Back before we knew for sure there were planets around other stars, the universe seemed likely to be ordered. If planet formation was common, then we’d see systems more or less like our own, with rocky inner worlds and gas giants in outer orbits. And if planet formation was a fluke, we’d find few planets to study. All that has, of course, been turned on its head by the abundant discoveries of exoplanets galore. And our Solar System turns out to be anything but a model for the rest of the galaxy. In today’s essay, Don Wilkins looks at several recent discoveries that challenge planet formation theory. We can bet that the more we probe the Milky Way, the more we’ll find anomalies that challenge our preconceptions.
by Don Wilkins
The past few decades have not been easy on planet formation theories. Concepts formed on the antiquated Copernican speculation, the commonality of star systems identical to the Solar System, have given way to the strangeness and variety uncovered by Kepler, Hubble, and the other space borne telescopes. The richness of the planetary arrangements defies easy explanation.
Penn State University researchers uncovered another oddity challenging current understanding of stellar system development. [1] Study of the LHS 3154 system reveals a planet so massive in comparison to its star that generally accepted theories of planet formation cannot explain the existence of the planet, Figure 1. LHS 3154, an “ultracool” star with a “chilly” surface temperature of 2,700 °K (2,430 °C; 4,400 °F), is an M-dwarf, a category that comprises three quarters of the stars in the Milky Way. Most of the light of LHS 3154 is in the infrared band. The M- dwarf star is nine times less massive than the Sun yet it hosts a planet 13 times more massive than Earth.
Figure 1. An artist rendition of the mass comparison between the Earth and Sun and the star LHS 3154, and its companion, LHS 3154b. Credit: Pennsylvania State University.
In current theories, stars form from condensing large clouds of gas and dust into smaller volumes. After the star forms, the left-over gas and dust which is a much smaller fraction of the original cloud, settles into a disk around the new star. From this much smaller mass, planets will condense, completing the star system. In these theories, the star consumes the major proportion of the progenitor clouds.
The Sun, for example, contains an estimated 99.8% of the mass of the Solar System. Only 0.2% is left over for the eight planets, various moons and asteroids.
The mass ratio comparing LHS 3154b to LHS 3154 is 117 times greater than mass ratio comparing the Earth to the Sun. LHS 3154b probably is Neptune-like in composition, completes its orbit in 3.7 Earth days and, the researchers believe, is a very rare world. Typically M-dwarves host small rocky bodies rather than gas giants.
According to current theories, once the star formed, there should not have been enough mass to form a planet as large as LHS 3154b. A young LHS 3154 disk dust-mass and dust-to-gas ratio must be ten times greater than what is typically observed surrounding an M-dwarf star to birth a giant such as LHS 3154b.
“The planet-forming disk around the low-mass star LHS 3154 is not expected to have enough solid mass to make this planet,” Suvrath Mahadevan, the Verne M. Willaman Professor of Astronomy and Astrophysics at Penn State and co-author on the paper said. “But it’s out there, so now we need to reexamine our understanding of how planets and stars form.”
Mahadevan’s team built a novel spectrograph, the Habitable Zone Planet Finder (HPF), with the intention of detecting planets orbiting the coolest of stars. Planets orbiting low temperature stars might have surfaces cool enough to support liquid water and life. In looking for planets with liquid water, the team found, as often happens in research, something new, a massive planet to challenge current theories of stellar system formation.
Another discovery, this time by a Carnegie Institution for Science team, uncovered another challenging world. [2]
Figure 2. Artist’s conception a small red dwarf star, TOI-5205, and its out-sized companion TOI-5205b. Credit: Katherine Cain, the Carnegie Institution for Science.
“The host star, TOI-5205, is just about four times the size of Jupiter, yet it has somehow managed to form a Jupiter-sized planet, which is quite surprising,” observed Shubham Kanodia, who led the team which found TOI-5205b.
When TOI-5205b crosses in front of TOI-5205, the planet blocks about seven percent of the star’s light—a dimming among the largest known exoplanet transit signals.
The rotating disk of gas and dust that surrounds a young star gives birth to its planetary companions. More massive planets require more of the gas and dust left over as the star ignites. Gas planet formation, in the accepted theories, requires about 10 Earth masses of rocky material to produce the massive rocky core of the gas giant. Once the core is formed, it gathers gas from the surrounding clouds, resulting in the mammoth atmosphere of the giant planet.
“TOI-5205b’s existence stretches what we know about the disks in which these planets are born,” Kanodia explained. “In the beginning, if there isn’t enough rocky material in the disk to form the initial core, then one cannot form a gas giant planet. And at the end, if the disk evaporates away before the massive core is formed, then one cannot form a gas giant planet. And yet TOI-5205b formed despite these guardrails. Based on our nominal current understanding of planet formation, TOI-5205b should not exist; it is a ‘forbidden’ planet.”
