An international team led by a young researcher from the Institut d’astrophysique de Paris announces the discovery of a particularly interesting planetary system around a star other than Sun. It includes a planet with radius and mass between those of the Earth and Neptune, possibly transiting in front of its bright host star each 146 days, as well as an outer, more massive companion. Such rare systems are particularly interesting to better understand planetary formation and evolution.

Since the first discovery in 1995 of a planet around a star other than our Sun, more than 5000 of those exoplanets have been detected. Exoplanets smaller and lighter than Neptune or Uranus orbiting far from their host stars are particularly difficult to detect, and only few are known today. Yet, they are interesting for the path toward detection of Earth-like planets around stars.

In that context, a collaboration of worldwide researchers announces the new planetary system discovered around the star HD88986. The team includes researchers at the Institut d'astrophysique de Paris (IAP) and 30 other institutes from nine countries. It is led by Neda Heidari, an Iranian postdoctoral fellow at the IAP. She started that analysis during her PhD at the Laboratoire d'astrophysique de Marseille, the Observatoire de la Côte d’Azur, and the Shahid Beheshti University in Tehran.

First, the planetary system includes a cold sub-Neptune, HD88986b. This planet has the longest orbital period (146 days) among known exoplanets smaller than Neptune or Uranus with precise mass measurements. It was detected using the SOPHIE high-precision spectrograph at the Haute-Provence Observatory, France. SOPHIE does detect and characterize exoplanets using the so-called radial-velocity method. It consists in measuring tiny motion variations of the star induced by planets orbiting it. Those SOPHIE observations revealed the planet and allowed the team to estimate its mass to approximately 17 times that of the Earth. Complementary observations obtained with NASA’s TESS (Transiting Exoplanet Survey Satellite, NASA) and ESA’s space telescope CHEOPS (CHaracterising ExOPlanet Satellite, ESA) indicate that the planet probably “transits” in front of it host star: this occurs when its orbit passes on the line of sight between the Earth and the star, partially occulting the star hence causing a decrease in its brightness that can be observed and quantified. These observations by both satellites allowed the team to directly estimate the diameter of the planet as about twice that of the Earth. Moreover, with an atmosphere temperature of 190 Celsius degrees only, HD88986b provides a rare opportunity for studying the composition of the so-called “cold” atmospheres, as most of the detected atmospheres for exoplanets are above 1000 Celsius degrees.

Figure 1: The T193 Telescope at Haute-Provence Observatory

Figure 1: The T193 Telescope at Haute-Provence Observatory. That facility was used for the discovery and the characterization of the cold sub-Neptune HD88986b and its massive, outer companion HD88986c.
Credits: S. Ilovaisky /OSU Pytheas, OHP, CNRS, AMU.
Figure 2: The SOPHIE spectrograph

Figure 2: The SOPHIE spectrograph. It is mounted at T193 Telescope at Haute-Provence Observatory and is regulated in temperature and pressure. Its particularly high stability and sensitivity allowed small motions of the star HD88986 to be detected overs weeks and years, revealing its hosted planetary system.
Credits: OSU Pytheas, CNRS, AMU.

Figure 3: The TESS satellite

Figure 3: The TESS satellite.That space observatory uses the transit method, that is measuring the small drops of a stellar light when its hosted exoplanet does transit in front of it. That method provided a more comprehensive study of the sub-Neptune HD88986b, in particular allowing its diameter to be measured.
Credits: NASA’s Goddard Space Flight Center.

The whole analysis uses more than 25 years of observations, and also includes data from the Gaia satellite Gaia and the Keck Telescope in Hawaii. It also revealed a second, outer companion around the central star. That companion is particularly massive (more than 100 times the mass of Jupiter) and its orbit has a period of several tens of years. Further observations are needed to understand its nature and better determine its properties.

One important aspect of this new planetary system is the wide orbit of the sub-Neptune HD88986b (as large as 60% of the Earth-Sun distance). As a result, HD88986b probably underwent rare interactions with other planets that may exist in the planetary system, and weak mass loss from the strong ultraviolet radiation of the central star. It may therefore have retained its original chemical composition, allowing scientists to explore the possible scenarios for the formation and evolution of this planetary system, which must take into account the presence of the massive companion planet.


puce The scientific article published today in the journal Astronomy & Astrophysics by Heidari, Boisse, Hara, Wilson, Kiefer, Hébrard, Philipot et al. (2024), « The SOPHIE search for northern extrasolar planets. XIX. A system including a cold sub-Neptune potentially transiting a V = 6.5 star HD8898 »

puce The SOPHIE spectrograph OHP

puce Press Release from CNRS-INSU

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January 2024

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