A lot of exoplanets are found using a technique understood as the transit method, where the exoplanet passes in front of its star, causing the star to dim slightly. It takes numerous transits to confirm an exoplanet, so its not unexpected that the majority of known exoplanets have a fairly brief orbital period. Months or days rather than years. Theres also an observational predisposition because most known stars are red overshadows, so its usually not unexpected that weve discovered yet another exoplanet closely orbiting a red dwarf star. However often what we discover is so severe, it actually is unexpected.
Just recently astronomers found a world orbiting a red dwarf referred to as GJ 367. This particular star has to do with 31 light-years from Earth and isnt particularly unique, however its planet orbits the star at a range of about one-sixtieth that of the orbital radius of Mercury. The world is so near GJ 367 that it makes a total orbit every 8 hours.
One of the downsides of the transit approach is that it can only inform you the size of an exoplanet relative to its star. Based on the transit data we have, we understand the world is approximately the size of Mars.
Based on the temperature of the star and the planets orbital distance, the group approximates that its surface temperature level might be about 1,500 degrees Celsius. One idea about close-orbiting planets like this is that most of the environment boils away, leaving a rocky iron core.
The size and mass of GJ 367b is consistent with an iron planet. Credit: Lam, et al (2021 )
To check this concept, the group utilized information from the HARPS instrument on the European Southern Observatorys 3.6 m telescope. HARPS can measure the relative motion of GJ 367. Part of that movement is the wobble triggered by the gravitational pull of its planet. The bigger the wobble, the more huge the world. When the team analyzed the information they found the planet has a mass about 5 times that of Mars.
This is surprisingly massive and means the world has a density just under that of pure iron. Based on planetary designs, the world is likely a thin crust covering a big iron core, which includes weight to the idea that it may have as soon as been a gassy super-Earth that has been removed of its environment and outer crust.
We need to beware not to presume too much about this world simply understanding its size and mass. Astronomers have actually only found a handful of these ultra-short period worlds, so we still have much to learn. It is clear that the universe is filled with some very odd worlds.
Reference: Lam, Kristine WF, et al. “GJ 367b: A thick, ultrashort-period sub-Earth planet transiting a neighboring red dwarf star.” Science 374.6572 (2021 ): 1271-1275.
Referral: Armstrong, David J., et al. “A remnant planetary core in the hot-Neptune desert.” Nature 583.7814 (2020 ): 39-42.
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The majority of exoplanets are found using a strategy understood as the transit technique, where the exoplanet passes in front of its star, triggering the star to dim a little. It takes numerous transits to validate an exoplanet, so its not surprising that the majority of understood exoplanets have a fairly short orbital duration. Theres also an observational predisposition in that most known stars are red dwarfs, so its generally not surprising that weve discovered yet another exoplanet carefully orbiting a red dwarf star. One of the disadvantages of the transit approach is that it can just tell you the size of an exoplanet relative to its star. Based on the temperature level of the star and the worlds orbital distance, the group approximates that its surface area temperature level could be about 1,500 degrees Celsius.
