HD 63433 c

HD 63433 c (TOI-1726 c) is a mini-Neptune exoplanet orbiting the Sun-like star HD 63433. It is the outermost planet in its planetary system, being located 0.1448 AU from its star, and completing one orbit every 21 days. 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. HD 63433 c is about 2.7 times larger than Earth and 15.5 times more massive, but still smaller and less massive than Neptune. 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.

Characteristics
HD 63433 c is classified as a mini-Neptune planet, a class of planets that are smaller than Neptune but still have an atmosphere of hydrogen and/or helium, just like Neptune. 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. 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. Its mass is about 15.5, being situated between the masses of Uranus and Neptune, which are 14.54 and 17.15 respectively. The density of HD 63433 c is calculated at 4.6 g/cm3, slightly lower than Earth's, but higher than the densities of Neptune and Uranus. The age of the planet is estimated at 400 million years.

It is the outermost planet in its planetary system, orbiting its star at a distance of 0.145 AU, with an orbital period of 21 days. Despite being the outermost planet, its distance is about 37% of Mercury's distance from the Sun, in addition to having a high temperature, estimated between 267 and 406 degrees Celsius.

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. It was discovered in 2020 by Mann et al., using the Transiting Exoplanet Survey Satellite (TESS), together with HD 63433 b. Both planets were discovered through the transit method.

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. 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. 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.

The same effect was not identified on the sibling planet HD 63433 b, which could indicate that it has already lost its primordial atmosphere.

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. The properties of this star, such as its radius (0.912 ) and its mass (0.99 ) are very similar to those of the Sun, which makes it classified as a Solar analog. The star is part of the Ursa Major moving group, allowing its age to be estimated at $2.692 Earth radius$ million years. There are other exoplanets orbiting this star, HD 63433 b, also a mini-Neptune, and HD 63433 d, an Earth-sized planet.

Notes and references