Not all mysteries are confined to M-dwarfs. A sun-like star, an infant of 14 million years some 360 light years from Earth, hosts a gas giant six times more massive than Jupiter, that orbits the star at a distance twenty times greater than the distance separating Jupiter and the Sun, Figure 3. [3]
Figure 3. A direct image of the exoplanet YSES 2b (bottom right) and its star (center). The star is blocked by a coronagraph. Credit: ESO/SPHERE/VLT/Bohn et al.
The large distance from YSES 2b to the star does not fit either of the two most well-known models describing large gaseous planet formation. If YSES 2b formed by means of core accretion at such an enormous distance far from the star, the planet should be much lighter than what is observed as a result of scarcity of disk material at that distant location. YSES 2b is too massive to satisfy this theory.
Gravitationally instability, the second theorized method for producing gas giants, postulates very massive protostellar disks that are unstable, splintering into large clumps from which gas giants are directly formed. YSES 2b appears not massive enough to have been formed in this fashion.
In a third possibility, YSES 2b might have formed by core accretion much closer to its host star and migrated outwards. A second planet is needed to pull YSES 2b into the outer regions of the system, but no such planet has been located.
Observations by the current generation of space-borne telescopes have upset the theories of planet formation. Hot Jupiters, worlds orbiting pulsars, odd arrangements of worlds, super Earths, and wandering worlds flung close to a star then flying back have complicated the ideas of Laplace, See, Chamberlin and Moulton. Further study by the James Webb Space Telescope and its successors will only enliven the debate surrounding the origin of the planets.
References
[1] Guðmundur Stefánsson, Suvrath Mahadevan, Yamila Miguel, et al, “A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models,” Science, 30 Nov 2023, Vol. 382, Issue 6674, pp. 1031-1035, DOI: 10.1126/science.abo0233.
[2] Shubham Kanodia et al, “TOI-5205b: A Short-period Jovian Planet Transiting a Mid-M Dwarf,” The Astronomical Journal (2023). DOI: 10.3847/1538-3881/acabce
[3] Alexander J. Bohn et al. “Discovery of a directly imaged planet to the young solar analog YSES 2.” Accepted for publication in Astronomy & Astrophysics, www.aanda.org/10.1051/0004-6361/202140508
After reading this, I am tempted to choose a different title: “Observations Clash With Forbidden World Formation Theories”.
“The mass ratio comparing LHS 3154b to LHS 3154 is 117 times greater than mass ratio comparing the Earth to the Sun.”
Yet Jupiter is more than 300x more massive than Earth. So these ratios are certainly allowed given the right circumstances. LHS 3154b is not quite forbidden!
There is no good reason to forbid the angular momentum of a large condensing nebula from forming more than one nexus of body formation. We see it all the time with binary and trinary systems. The critical question is perhaps how close these can be versus the mass in each nexus. The smaller one will either be disrupted or continue with formation of a body, small or large.
In so many other of these systems, there may be a bias due to nomenclature; that is, what we choose to call a planet versus a star, brown dwarf, super-Jovian planet, etc. We now that all of these exist and they didn’t come about in isolation. In most cases they appear to form together from one condensing nebula. Some are ejected and some stay.
We need to replace forbidden theories with new ones. There’s nothing wrong with the existing theories, we just refine and replace as necessary as our understanding and our observations improve. It’s science.
I should probably add that the title wasn’t Don’s, but mine. I sort of liked the nod to ‘Forbidden Planet.’ I take your point, though.
Your points are well taken, Ron. Let me add another. All of our methods for detecting exoplanets are extremely sensitive to very massive worlds orbiting very small stars at very close distances, and these M-dwarfs are further oversampled because there are so many of them! Any conclusions on planetary formation mechanisms based on this selection effect are bound to be biased.
We also seem to have made the assumption somewhere along the line that planetary systems are the result of processes fundamentally different to those that form binary stars. Perhaps there is no discrete functional boundary separating multiple star systems from planetary systems, one simply blends gradually into the other. Even if the history that leads to planets (as opposed to multiple star systems) is indeed distinct, what are the conditions that both systems might be operating simultaneously, perhaps overlapping around a distinct class of intermediate bodies?
The point is, we simply don’t have a good sample of objects and circumstances upon which to base a comprehensive theory of stellar system formation yet, the data we have is extensive, but highly biased.
And we must not forget that at one time it was feared solar systems were impossible because no mechanism was known to bleed off the excess angular momentum. Today we know better, as always, theory eventually seems to catch up to the observations. The good news is that well-behaved planetary systems circling suitable stars for life and civilization still cannot be ruled out. In fact, they may very well be the norm, not the exception.
One way to look at what is happening when these systems form, is like an ecosystem. The clouds from exploding stars form patterns like rivers and valleys where rogue planets may outnumber stars forming a thousand to one and the mountains where large stars form with few smaller stars or rogue planets forming there. This would make the rivers and valleys much more chaotic for larger stars. Then we have a period when all these systems go through the galaxy arms and become chaotic again. Through all of this the small tightly packed systems would be the least affected.
