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HD 63433 c
Mini-Neptune orbiting the Sun-like star HD 63433 From Wikipedia, the free encyclopedia
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HD 63433 c (TOI-1726 c) is a mini-Neptune exoplanet orbiting the Sun-like star HD 63433.[4] It is the outermost planet in its planetary system, being located 0.1448 astronomical units (21,660,000 km) from its star, and completing one orbit every 21 days.[1] Despite being the outermost planet in the system, it is still located close to its star, meaning that its temperature is hot, being estimated between 267 and 406 °C.[3] HD 63433 c is about 2.7 times larger than Earth and 15.5 times more massive,[3] but still smaller and less massive than Neptune.[note 1] In 2022, a study showed that its atmosphere, made up of hydrogen, is being evaporated by the strong radiation from its star, causing it to slowly turn into a super-Earth planet.[6][7]
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Characteristics
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HD 63433 c is classified as a mini-Neptune planet,[6] a class of planets that are smaller than Neptune but still have an atmosphere of hydrogen and/or helium, just like Neptune.[6] According to theoretical models, its composition is mainly of silicate and water, surrounded by a gaseous envelope that makes up about 2% of the planet.[3] HD 63433 c is 2.7 times larger than Earth, but still 1/3 smaller than the Solar System's ice giants, Neptune and Uranus.[3][note 2] Its mass is about 15.5 ME, being situated between the masses of Uranus and Neptune, which are 14.54 and 17.15 ME respectively.[5][8] The density of HD 63433 c is calculated at 4.6 g/cm3,[3] slightly lower than Earth's,[note 3] but higher than the densities of Neptune and Uranus.[note 4] The age of the planet is estimated at 400 million years.[4]
It is the outermost planet in its planetary system, orbiting its star at a distance of 0.145 astronomical units (21,700,000 km), with an orbital period of 21 days.[1] Despite being the outermost planet, its distance is about 37% of Mercury's distance from the Sun,[c] in addition to having a high temperature, estimated between 267 and 406 degrees Celsius.[3][d]
The discovery of HD 63433 c, as well as all planets orbiting HD 63433, is part of a project called TESS Hunt for Young and Maturing Exoplanets (THYME), whose objective is to discover transiting exoplanets in stellar associations, moving groups and open clusters.[4][1] It was discovered in 2020 by Mann et al., using the Transiting Exoplanet Survey Satellite (TESS), together with HD 63433 b.[11] Both planets were discovered through the transit method.[11]
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Evaporating atmosphere
In 2022, a study led by Michael Zhang observed two mini-Neptunes from distinct planetary systems: HD 63433 c, using the Hubble Space Telescope, and TOI-560.01, using the W. M. Keck Observatory.[7] This study revealed that the atmospheres of the two planets – made up mainly of hydrogen (HD 63433 c) and helium (TOI-560.01) – are evaporating and transforming them into super-Earths, as the strong radiation of their host stars is slowly stripping away their puffy atmospheres, causing them to escape into space, like a hot air balloon.[7][2] In HD 63433 c in particular, these gases are escaping at a speed of 50 km/s, and forming a gas cocoon 12 times larger than the planet itself.[7]
The same effect was not identified on the sibling planet HD 63433 b, which could indicate that it has already lost its primordial atmosphere.[7]
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Host star
The host star of HD 63433 c is HD 63433, a G-type main-sequence star that is located 73 light-years from Earth in the constellation Gemini.[4][note 5] The properties of this star, such as its radius (0.912 R☉) and its mass (0.99 M☉) are very similar to those of the Sun, which makes it classified as a Solar analog.[4] The star is part of the Ursa Major moving group, allowing its age to be estimated at 414±23 million years.[4] There are other exoplanets orbiting this star, HD 63433 b, also a mini-Neptune, and HD 63433 d, an Earth-sized planet.[1]
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Notes and references
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