute

Core accretion also consistently explains most exoplanet properties.

5000 exoplanets
The mass, period, and discovery/measurement method used to determine the properties of the first 5000+ (technically, 5005) exoplanets ever discovered. Although there are planets of all sizes and periods, we are presently biased toward larger, heavier planets that orbit smaller stars at shorter orbital distances. The outer planets in most stellar systems remain largely undiscovered, but those that have been discovered, largely through direct imaging, are difficult to explain the way we think most exoplanets form: via the core accretion scenario.
Credit: NASA/JPL-Caltech/NASA Exoplanet Archive

Protoplanetary disks have been discovered, possessing the anticipated gaps.

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. The gaps between the rings are likely locations of newly forming planets.
(Credits: Visible: VLT/MUSE (ESO); Radio: ALMA (ESO/NAOJ/NRAO))

Young exoplanet PDS 70c even displays a moon-forming, circumplanetary disk.

But the AB Aurigae system is a “smoking gun” counterexample.

A dusty disk of protoplanetary material (red) surrounds the inner stellar system (blue) around the young star AB Aurigae (yellow star), with a candidate planet revealed in the location identified by the green arrow. This object has properties that render it incompatible with the standard core accretion scenario.
(Credit: T. Currie et al., Nature Astronomy, 2022)

Subaru and Hubble data, combined, revealed a giant planet at thrice the Sun-Neptune distance.

The combination of Subaru data (red image) and Hubble data (blue image) reveals the presence of an exoplanet at a distance of 93 Astronomical Units (where 1 A.U. is the Earth-Sun distance) from its parent star. The luminosity of the massive object indicates reflected stellar emission rather than unimpeded direct emission, while the lack of a polarization signal is highly suggestive of a formation scenario other than core accretion. This is one of more than 5000 exoplanets presently known.
(Credit: T. Currie et al., Nature Astronomy, 2022)

However, the polarization data shows no signal.

These two synthetic images are derived from simulations of the AB Aurigae system, including a protoplanetary disk and an embedded super-Jupiter planet. This planet represents the densest, hottest clump in a gravitational instability scenario.
(Credit: T. Currie et al., Nature Astronomy, 2022)

This suggests that gravitational instability and rapid, early collapse made these planets, not core accretion.

Additional planet formation sites around AB Aurigae, at distances between 400 and 600 A.U., have some suggestive features in legacy Hubble data that could indicate the presence of two further, yet unconfirmed, giant planets in this system.
(Credit: T. Currie et al., Nature Astronomy, 2022)

Two more distant planet formation sites ⁠— at hundreds of Astronomical Units ⁠— are suggested by the imagery.

A series of spiral arm features have been revealed in protoplanetary disks, but it remains an open question just what causes them. Two leading candidates is that they’re driven by planets, but also that they’re driven by gravitational instabilities.
(Credit: Ruobing Dong (董若冰) et al., ApJ, 2018)

Widely-separated planets create additional protoplanetary features, including spiral arms.

If the light from a parent star can be obscured, such as with a coronagraph or a starshade, the terrestrial planets within its habitable zone could potentially be directly imaged, allowing searches for numerous potential biosignatures. Our ability to directly image exoplanets is presently limited to giant exoplanets at great distances from bright stars, but this will improve with better telescope technology.
(Credit: J. Wang (UC Berkeley) & C. Marois (Herzberg Astrophysics), NExSS (NASA), Keck Obs.)

Formation via gravitational instability is consistent with other directly imaged exoplanets.

51 Eri b was discovered in 2014 by the Gemini Planet Imager. At 2 Jupiter masses, it is the coolest and lowest mass imaged exoplanet to date, and orbits only 12 Astronomical Units from its parent star. To image beings on the surface of this world would require a telescope with billions of times our present best resolution.
(Credit: Jason Wang (Caltech)/Gemini Planet Imager Exoplanet Survey)

Perhaps there are multiple ways to make a giant planet, after all.

dust ring
Two images of the planet-forming disk taken with ALMA around GW Orionis. Three independent, misaligned rings can be seen, raising questions about the nature of stars and planets that are present in this young system. Overall, the planets that lie at large separation distances from their parent stars may have a different formation mechanism than the closer-in ones.
(Credit: ALMA (ESO/NAOJ/NRAO), S. Kraus & J. Bi; NRAO/AUI/NSF, S. Dagnello)

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


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