- Researchers recently discovered a planet that, based on what we know about planetary formation, shouldn’t exist.
- This planet, known as LHS 3154b, orbits a star that should be too small to have brought such an object into existence.
- Scientists are excited to find a new edge case to use in testing theories of planetary formation, and look forward to learning more about how this planet’s existence was made possible in the first place.
There’s a standard way that planetary systems operate. Big star, one to many planets of various sizes, maybe some moons and some additional smaller celestial bodies getting in the orbital mix. In general, larger stars have more gravity and can thus hold onto more stuff. This leads to larger protoplanetary disks—disks of dust and gas that eventually coalesce into planets and other massive objects—and eventually, bigger objects orbiting that central star.
But the fun thing about standards—especially ones that we try to apply to processes that we don’t fully understand, like planetary formation—is that they don’t always hold. This recently became very obvious to a group of researchers, who noticed a planet orbiting a faraway ultracool dwarf star.
At this point in the history of astronomy, finding a new exoplanet is cool, but often nothing to write home about. This one, however—this one was a weirdo. Simply put, according to what we know about physics and the process of planetary formation, this planet is just way too big to be orbiting around its very little star.
The researcher team published a paper on their discovery in the journal Science.
“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, one of the authors on the paper, said in a press release. “But it’s out there, so now we need to reexamine our understanding of how planets and stars form.”
“This discovery,” Mahadevan said, “really drives home the point of just how little we know about the universe.”
The planet discovered, known as LHS 3154b, is about 13 times more massive than Earth. As planets go, that’s not unheard of. Our own Uranus is about 14 times the mass of Earth, and our largest planet Jupiter is about 316 times more massive than Earth.
If it were orbiting a host of other stars, LHS 3154b would be rather unremarkable. But it’s orbiting LHS 3154, and that star is tiny. Like, very tiny—it’s an ultracool dwarf, after all. LHS 3154 is only one ninth the mass of our Sun, and according to current models, a star as small as LHS 3154 should not have had a protoplanetary disk full of enough stuff to build a planet with as heavy a core as LHS 3154b seems to have. The team believes that ratios like “dust-mass” and “dust-to-gas,” which tell astronomers about the makeup of protoplanetary disks, must have been about 10 times higher than expected in order for this planet to have come into existence around this star.
“What we have discovered provides an extreme test case for all existing planet formation theories,” Mahadevan said in the press release. “This is exactly what we built HPF [Habitable Zone Planet Finder, the device used to make this discovery] to do, to discover how the most common stars in our galaxy form planets—and to find those planets.”
And it seems like researchers are finding them, alright. LHS 3154 may have placed quite a puzzle before scientists, but at least it was kind enough to make itself known.
The research team was actually looking for planets in the habitable zones of ultracool stars when they found LHS 3154b, and those habitable zones are very close to host stars when those stars are very cool. The easiest planets to spot are large planets orbiting very closely to stars—depending on which search method you’re using, they either block the most light or exert the most gravitational force from or on their stars, respectively—and LHS 3154b fits that bill perfectly.
Now, researchers just have to answer the question of how this thing can exist. “Based on current survey work with the HPF and other instruments, an object like the one we discovered is likely extremely rare, so detecting it has been really exciting,” Megan Delamer, one of the authors on the paper, said in a press release. “Our current theories of planet formation have trouble accounting for what we’re seeing.”
Sounds like it might be time to modify some theories.
Associate News Editor
Jackie is a writer and editor from Pennsylvania. She's especially fond of writing about space and physics, and loves sharing the weird wonders of the universe with anyone who wants to listen. She is supervised in her home office by her two cats.
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2023-12-12 19:38:00Z
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