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| Artist impression of the planetary system around the star LHS 1903 Image Credit: © ESA |
Scientific Frontline: "At a Glance" Summary
- Main Discovery: Identification of LHS 1903 e, a rocky planet located beyond gas giants in the LHS 1903 system, contradicting the standard inner-rocky/outer-gas planetary hierarchy.
- Methodology: Utilized high-precision photometry from the ESA CHEOPS satellite to detect the planet, followed by planetary formation simulations to confirm an "inside-out" formation sequence and exclude migration or collision hypotheses.
- Key Data: Located 116 light-years from Earth around an M-type red dwarf; the fourth planet shares a similar mass with the inner third planet (a gas giant) yet possesses a rocky composition.
- Significance: Provides observational evidence for the inside-out planet formation theory, indicating that planets can form sequentially after the dissipation of protoplanetary disk gas rather than simultaneously.
- Future Application: Refinement of planetary accretion simulations to incorporate asynchronous formation timelines and better characterization of atypical planetary system architectures.
- Branch of Science: Astrophysics and Exoplanetology
- Additional Detail: Analysis indicates LHS 1903 e formed significantly later than its gas giant siblings, occurring only after the protoplanetary disk had been depleted of gas.
First the rocky planets, very close to their star, then the gas giants: this is the order in which scientists have hitherto conceived of the hierarchy of planets. A conception that corresponds to our Solar System but also to most of the planetary systems identified to date. However, the recent discovery of a new planet around the star LHS 1903 by the CHEOPS space telescope is now challenging this theory. The international team of astronomers behind the discovery includes scientists from the University of Bern and the University of Geneva who are also members of the National Centre of Competence in Research PlanetS.
The eight planets of our Solar System are classified into two types: rocky and gaseous. The inner planets closest to the Sun - Mercury to Mars - are rocky, while the outer planets - Jupiter to Neptune
- are gaseous. This configuration is commonly observed in our Galaxy. However, the discovery of a planetary system around a star called LHS 1903, located 116 light-years from Earth, has overturned this understanding.
LHS 1903 is a small red dwarf star of type M, cooler and less luminous than our Sun. Three planets - respecting the established order - had initially been detected around the star. Using the European Space Agency's (ESA) CHEOPS satellite scientists including researchers from the University of Bern and the University of Geneva who are also members of the NCCR PlanetS have now made a strange discovery: a fourth planet, the most distant from LHS 1903, which appears to be rocky. The findings have just been published in Science.
The precision of CHEOPS reveals a rule‑breaking planet
CHEOPS is a joint mission by the European Space Agency ESA and Switzerland, under the leadership of the University of Bern in collaboration with the University of Geneva, where its operations center is located within the Department of Astronomy. "It is thanks to the precision of CHEOPS that we were able to detect this new planet," says Monika Lendl, associate professor in the Department of Astronomy at the Faculty of Science of the University of Geneva, mission scientist of the CHEOPS mission and co-author of the study. "Since rocky planets do not usually form beyond gas giants, this one completely overturns our theories!"
Near the star, the very high temperature in the protoplanetary disk - where planets form - prevents the formation of a gaseous envelope around the rocky cores of the planets. Conversely, far from the star, the temperature is low enough for a thick atmosphere to accumulate, persist, and form a permanent gas planet. This fourth planet, LHS 1903 e, should therefore be a gas planet.
A late bloomer defying expectations
Before questioning the established model, the team of scientists ruled out several hypotheses: was the planet, for example, struck at some point in its history by a giant asteroid, a comet, or another large object that would have swept away its atmosphere? Did the planets around LHS 1903 change position at some point during their evolution?
The accepted explanation is even more intriguing: the planets formed one after the other, rather than simultaneously as current theories suggest. This idea, known as inside-out planet formation, was proposed by scientists about ten years ago but had never been proven.
"Based on the planetary formation simulations we've been developing at the University of Bern for several decades, we were able to show that LHS 1903 e must have formed much later than its two gas giant siblings," explains Yann Alibert, professor at the Space Research and Planetary Sciences Division (WP) at the University of Bern and co-author of the study. "Indeed, the fourth planet - with a mass equivalent to that of the third, which contains a massive envelope of gas - should have accumulated and retained a large amount of gas. Our hypothesis is therefore that it formed after the gas disappeared from the protoplanetary disk, and thus after the second and third planets of the system, which are gas giants."
Diversity of planetary systems
As instruments improve, scientists continue to discover more and more "strange" planetary systems that force them to question established theories about planet formation. Ultimately, these discoveries also help us understand how the solar system fits into the diverse family of planetary systems.
"CHEOPS demonstrates here how new ultra-precise instruments can lead us to revise our understanding of the Universe. The diversity of planetary systems confirms that our Solar System does not appear to be a universal model," concludes David Ehrenreich, associate professor in the Department of Astronomy at the Faculty of Science of the University of Geneva, Chairman of the Science Team of the CHEOPS mission and co-author of the study.
Published in journal: Science
Title: Gas-depleted planet formation occurred in the four-planet system around the red dwarf LHS 1903
Authors: 166 listed on paper
Source/Credit: University of Bern
Reference Number: asph021226_01