In a planetary arrangement that looks like a “double‑stuffed Oreo,” the red dwarf star LHS 1903 hosts two rocky worlds sandwiched between two gas‑rich mini‑Neptunes. This inside‑out planetary system defies the conventional expectation that dense, rocky planets form closest to their star while lighter, gas‑enveloped planets settle farther out.
Discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) in 2019, the four planets orbit their host star in less than 30 days, each ranging from roughly 1.4 to 2.5 times Earth’s radius—a size bracket that straddles the line between super‑Earths and mini‑Neptunes. Precise mass and density measurements from follow‑up observations with ground‑based telescopes and the Spitzer Space Telescope have revealed an unexpected reversal: the innermost and outermost planets are dense and rocky, while the two planets in the middle retain thick gaseous envelopes.
Why the arrangement is puzzling
Planet formation models predict that, as a protoplanetary disc cools, rocky planets emerge close to the star where intense radiation strips volatile gases, while gas giants and mini‑Neptunes form farther out where icy material can coalesce and capture hydrogen‑helium envelopes planetary migration. LHS 1903 follows this pattern for its first three planets, but the fourth planet—situated farthest from the star—appears to be rocky, a configuration likened to finding a Venus‑like world beyond Neptune’s orbit.
“Bad stuff does happen in young planetary systems,” said Andrew Cameron, an astronomer at the University of St Andrews. “This one has the look of something that’s been turned inside out.” University of St Andrews press release
Evidence from mass and density
Masses derived from radial‑velocity measurements and transit‑timing variations indicate that the outermost planet, LHS 1903 d, has a density comparable to Earth’s, implying a silicate‑rich composition with little to no atmosphere. By contrast, the two intermediate planets, LHS 1903 b and LHS 1903 c, possess densities only a fraction of Earth’s, consistent with substantial gaseous envelopes. The innermost planet, LHS 1903 e, is too dense and rocky.
These findings are catalogued in the NASA Exoplanet Archive, which lists the planets’ radii, orbital periods, and inferred compositions NASA Exoplanet Archive.
Possible formation scenarios
One plausible explanation is that the outer planets migrated inward early in the system’s history, a process thought to have shaped our own solar system during its first few hundred million years. In that era, gravitational interactions among the giant planets caused Jupiter and Saturn to shift sunward, scattering smaller bodies and even swapping the orbits of Uranus and Neptune. A similar dynamical upheaval could have driven LHS 1903’s gas‑rich planets toward the star, leaving the outermost rocky planet either untouched or stripped of its atmosphere by a high‑energy impact.
Two specific mechanisms have been proposed:
- Atmospheric erosion by a giant impact: A large body colliding with the outermost planet could have blasted away any primordial atmosphere, leaving a bare rocky core.
- Late‑stage accretion after gas dispersal: The outermost planet may have formed after the protoplanetary disc’s gas had largely vanished, preventing it from capturing a thick envelope.
Both scenarios align with recent theoretical work on planetary migration and atmospheric loss, suggesting that violent early histories may be more common than previously thought.
Implications for exoplanet science
The LHS 1903 system offers a rare laboratory for testing models of planet formation and migration. Its compact architecture—four planets packed within a quarter of an astronomical unit—allows astronomers to study how closely spaced planets interact gravitationally over time. The clear density contrast between the inner/outer rocky worlds and the middle gas‑rich planets provides a direct test of atmospheric stripping mechanisms.
Future observations with the James Webb Space Telescope (JWST) could probe the atmospheric composition of the mini‑Neptunes, while high‑precision radial‑velocity campaigns may refine the masses of the rocky planets. Such data would help determine whether the outermost rocky planet truly lost an atmosphere or simply never acquired one.
What comes next
Continued monitoring of LHS 1903 will focus on two fronts: (1) high‑resolution spectroscopy to search for lingering atmospheric traces on the inner rocky planets, and (2) dynamical modeling to reconstruct the system’s migration history. As more “inside‑out” architectures are uncovered, astronomers will be better equipped to assess how common violent early planetary evolution is across the galaxy.
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