While outliers are interesting, I don’t think they are much help in coming up with a general theory of planetary formation. We will learn far more from general patterns in the statistical data. Unfortunately, this is heavily skewed towards the large and the close.
I think one of the best first steps would be to classify systems into types. For instance, there appears to be a type of system that has multiple similar-sized planets, the Trappist 1 system being the most notable. This arrangement turns up over a broad mass range of stars, scaling approximately with the star. Small Red dwarfs produce approximately Earth-sized planets, and as the stellar mass goes up the planets scale to Neptune size with a sol-mass star.
Astronomers have also noted a range of star-forming nebulas, classified as either compact or large. I would assume that a compact nebula would be slow rotating allowing material to fall rapidly into the star while the large ones would be rapidly rotating. This should produce measurable differences in stellar rotation, so we might be able to correlate planetary system type with its primaries’ rotation rate adjusted for its age.
It’s our observations that our limited. Our data and sample of exoplanets around red dwarfs is still small and our knowledge is based only a little data. These might add up with the discovery of more red dwarf systems with exoplanets.
A Jupiter around a red dwarf is not that unusual if we consider the mass of a red dwarf compared to a Jupiter sized gas giant. It’s only one planet. What we won’t see is a solar system the size of ours with a red dwarf star,with nine planets, two gas giants, two ice giants and four inner plant, etc. because the accretion disk is much smaller and mass of the protoplanetary gas cloud with much less mass.
If we discover some rocky planets around this red dwarf also that would be surprising as it grabbed up most of the gas, dust and water.
Hi Paul
The Capture Theory, developed by Michael Woolfson would explain such worlds, since the collapsing streamer of gas pulled from a proto-star disk doesn’t belong to the star of origin. Self-gravitating clumps are formed, then captured by dynamical friction with the capturing star’s protostellar disk. Woolfson has modelled the interactions within birth nebulas and gets quite high fractions of stars that form this way. Alternatively disk instability proto-planets can be tidally stripped if their approach to their home-star is sufficiently close, a process studied by Sergei Nayakshin.
As you see, there’s more than one way to flense the feline.
Woolfson and Nayakshin are unknown to me. Can you tell us more about them?
Here are the first sets from a plain Google search:
Capture Theory by Michael Woolfson
and
disk instability proto-planets can be tidally stripped Sergei Nayakshin
Well done, Robin. Thanks.
https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs
Not as astrometric data- detailed as the RASC Observer’s Handbook list I often refer to here (5 pc), but it does extend out a little further, to about 20 ly.
The book I had when I was a kid i think it was Universe by Time Life books had two theories of solar system formation. The Nebular hypothesis and the Capture theory. I liked the capture theory. Of course, it’s completely obsolete today based on actual telescopic images of the accretion disks of other solar system which we have only obtained in relatively modern times. If life was based on the Capture theory, then life really would be very rare.
We are talking about planetary capture, not the Solar Capture theory. However, it is important for us to consider the size of the solar system due to the mass differences between our Sun and a red dwarf star. It is this mass difference that makes the need for a capture of a Jupiter size object not a necessity. I assume a Jupiter near a G class star is more rare than one around a red dwarf star. How fast the gas giant forms is also a factor. I will admit I am biased against the hot Jupiter capture theory around G class stars and think they formed in situ from the accretion of the proto planetary disk.
Jupiter-class planets are rare as hens’ teeth around red dwarfs. One relevant article:
https://www.sciencenews.org/article/jupiter-planets-rare-red-dwarf-stars#:~:text=Fewer%20than%202%20percent%20of,according%20to%20a%20new%20study.
In the academic literature, the above cites two recent papers on types of planets around M-dwarfs:
E.K. Pass et al. Mid-to-late M dwarfs lack Jupiter analogs. The Astronomical Journal. Vol. 166, July 2023, p. 11. doi: 10.3847/1538-3881/acd349.
K. Ment and D. Charbonneau. The occurrence rate of terrestrial planets orbiting nearby mid-to-late M dwarfs from TESS sectors 1–42. The Astronomical Journal. Vol. 165, June 2023, p. 265. doi: 10.3847/1538-3881/acd175.
Seems to me that planet formation theory and binary/multiple star formation theory are on a continuum where multiple physics mechanisms are all at play to varying degrees. To date, the various ‘subtheories’ have focused on cases where one mechanism or another predominates.
So, we may not even know all the mechanisms, plus we need more study on cases where no single mechanism predominates. (Honesty moment: I’ve not kept up with the literature of late, so perhaps such studies have been done and I’ve missed them.)
In the meantime, we should not be surprised when in our envisioned world of beavers and ducks, we occasionally find a platypus.