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Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903.

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Title: Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903.
Authors: Wilson, Thomas G. (AUTHOR), Simpson, Anna M. (AUTHOR), Collier Cameron, Andrew (AUTHOR), Cloutier, Ryan (AUTHOR), Adibekyan, Vardan (AUTHOR), John, Ancy Anna (AUTHOR), Alibert, Yann (AUTHOR), Stalport, Manu (AUTHOR), Egger, Jo Ann (AUTHOR), Bonfanti, Andrea (AUTHOR), Billot, Nicolas (AUTHOR), Guterman, Pascal (AUTHOR), Maxted, Pierre F. L. (AUTHOR), Simon, Attila E. (AUTHOR), Sousa, Sérgio G. (AUTHOR), Fridlund, Malcolm (AUTHOR), Beck, Mathias (AUTHOR), Bekkelien, Anja (AUTHOR), Salmon, Sébastien (AUTHOR), Van Grootel, Valérie (AUTHOR)
Source: Science. 4/16/2026, Vol. 392 Issue 6795, p1-12. 12p.
Subjects: Origin of planets, Protoplanetary disks, Dwarf stars, Extrasolar planets, Radial velocity of stars, Photometry
Abstract: The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This feature could be produced by the removal of atmospheres by stellar or internal heating or by the lack of an initial envelope. We used transit photometry and radial velocity measurements to detect and characterize four exoplanets orbiting LHS 1903, a red dwarf star in the Milky Way's thick disk. These four planets have orbital periods ranging from 2.2 to 29.3 days and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, whereas LHS 1903 c and LHS 1903 d have extended atmospheres. The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating that it formed from gas-depleted material. Editor's summary: The radii of midsized exoplanets show a bimodal distribution, with rocky super-Earths and gas-rich sub-Neptunes separated by a radius valley. The origin of this dichotomy is unclear: It could be due to different levels of atmospheric loss driven by stellar activity or it might reflect different conditions during the planet formation process. Wilson et al. identified and characterized four exoplanets orbiting a red dwarf star. Their properties span the radius valley within a single system. The outermost planet is rocky but cannot be explained by atmospheric loss, so it must have formed in a gas-poor region of the protoplanetary disk. —Keith T. Smith INTRODUCTION: Thousands of exoplanets have been observed, allowing for studies of their population-level, demographic trends. It has been shown that there is a dearth of exoplanets with radii between 1.6 and 1.8 times the radius of Earth, known as the radius valley. The position of the radius valley varies with planet orbital period and stellar mass, which could provide information on how it is produced. For planets orbiting low-mass red dwarf (M-dwarf) stars, proposed theoretical models that could explain the radius valley include thermally driven mass loss (T-DML) mechanisms, which drive atmospheric escape from a planet, or gas-depleted formation (G-DF) processes that restrict the availability of gaseous material in the protoplanetary disk where the planet forms. RATIONALE: Planet formation and evolution theories predict the composition (rocky or gaseous) of small planets. The T-DML scenario predicts that exoplanets with longer orbital periods have larger atmospheres than do planets closer to their host stars, owing to the effects of photoevaporation and core-powered heating. This prediction is consistent with gas-rich planet formation theory derived from first principles. Conversely, the G-DF scenario predicts that longer period planets should be rocky, owing to a lack of gas in the outer regions of the protoplanetary disk. These predictions can be tested by measuring the bulk densities of exoplanets across a range of orbital periods to determine their composition. If suitable bodies are identified in multiplanet systems, the effect of differences in host star mass and activity is removed. We identified four planets orbiting the M-dwarf star LHS 1903 and performed follow-up observations to determine their properties. RESULTS: By combining transit photometry and radial velocity data, we determined that the four planets (LHS 1903 b, c, d, and e) have orbital periods of 2.2, 6.2, 12.6, and 29.3 days and radii of 1.4, 2.0, 2.5, and 1.7 times Earth's radius, respectively. The