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The CHEOPS satellite enabled the discovery of a fourth exoplanet around the star LHS 1903, which would have formed after the others, challenging our understanding of planetary formation.
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 not only to our Solar System but also to most 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. This work, carried out by an international team of astronomers, including scientists from the University of Geneva (UNIGE), the University of Bern (UNIBE), and the National Centre of Competence in Research PlanetS, can be found in Science.
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. It was using the European Space Agency's (ESA) CHEOPS satellite – developed and built largely in Switzerland by UNIBE and UNIGE, with its operations center located in the UNIGE Department of Astronomy – that scientists made a strange discovery: a fourth planet, the most distant from LHS 1903, which appears to be rocky.
CHEOPS demonstrates here how new ultra-precise instruments can lead us to revise our understanding of the Universe.
"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 (UNIGE), CHEOPS mission scientist 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 gaseous planet. This fourth planet, LHS 1903 e, would therefore be expected to 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 UNIBE 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 UNIGE, Chairman of the Science Team of the CHEOPS mission and co-author of the study.
