December 5, 2023 | David F. Coppedge

Fat Star Child Too Big for Mom

Could a mother give birth to a 170-pound baby?
That’s what a new exoplanet is like



Astronomers are baffled again. A new exoplanet is so big in comparison to its host star, it’s throwing prevailing theories about planet formation into the trash can. Planets are thought to form as clumps in dust disks circling young stars, but planets as big as Neptune are not allowed to form around dwarf stars. LHS 3154b just broke the law.

This ‘forbidden’ exoplanet is way too massive for its star (, 30 Nov 2023). Robert Lea says the ratio between this star and its planet is 100 times that of the sun and Earth. It’s illegal. Stop existing!

“This discovery really drives home the point of just how little we know about the universe. We wouldn’t expect a planet this heavy around such a low-mass star to exist.”

With a planet having more material than thought possible, it challenges two theories: planet formation and star formation.

Thus, the discovery of this particular exoplanet also raises questions about the formation of stars. This is because the ratios of the dust-to-mass and dust-to-gas content of LHS 3154’s original protoplanetary disk would have had to be ten times higher than predictions in order to birth a Neptune-like world as massive as LHS 3154 b.

“Astronomers have discovered a massive extrasolar planet, or ‘exoplanet,’ orbiting an ultracool dwarf star that is way too small to host such a world, challenging scientists’ models of how planets and planetary systems are born.” — Image credit: Penn State

Tiny star found harbouring a huge planet that shouldn’t exist (New Scientist, 30 Nov 2023). Astronomers keep imagining things that they can’t see (like dark matter), and finding things that they can’t imagine. Science writer Leah Crane expresses the latest conundrum in typical Tontological form, sweeping everyone into the grand fallacy with royal “we” pronouns.

An enormous planet orbiting a tiny star may break our ideas about planet formation. Astronomers have found a world more than 13 times as massive as Earth orbiting a star nine times less massive than the sun, and our best predictions of how planets form say that such a world should not exist….

Generally, we think that planets form in one of two ways: either the protoplanetary disc of dust and gas surrounding a young star rapidly collapses under its own gravity to form clumps of material, or large rocks in the disc slowly accrete many smaller ones over a long time. The researchers performed hundreds of simulations of small stars with discs similar to ones that have been observed, and none of them formed a single world that was anything like LHS 3154b.

Discovery of planet too big for its sun throws off solar system formation models (Penn State, 30 Nov 2023). This press release comes from the researchers. Grad student Megan Delamer and advisor Suvrath Mahadevan explain the conundrum in an embedded video. They also explain the instruments and techniques used to measure the planet’s size.

A Neptune-mass exoplanet in close orbit around a very low-mass star challenges formation models (Stefánsson et al., Science, 30 Nov 2023). This is the research paper. Nearly 30 astronomers band together in this cry fest about the demise of theory.

Theories of planet formation predict that low-mass stars should rarely host exoplanets with masses exceeding that of Neptune. We used radial velocity observations to detect a Neptune-mass exoplanet orbiting LHS 3154, a star that is nine times less massive than the Sun. The exoplanet’s orbital period is 3.7 days, and its minimum mass is 13.2 Earth masses. We used simulations to show that the high planet-to-star mass ratio (>3.5 × 10−4) is not an expected outcome of either the core accretion or gravitational instability theories of planet formation. In the core-accretion simulations, we show that close-in Neptune-mass planets are only formed if the dust mass of the protoplanetary disk is an order of magnitude greater than typically observed around very low-mass stars.

If these measurements hold up, it’s back to square one. The disk instability theory was concocted to rescue the core accretion theory. Now, both theories need rescuing, because up till now, “Giant planets have not been observed on close orbits around very low-mass dwarfs.” Yet this one exists. “LHS 3154b is difficult to explain with core-accretion models.” But disk instability models are worse:

Although we cannot rule out the gravitational instability mechanism, if LHS 3154b formed through gravitational instability followed by inward migration, it would require even greater protoplanetary disk masses than we considered above for the core-accretion scenario.

Actually, both were in trouble before. Core accretion doesn’t work, because particles don’t stick together: they bounce (14 Sept 2017). Disk instability doesn’t work, because it is nonphysical. Astronomers invented it as a rescue device to keep gas giants from spiraling into the star (28 June 2018, 23 July 2021).

Observations like this are difficult to make. Because this exoplanet’s dimensions were measured by one method, caution should be exercised. A team of 29 authors, however, did not question the measurements, nor did science reporters.

Additional Unaddressed Questions in Planetary Science News

Asteroid Phaethon’s mysterious tail may finally have an explanation (, 29 Nov 2023). Astronomers are claiming success explaining why this “comet” shuts off its tail after leaving the sun and reaching Jupiter’s distance. The answer has to do with minerals and their vaporization temperatures. That’s fine, but why does this small body have any volatiles left after billions of years?

Our solar system’s planets aren’t weird after all. Exoplanets have tilted orbits, too (, 29 Nov 2023). What a relief: every planet is weird! But if everybody is weird, then weird is the new normal. It was thought that gravitational interactions caused planets to tilt their orbits.

But fresh research led by Yale University shows that even “pristine” star systems far away from us, which haven’t been affected by such chaos, can have planets with tilted orbits. In fact, planets in those conditions may still slant as far as 20 degrees from the horizontal.

“It’s reassuring,” Malena Rice, lead author and Yale astronomer, said in a Tuesday (Nov. 28) statement about the research. “It tells us that we’re not a super-weird solar system. This is really like looking at ourselves in a funhouse mirror, and seeing how we fit into the bigger picture of the universe.

The unaddressed question, though, is why weird is the new normal. What happens to the old theory that gravitational interactions caused planetary tilts? Yale astronomer Malena Rice is puzzled.

“I’m trying to figure out why systems with hot Jupiters have such extremely tilted orbits,” Rice said. “When did they get tilted? Can they just be born that way?

Comparison of tilted planets with non-tilted ones may help answer the question. Planetary formation theories, however, predicted that inclinations would match those of the dust disks around their parent stars, unless and until they were knocked sideways by close encounters from other planets. By some weird cosmic stroke of good luck, Earth’s tilt is just right for habitability and complex life, if one accepts the Stuff Happens Law.



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