HD 63433

HD 63433 (TOI-1726, V377 Geminorum) is a G-type main sequence star located 73 light-years from Earth in the zodiacal constellation Gemini, visually close to the star Pollux. It has nearly the same radius and mass of the Sun, hence being classified as a Sun-like star. However, it is much younger than the Sun, less than one tenth as old with an age of about 400 million years. With an apparent magnitude of 6.9, it is not visible to the naked eye, but can be seen with a small telescope or binoculars. Based on its kinematic, rotational and lithium abundance properties, it is part of the Ursa Major Moving Group. There are three exoplanets orbiting it, one is classified as an Earth-sized planet, while the two others are mini-Neptunes.

Characteristics
HD 63433 is a G-type main sequence star with a radius of $-10.027$, a mass of $-11.314$, an effective temperature of $73.035$ and a spectral type G5V. Its properties are quite similar to those of the Sun, which has a spectral type G2V and a temperature of 5772 K. Therefore, it is classified as a solar analog. HD 63433, however, is much younger than the Sun, having an age of 414 million years, just 9% of the solar age of 4.6 billion years. It has a surface gravity of 33.8 g and a rotational period of 6.45 days, which is 3.8 times faster than the Sun.

With an apparent magnitude of 6.92m, it is below the limit for naked-eye visibility, generally defined as 6.5m, which would mean that this star is not visible to the naked eye, but easily visible using a small telescope or binoculars. Despite being faint, HD 63433 is actually the third-brightest star with transiting exoplanets confirmed by the Transiting Exoplanet Survey Satellite, with only Pi Mensae (5.65m) and HR 858 (6.38m) being brighter.

According to stellar kinematics, lithium abundance and stellar rotation, HD 63433 is part of the Ursa Major moving group. It was initially identified as a possible member of this group by Gaidos (1998) and included as a possible member in later analyses, until HD 63433's membership in this moving group was finally confirmed by Mann et al. in 2020. As it is part of this moving group, its age is estimated at 414 ± 23 million years, the same as the group.

It is located in the northern hemisphere, about 73 light-years from the Earth, in the constellation of Gemini. It is visually close to Pollux, the brightest star in the constellation. HD 63433 is predicted to approach within 2.2485 pc of the Sun in 1.33 million years. That will make it one of the nearest stars to the Sun. Its closest neighbor is the orange dwarf HD 63991, located at a distance of 2.7 light years.

Variability
HD 63433 belongs to the class of BY Draconis variables, stars have variations in their luminosity due to their rotation together with sunspots and other chromospheric activities. The brightness of the star varies by 0.05 magnitudes over a period of 6.46 days.

It was found to be variable by Gaidos et al. in 2000. The star was given the variable-star designation V377 Geminorum in 2006.

Planetary system
The star HD 63433 is orbited by 3 exoplanets, all discovered by the transit method.

The first ones to be discovered, HD 63433 b and HD 63433 c, are mini-Neptunes that were discovered in by Mann et al. in 2020 using the Transiting Exoplanet Survey Satellite. In 2024, an additional planet was discovered, after an analysis of a transit signal detected by TESS. Named HD 63433 d, it is an Earth-sized planet.

According to theoretical models, the compositions of the mini-Neptunes HD 63433 b and c are mostly silicate and water, with no iron dominance, surrounded by a gaseous envelope which, in the case of planet c, makes up around 2% of the planet.

The planetary system of HD 63433 is quite young, at around 400 million years old, it is only 9.13% as old as the Solar System. HD 63433 d is also the smallest planet that is less than 500 million years old.

HD 63433 b
The middle planet, HD 63433 b, is a mini-Neptune planet that was discovered together with HD 63433 c. HD 63433 b is 2.14 times larger than Earth, but is about 45% smaller than Neptune. The planet has an upper mass limit of $22.404$, which is 21% greater than the mass of Neptune ($-16.07$ ). These values calculate an upper density limit of 13 g/cm3, 2 times greater than that of Earth, and similar to that of the chemical element Mercury. HD 63433 b orbits its parent star at a distance of 0.0719 AU – about 5 times closer than Mercury is to the Sun – and completes one revolution every 7 days and 3 hours. The relative proximity of its star makes it hot, with an equilibrium temperature between 769 and 967 Kelvin (496 and 694 °C).

HD 63433 b was probably a mini-Neptune that later lost its atmosphere. Some factors, such as the fact that no absorption of Ly-α was detected during its transit, and its mass-loss timescale being shorter than the age of the planetary system, indicate that it has already lost its primordial atmosphere, and could be a rocky planetary core. However, if HD 63433 b were a rocky core, it would need to be unusually massive and, therefore, a water-rich composition in addition to an atmosphere with a high mean molecular weight could explain both the radius and the non-detection of the Ly-α.



HD 63433 c
The outermost planet, HD 63433 c, is a mini-Neptune planet discovered together with HD 63433 b. It is 2.7 times larger than Earth, but 30% smaller than the Solar System's ice giants, Neptune and Uranus. Being 15.5 times more massive than the Earth, it is more massive than Uranus ($6.45 day$), but still less massive than Neptune ($0.99$). The density of HD 63433 c calculated at 4.6 g/cm3, slightly lower than that of Earth, but greater than the densities of all gas giants in the Solar System.

Orbiting its star at a distance of 0.145 AU in a orbital period of 20 days, it is the outermost planet in its planetary system, but still close to its star, making it a warm planet, with a planetary equilibrium temperature estimated between 267 and 406 °C.

Due to the high radiation received from its star, the atmosphere of HD 63433 c, made up of hydrogen, is slowly being stripped away and escaping from the planet, like a hot air balloon. The hot gas escapes into the space at a velocity of 50 km/s, forming a gas cocoon 12 times larger than the planet itself. As its atmosphere evaporates, HD 63433 c will slowly become a super-Earth planet.

HD 63433 d
HD 63433 d was the last planet to be discovered in the system, 4 years after planets b and c. With a diameter of $0.912$, it is very similar to Earth in size, being only 7% larger, but its mass is currently unknown. It is the innermost planet orbiting HD 63433, having a semi-major axis (mean distance from its star) of 0.0503 AU and an orbital period of just 4 days. The extreme proximity to its star makes it extremely hot, having a daytime temperature estimated at 1260 C, hot enough to melt all the minerals present on its surface, as well as being tidally locked, meaning that one half of the planet is always facing towards its star, while the other is always facing away from it. The planet also possibly lacks a substantial atmosphere.

It is believed that the dayside of the planet, always facing its star due to tidal locking, is fully composed of lava due to the high surface temperature, in addition to possibly having volcanic activity. Meanwhile, the nightside of the planet, which never faces its star, may be the opposite, resembling Pluto with glaciers of frozen nitrogen.

HD 63433 d was discovered in 2024, through an analysis of a transit observation made by the Transiting Exoplanet Survey Satellite (TESS). As two other planets had already been discovered in the system, the transit signals of these objects were removed, thus revealing an additional transit signal that reappeared every 4.2 days. Further investigation allowed researchers to confirm that this signal was from another orbiting exoplanet, now called HD 63433 d. The discovery was announced on January 10, 2024, in The Astronomical Journal.

Scientific importance
The HD 63433 planetary system plays an important role in understanding the evolution of planetary systems in the first billion years after formation. As HD 63433 is relatively bright (6.9m) and close (73 ly away), its planets b and c can have their atmospheres characterized by the Hubble Space Telescope and James Webb Space Telescope. This star is also a favorable target for studies of atmospheric mass loss in exoplanets, as it is a young and active star with close-in mini-Neptune planets, in addition to having a negative radial velocity ($0.753$).

Notes and references