planets span the expected position of the radius valley. The bulk densities and an internal structure analysis indicate that LHS 1903 b is rocky, whereas planets c and d are gas rich. The outermost planet, LHS 1903 e, is found to be gas poor. This allowed us to discriminate between the T-DML and G-DF scenarios, which predict different compositions for this planet. CONCLUSION: The compositions of LHS 1903 b, c, and d can all be explained by both the T-DML and G-DF scenarios. LHS 1903 e is not compatible with the T-DML model, because of its lack of an extended atmosphere, but is consistent with the G-DF model. We propose that the properties of this four-planet system can be explained by an inside-out formation mechanism, in which the inner planet formed first and triggered the formation of the next planet, with the others following in sequence. This formation pathway could have occurred if there was fast migration of material within the protoplanetary disk, causing the outer disk (at the location where planet e formed) to be depleted of gas. Comparison with planet formation simulations indicated that LHS 1903 e formed in a more gas-depleted environment than did planets c and d. We conclude that LHS 1903 e formed under the G-DF scenario. Measured properties of the four planets orbiting LHS 1903 and a schematic illustration of our proposed formation scenario.: The measured radius (in Earth radii, R⊕) and equilibrium temperature of each planet are labeled along the bottom. The colored layers of each planet indicate the modeled internal structures: The core and mantle are brown, with blue representing water and gray representing the atmosphere. The layer uncertainties are shown as colored rectangles, and the total radius uncertainties are shown as white annuli. Arrows indicate our proposed inside-out formation mechanism, with red shading indicating the gas density in the protoplanetary disk. The rocky planet e formed later, in a gas-depleted environment. [ABSTRACT FROM AUTHOR]
Copyright of Science is the property of American Association for the Advancement of Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903.
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  Data: <searchLink fieldCode="AR" term="%22Wilson%2C+Thomas+G%2E%22">Wilson, Thomas G.</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Simpson%2C+Anna+M%2E%22">Simpson, Anna M.</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Collier+Cameron%2C+Andrew%22">Collier Cameron, Andrew</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Cloutier%2C+Ryan%22">Cloutier, Ryan</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Adibekyan%2C+Vardan%22">Adibekyan, Vardan</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22John%2C+Ancy+Anna%22">John, Ancy Anna</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Alibert%2C+Yann%22">Alibert, Yann</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Stalport%2C+Manu%22">Stalport, Manu</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Egger%2C+Jo+Ann%22">Egger, Jo Ann</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Bonfanti%2C+Andrea%22">Bonfanti, Andrea</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Billot%2C+Nicolas%22">Billot, Nicolas</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Guterman%2C+Pascal%22">Guterman, Pascal</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Maxted%2C+Pierre+F%2E+L%2E%22">Maxted, Pierre F. L.</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Simon%2C+Attila+E%2E%22">Simon, Attila E.</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Sousa%2C+Sérgio+G%2E%22">Sousa, Sérgio G.</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Fridlund%2C+Malcolm%22">Fridlund, Malcolm</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Beck%2C+Mathias%22">Beck, Mathias</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Bekkelien%2C+Anja%22">Bekkelien, Anja</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Salmon%2C+Sébastien%22">Salmon, Sébastien</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Van+Grootel%2C+Valérie%22">Van Grootel, Valérie</searchLink> (AUTHOR)
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  Data: <searchLink fieldCode="JN" term="%22Science%22">Science</searchLink>. 4/16/2026, Vol. 392 Issue 6795, p1-12. 12p.
