Astronomers make a rare find: a small, cold starExoplanets are accompanied by a much larger outer companion, providing new insights into how Earth-like planets form. The discoveries include a planet with a radius and mass between Earth andNeptune's potential orbit around its host star is 146 days. The star system also contains a large outer companion star with a mass of100 times that of Jupiter.

The space-based TESS satellite uses the transit method to measure tiny droplets of starlight as exoplanets pass in front of them. The TESS satellite determined the characteristics of sub-Neptunian HD88986b by precisely measuring the size of the planet. Source: NASA Goddard Space Flight Center

This is a rare discovery because exoplanets smaller and lighter than Neptune and Uranus are difficult to detect, and only a few have been discovered so far. Such rare systems are particularly interesting for better understanding planet formation and evolution; they are considered a key step toward detecting Earth-like planets around stars.

The new planetary system was discovered around the star HD88986. The star has a temperature similar to that of the Sun, has a slightly larger radius, and is bright enough to be seen by keen observers at black-sky observing sites across the UK, such as Bannau Brycheiniog National Park (Brecon Beacons).

The research, published in the journal Astronomy & Astrophysics, was led by Neda Heidari, an Iranian postdoctoral fellow at the Institute of Astrophysics in Paris (IAP). In the UK, Thomas Wilson, a senior researcher at the University of Warwick, co-led the analysis of satellite data, including the search for new planets. The team also includes researchers from 29 other institutes in nine countries, including Switzerland, Chile and the United States.

A cold Neptune-like exoplanet

The system includes a cold planet smaller than Neptune, a so-called sub-Neptune, HD88986b. The planet has the longest orbital period of any known exoplanet (146 days), smaller than Neptune or Uranus.

Neda Heidari of the IAP explains: "Most of the planets we have discovered and measured their masses and radii have short orbits, usually less than 40 days. Compared to the Solar System, even Mercury, the closest planet to the Sun, takes 88 days to complete its orbit. The lack of detection of planets with longer orbits poses a challenge to understanding how planets in other galaxies and even our own solar system form and evolve. With an orbital period of 146 days, HD88986b may be the longest and precisely measured asteroid with a known orbit."

HD88986b was detected using the high-precision spectrometer SOPHIE (a machine that analyzes the wavelengths of light from exoplanets) at the Haute-Provence Observatory in France. SOPHIE uses the "radial velocity method" to detect exoplanets and determine their characteristics; measuring small changes in the star's motion caused by planets orbiting the star.

These observations revealed the planet's existence, allowing the team to estimate its mass to be about 17 times that of Earth.

Supplementary observations from NASA's Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency's (ESA) Characterizing Exoplanet Satellite (CHEOPS) suggest that the planet may "transit" in front of its host star. As its orbit passes between the line of sight of Earth and the star, it partially obscures the star - causing a diminution in the star's brightness that can be observed and quantified.

Through observations from these two satellites, the research team directly estimated that the diameter of the planet is about twice the diameter of the Earth. The study's findings relied on more than 25 years of observational data, which also included data from ESA's Gaia satellite and the Keck telescope in Hawaii.

In addition, HD88986b's atmospheric temperature is only 190 degrees Celsius, which provides a rare opportunity to study the composition of so-called "cold" atmospheres, since most detected exoplanet atmospheres are above 1,000 degrees Celsius.

Due to the wide orbit of sub-Neptunian HD88986b (up to 60% of the Earth-Sun distance), HD88986b is likely to have had rare interactions with other planets that may exist in this planetary system, and has experienced weak mass loss under the strong ultraviolet radiation of the central star. Therefore, it may have retained its original chemical composition, allowing scientists to explore the possible formation and evolution of this planetary system.

Thomas Wilson, from the Department of Physics at the University of Warwick, said: "HD88986b is essentially a smaller version of Neptune, located between the orbits of Mercury and Venus. It is one of the best-studied small cold exoplanets and provides a great opportunity for research. Studying its atmosphere paves the way to understanding its similarities to our own. It also orbits a star with a temperature similar to that of the Sun, making it a precursor to the Earth-like planets to be discovered by the PLATO space telescope, in which the University of Warwick plays a leading role."

second external companion

Astronomers also discovered a second exoplanet orbiting the central star. This exoplanet is extremely massive (more than 100 times the mass of Jupiter) and has an orbital period of several decades. Further observations are needed to understand its nature and better determine its properties.

Thomas-Wilson added: "We have collected over 25 years of data from telescopes pointed at HD 88986, making it one of the longest-studied exoplanetary systems. This wealth of data reveals a second exoplanet companion more massive than Jupiter, which may have been important in the formation of this Neptune-like planet, like Jupiter in our own solar system."

Compiled source: ScitechDaily