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An eight-hour Year
German Aerospace Center Press Release
Ultra-light and super-fast
As far as extrasolar planets go, ‘GJ 367 b’ is a featherweight. With half the mass of Earth, the newly discovered planet is one of the lightest among the nearly 5000 exoplanets known today. It takes the extrasolar planet approximately eight hours to orbit its parent star. With a diameter of just over 9000 kilometres, GJ 367 b is slightly larger than Mars. The planetary system is located just under 31 light years from Earth and is thus ideal for further investigation. The discovery demonstrates that it is possible to precisely determine the properties of even the smallest, least massive exoplanets. Such studies provide a key to understanding how terrestrial planets form and evolve.
An international group of 78 researchers led by Kristine W. F. Lam and Szilárd Csizmadia from the Institute of Planetary Research at DLR (German Aerospace Center) report on the results of their studies in the scientific journal Science. With an orbital period of only one-third of an Earth day, GJ 367 b is a fast mover. “From the precise determination of its radius and mass, GJ 367b is classified as a rocky planet,” reports Kristine Lam. “It seems to have similarities to Mercury. This places it among the sub-Earth sized terrestrial planets and brings research one step forward in the search for a ‘second Earth’.”
More precise exoplanet trackers possible
A quarter of a century after the first discovery of an extrasolar planet, the focus has shifted to characterising these planets more precisely, in addition to making new discoveries. At present, it is possible to construct a much more precise profile for most known exoplanets. Many exoplanets were discovered using the transit method – the measurement of minute differences in the emitted light, or its apparent magnitude, of a star as a planet passes in front of it (with respect to the observer). GJ 367 b was also discovered using this method, with the help of NASA’s Transiting Exoplanet Survey Satellite (TESS).
Origin of the small fast-moving planets still unknown
GJ 367 b belongs to the ‘ultra-short period’ (USP) group of exoplanets that orbit their star in less than 24 hours. “We already know a few of these, but their origins are currently unknown,” says Kristine Lam. “By measuring the precise fundamental properties of the USP planet, we can get a glimpse of the system’s formation and evolution history.” Following the discovery of this planet using TESS and the transit method, the spectrum of its star was then studied from the ground using the radial velocity method. The mass was determined using the HARPS instrument on the European Southern Observatory’s 3.6m telescope. With the meticulous study and combination of different evaluation methods, the radius and mass of the planet were precisely determined: its radius is 72 percent of Earth’s radius, and its mass 55 percent of Earth’s mass.
Highest precision for radius and mass
By determining its radius and mass with precisions of 7 and 14 percent respectively, the researchers were also able to draw conclusions about the exoplanet’s inner structure. It is a low-mass rocky planet, but has a higher density than the Earth. “The high density indicates the planet is dominated by an iron core,” explains Szilárd Csizmadia. “These properties are similar to those of Mercury, with its disproportionately large iron and nickel core that differentiates it from other terrestrial bodies in the Solar System.” However, the planet’s proximity to its star means it is exposed to an extreme high level of radiation, more than 500 times stronger than what the Earth experiences. The surface temperature could reach up to 1500 degrees Celsius – a temperature at which all rocks and metals would be melted. GJ 367 b therefore cannot be considered a ‘second Earth’.
Parent star is a ‘red dwarf’
The parent star of this newly discovered exoplanet, a red dwarf called GJ 367, is only about half the size of the Sun. This was beneficial for its discovery as the transit signal of the orbiting planet is particularly significant. Red dwarfs are not only smaller, but also cooler than the Sun. This makes their associated planets easier to find and characterise. They are among the most common stellar objects in our cosmic neighbourhood and are therefore suitable targets for exoplanet research. Researchers estimate that these red dwarfs, also known as ‘class M stars’, are orbited by an average of two to three planets.
Planet GJ 367 as an artistic representation
The exoplanet GJ 367b orbits its star in an extremely short time. Only 8 hours, then a year has passed on this planet. We don’t know anything like this from our solar system: Mercury is the fastest planet here with an orbital period of 88 days, compared to GJ 367b it is a lame snail.
GJ 367b is a rocky planet that is much denser than Earth and similar in structure to Mercury. It probably has a large iron core. This precise characterisation is based on high-precision measurements of radius and mass – not a matter of course for exoplanets.GJ 367b orbits a dwarf star that is about half the size of the Sun. The radiation on the planet is enormous because of the small distance to the star: On the side of the planet facing the star, the temperature is between 1300° and 1500° Celsius. At such temperatures, iron and rocks melt.
Mass and radius of known small planets
The diagram shows the ratio of mass (horizontal scale) and radius (vertical scale) of exoplanets which are smaller then two Earth radii and have a mass and radius precisions better than 30%. The radius and mass of the planets are given as multiples or fractions of the Earth’s radius and mass.
Circle symbols show planets whose mass was determined using the radial velocity method. Triangles show planets whose mass was determined using transit time variations. The crosses at the data points indicate the measurement precision. GJ367 is one of the smallest planets and its mass was determined with the radial velocity with the highest precision.
For comparison, the terrestrial planets of the solar system, Mercury, Venus, Earth and Mars, are plotted. The colours of the data points indicate the equilibrium temperatures of the day side illuminated by the star.
Theoretical mass-radius relationships can be used to infer the interior of the planet. For orientation, five types of planetary cores are drawn: a pure iron core (100% Fe), cores with decreasing iron content (80% and 50%), pure rocky planets (100% MgSiO3) and a planetary core consisting of 50% water.
The newly discovered exoplanet GJ 367b is probably an iron-dominated planet.
Reference
Kristine W. F. Lam, Szilárd Csizmadia, Nicola Astudillo-Defru, Xavier Bonfils et al. GJ 367b: A dense, ultrashort-period sub-Earth planet transiting a nearby red dwarf star Science 2 Dec 2021 Vol 374, Issue 6572 pp. 1271-1275
DOI: 10.1126/science.aay3253
ArXiv
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This article was first published by German Aerospace Center
Updated on 20 December 2021