HARPS-N helps to discover and characterize an "inside out" planetary system around the star LHS 1903.

Thanks to 108 precise radial-velocity measurements obtained with the high-resolution spectrograph HARPS-N, installed at the Telescopio Nazionale Galileo (between 2 October 2020 and 19 February 2023), together with data from ESA's CHEOPS satellite (Characterising ExOPlanet Satellite), astronomers were able to characterize the unusual planetary system known as LHS 1903.

Within the framework of the HARPS-N Collaboration GTO and OPTICON programs, an international team of scientists led by Dr. Thomas Wilson (University of Warwick) carried out an in-depth study of the star LHS 1903. Their results, published in Science, reveal a surprising architecture: the most distant outer planet appears to be rocky and may have formed later, in a different environment from the other planets in the system.

In our Solar System, planets are broadly divided into two types: rocky and gaseous. The inner planets closest to the Sun are rocky, while the outer planets are gas giants. This “rock-then-gas” pattern is commonly observed across the Milky Way.

However, the present study shows that the cool, faint red dwarf star LHS 1903 hosts a planetary system that first features a close-in rocky planet, followed by two outer gas giants, consistent with current models of planetary formation, and then, surprisingly, ends with a rocky planet at the outer edge of the system.

According to current theories, planets form within protoplanetary discs, where material clumps together into planetary embryos at roughly the same time. Over millions of years, these embryos grow into planets of different sizes and compositions. Traditional models predict that rocky planets form close to the star, where stellar radiation strips away gaseous envelopes, leaving dense solid cores behind. Gas giants, instead, form farther out in cooler regions where gas can accumulate and be retained.

“This strange disorder makes it a unique inside-out system,” says Dr. Thomas Wilson. “Rocky planets do not usually form far from their host star, beyond gaseous worlds.”

This study also suggests that the star may have formed its four planets sequentially rather than simultaneously. The distant rocky world orbiting LHS 1903 appears to have either lost its gaseous atmosphere or never acquired one.

The idea, known as inside-out planet formation, was proposed by scientists about a decade ago, when HARPS-N reveled, for the first time, the same pattern, in the planetary system named Kepler 101.

“The role of HARPS-N has been crucial”, said A. Ghedina director of the Telescopio Nazionale Galileo, "enabling the detection and precise measurement of the planet's masses. We have to point out that the first part of these measurments were possible thanks to the dedication of the TNG staff who carried out the observations at a time when travel was banned across most of the world due to COVID-19 pandemic restrictions".

"It is fantastic to see how the TNG (that this year celebrates its 30th anniversary) is a worldwide reference in exoplanetary science thanks to the high precision measurements in Radial Velocities of HARPS-N.” concludes Ghedina.

Link to the paper.