Astronomers Discover Unusual 'Inside-Out' Planetary System

Four worlds around a small, dim star challenge theories of planet formation

Published on Feb. 12, 2026

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Researchers have discovered a planetary system around the M-dwarf star LHS 1903 that appears to have been built 'inside-out', with larger worlds closer to the star and a smaller, rocky world farther out - the opposite of the typical pattern seen in most planetary systems. This unexpected configuration has left astronomers puzzled about how this system formed.

Why it matters

The discovery of this unusual planetary system challenges current theories of how planets form and evolve around M-dwarf stars, which are the most common type of star in the universe. Understanding the formation of this 'inside-out' system could provide key insights into the 'radius valley' - the gap in size between smaller rocky worlds and larger gaseous planets - that has been a topic of debate for M-dwarf systems.

The details

The international team of astronomers, led by Thomas Wilson of the University of Warwick, used data from the TESS and CHEOPS space telescopes to identify four planets orbiting the ancient, low-mass M-dwarf star LHS 1903. The innermost planet, LHS 1903 b, is a dense, rocky super-Earth. The next two planets, LHS 1903 c and d, are sub-Neptunes with thick, gaseous atmospheres. But the biggest surprise was the discovery of a fourth planet, LHS 1903 e, which is a rocky world with no atmosphere located at the outer edge of the system - the opposite of the typical pattern seen in most planetary systems.

  • The TESS space telescope first spotted three planets around LHS 1903 in 2023.
  • The international team used the CHEOPS space telescope to take a closer look at the system in 2025.
  • The results of their analysis were published in the journal Science on February 12, 2026.

The players

Thomas Wilson

An astrophysicist at the University of Warwick in England who led the international team of researchers studying the LHS 1903 planetary system.

LHS 1903

An ancient, low-mass M-dwarf star around seven billion years old, only about half the mass of our Sun, that hosts an unusual 'inside-out' planetary system.

TESS

NASA's Transiting Exoplanet Survey Satellite, which first detected three planets orbiting LHS 1903 in 2023.

CHEOPS

The European Space Agency's Characterizing Exoplanet Satellite, which was used by the research team to take a closer look at the LHS 1903 planetary system in 2025.

Lauren Weiss

An astrophysicist at the University of Notre Dame who was not involved in the study but provided commentary on the research.

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What they’re saying

“The sequential formation mechanism would mean that the inner planets were built early on, in a resource-rich environment, whereas the outer body was built last in a poorer region”

— Thomas Wilson, Astrophysicist, University of Warwick (Science)

“As for their conclusion that LHS 1903 e formed in a gas-depleted environment, I would have liked to see a more detailed experiment exploring the giant-impact scenario”

— Lauren Weiss, Astrophysicist, University of Notre Dame (Scientific American)

“This study opens new insights into the formation process of multiplanet systems orbiting M-dwarf stars. Finding more of these systems will really help us refine and constrain planet formation models in the near future.”

— Kevin Hardegree-Ullman, Astronomer, NASA Exoplanet Science Institute (Scientific American)

What’s next

The research team plans to use the James Webb Space Telescope to study the atmospheric composition of the planets in the LHS 1903 system, which could provide key evidence to support their 'gas-depleted formation' hypothesis.

The takeaway

The discovery of this unusual 'inside-out' planetary system around the M-dwarf star LHS 1903 challenges current theories of planet formation and evolution, and could provide important insights into the long-standing mystery of the 'radius valley' that separates smaller rocky worlds from larger gaseous planets around low-mass stars.