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  Data: <searchLink fieldCode="DE" term="%22Origin+of+planets%22">Origin of planets</searchLink><br /><searchLink fieldCode="DE" term="%22Protoplanetary+disks%22">Protoplanetary disks</searchLink><br /><searchLink fieldCode="DE" term="%22Dwarf+stars%22">Dwarf stars</searchLink><br /><searchLink fieldCode="DE" term="%22Extrasolar+planets%22">Extrasolar planets</searchLink><br /><searchLink fieldCode="DE" term="%22Radial+velocity+of+stars%22">Radial velocity of stars</searchLink><br /><searchLink fieldCode="DE" term="%22Photometry%22">Photometry</searchLink>
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  Data: The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This feature could be produced by the removal of atmospheres by stellar or internal heating or by the lack of an initial envelope. We used transit photometry and radial velocity measurements to detect and characterize four exoplanets orbiting LHS 1903, a red dwarf star in the Milky Way's thick disk. These four planets have orbital periods ranging from 2.2 to 29.3 days and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, whereas LHS 1903 c and LHS 1903 d have extended atmospheres. The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating that it formed from gas-depleted material. Editor's summary: The radii of midsized exoplanets show a bimodal distribution, with rocky super-Earths and gas-rich sub-Neptunes separated by a radius valley. The origin of this dichotomy is unclear: It could be due to different levels of atmospheric loss driven by stellar activity or it might reflect different conditions during the planet formation process. Wilson et al. identified and characterized four exoplanets orbiting a red dwarf star. Their properties span the radius valley within a single system. The outermost planet is rocky but cannot be explained by atmospheric loss, so it must have formed in a gas-poor region of the protoplanetary disk. —Keith T. Smith INTRODUCTION: Thousands of exoplanets have been observed, allowing for studies of their population-level, demographic trends. It has been shown that there is a dearth of exoplanets with radii between 1.6 and 1.8 times the radius of Earth, known as the radius valley. The position of the radius valley varies with planet orbital period and stellar mass, which could provide information on how it is produced. For planets orbiting low-mass red dwarf (M-dwarf) stars, proposed theoretical models that could explain the radius valley include thermally driven mass loss (T-DML) mechanisms, which drive atmospheric escape from a planet, or gas-depleted formation (G-DF) processes that restrict the availability of gaseous material in the protoplanetary disk where the planet forms. RATIONALE: Planet formation and evolution theories predict the composition (rocky or gaseous) of small planets. The T-DML scenario predicts that exoplanets with longer orbital periods have larger atmospheres than do planets closer to their host stars, owing to the effects of photoevaporation and core-powered heating. This prediction is consistent with gas-rich planet formation theory derived from first principles. Conversely, the G-DF scenario predicts that longer period planets should be rocky, owing to a lack of gas in the outer regions of the protoplanetary disk. These predictions can be tested by measuring the bulk densities of exoplanets across a range of orbital periods to determine their composition. If suitable bodies are identified in multiplanet systems, the effect of differences in host star mass and activity is removed. We identified four planets orbiting the M-dwarf star LHS 1903 and performed follow-up observations to determine their properties. RESULTS: By combining transit photometry and radial velocity data, we determined that the four planets (LHS 1903 b, c, d, and e) have orbital periods of 2.2, 6.2, 12.6, and 29.3 days and radii of 1.4, 2.0, 2.5, and 1.7 times Earth's radius, respectively. The planets span the expected position of the radius valley. The bulk densities and an internal structure analysis indicate that LHS 1903 b is rocky, whereas planets c and d are gas rich. The outermost planet, LHS 1903 e, is found to be gas poor. This allowed us to discriminate between the T-DML and G-DF scenarios, which predict different compositions for this planet. CONCLUSION: The compositions of LHS 1903 b, c, and d can all be explained by both the T-DML and G-DF scenarios. LHS 1903 e is not compatible with the T-DML model, because of its lack of an extended atmosphere, but is consistent with the G-DF model. We propose that the properties of this four-planet system can be explained by an inside-out formation mechanism, in which the inner planet formed first and triggered the formation of the next planet, with the others following in sequence. This formation pathway could have occurred if there was fast migration of material within the protoplanetary disk, causing the outer disk (at the location where planet e formed) to be depleted of gas. Comparison with planet formation simulations indicated that LHS 1903 e formed in a more gas-depleted environment than did planets c and d. We conclude that LHS 1903 e formed under the G-DF scenario. Measured properties of the four planets orbiting LHS 1903 and a schematic illustration of our proposed formation scenario.: The measured radius (in Earth radii, R⊕) and equilibrium temperature of each planet are labeled along the bottom. The colored layers of each planet indicate the modeled internal structures: The core and mantle are brown, with blue representing water and gray representing the atmosphere. The layer uncertainties are shown as colored rectangles, and the total radius uncertainties are shown as white annuli. Arrows indicate our proposed inside-out formation mechanism, with red shading indicating the gas density in the protoplanetary disk. The rocky planet e formed later, in a gas-depleted environment. [ABSTRACT FROM AUTHOR]
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  Data: <i>Copyright of Science is the property of American Association for the Advancement of Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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