HD_63433_b

HD 63433

HD 63433

Sun-like star in the constellation of Gemini

HD 63433 (TOI-1726, V377 Geminorum) is a G-type main sequence star located 73 light-years from Earth in the zodiacal constellation Gemini,[9][note 1] visually close to the star Pollux.[7] It has nearly the same radius and mass of the Sun, hence being classified as a Sun-like star.[9] However, it is much younger than the Sun, less than one tenth as old with an age of about 400 million years.[9][lower-alpha 1] With an apparent magnitude of 6.9, it is not visible to the naked eye, but can be seen with a small telescope[12] or binoculars.[13] Based on its kinematic, rotational and lithium abundance properties, it is part of the Ursa Major Moving Group.[9] There are three exoplanets orbiting it, one is classified as an Earth-sized planet, while the two others are mini-Neptunes.[4]

Quick Facts Observation data Epoch J2000 Equinox J2000, Constellation ...

Characteristics

HD 63433 is a G-type main sequence star with a radius of 0.912 R☉, a mass of 0.99 M☉, an effective temperature of 5640 K and a spectral type G5V.[9] Its properties are quite similar to those of the Sun, which has a spectral type G2V and a temperature of 5772 K.[14] Therefore, it is classified as a solar analog.[9] HD 63433, however, is much younger than the Sun, having an age of 414 million years,[9] just 9% of the solar age of 4.6 billion years.[11] It has a surface gravity of 33.8 g[lower-alpha 2] and a rotational period of 6.45 days,[9] which is 3.8 times faster than the Sun.

With an apparent magnitude of 6.92m,[3] it is below the limit for naked-eye visibility, generally defined as 6.5m,[15] which would mean that this star is not visible to the naked eye, but easily visible using a small telescope[12] or binoculars.[16][13] 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)[17] and HR 858 (6.38m)[18] being brighter.[9][19]

According to stellar kinematics, lithium abundance and stellar rotation, HD 63433 is part of the Ursa Major moving group.[9] It was initially identified as a possible member of this group by Gaidos (1998)[20] 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.[9] As it is part of this moving group, its age is estimated at 414 ± 23 million years, the same as the group.[9]

It is located in the northern hemisphere, about 73 light-years from the Earth, in the constellation of Gemini.[12] It is visually close to Pollux, the brightest star in the constellation.[7] HD 63433 is predicted to approach within 7.33 light-years of the Sun in 1.33 million years.[21] 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.[12]

More information Name, Distance (light-years) ...

Variability

HD 63433 belongs to the class of BY Draconis variables,[5][6] 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.[6]

It was found to be variable by Gaidos et al. in 2000.[22] The star was given the variable-star designation V377 Geminorum in 2006.[23]

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[24] that were discovered in by Mann et al. in 2020 using the Transiting Exoplanet Survey Satellite.[25] 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.[26]

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.[24]

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.[4][26][lower-alpha 3] HD 63433 d is also the smallest planet that is less than 500 million years old.

Temperature
comparisons		 HD 63433 b[24] HD 63433 c[24] HD 63433 d[4]
Global equilibrium temperature 769 – 967 K
496 – 694 °C
925 – 1281 °F		 540 – 679 K
267 – 406 °C
513 – 763 °F		 1040 K
767 °C
1413 °F
More information Companion (in order from star), Mass ...

HD 63433 b

The middle planet, HD 63433 b, is a mini-Neptune planet that was discovered together with HD 63433 c.[4] HD 63433 b is 2.14 times larger than Earth,[24] but is about 45% smaller than Neptune.[lower-alpha 4] The planet has an upper mass limit of 21.7 M🜨,[24] which is 21% greater than the mass of Neptune (17.15 M🜨 [27]). These values calculate an upper density limit of 13 g/cm3,[24] 2 times greater than that of Earth,[note 2] and similar to that of the chemical element Mercury. HD 63433 b orbits its parent star at a distance of 0.0719 astronomical units (10,760,000 km) – 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).[24][lower-alpha 5]

HD 63433 b was probably a mini-Neptune that later lost its atmosphere.[29] 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.[29] 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.[29]

HD 63433 c

Main article: HD 63433 c

The outermost planet, HD 63433 c, is a mini-Neptune planet discovered together with HD 63433 b.[9] It is 2.7 times larger than Earth,[24] but 30% smaller than the Solar System's ice giants, Neptune and Uranus. Being 15.5 times more massive than the Earth,[24] it is more massive than Uranus (14.54 M🜨), but still less massive than Neptune (17.15 M🜨). The density of HD 63433 c calculated at 4.6 g/cm3, slightly lower than that of Earth,[24] but greater than the densities of all gas giants in the Solar System.

Orbiting its star at a distance of 0.145 astronomical units (21,700,000 km) in a orbital period of 20 days,[24] 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.[24][lower-alpha 6]

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.[30] 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.[30] As its atmosphere evaporates, HD 63433 c will slowly become a super-Earth planet.[30]

HD 63433 d

Main article: HD 63433 d

HD 63433 d was the last planet to be discovered in the system, 4 years after planets b and c.[4] With a diameter of ~13,690 km, it is very similar to Earth in size, being only 7% larger, but its mass is currently unknown.[4] It is the innermost planet orbiting HD 63433, having a semi-major axis (mean distance from its star) of 0.0503 astronomical units (7,520,000 km) and an orbital period of just 4 days.[4] The extreme proximity to its star makes it extremely hot, having a daytime temperature estimated at 1,260 °C (2,300 °F),[4][lower-alpha 7] hot enough to melt all the minerals present on its surface,[31] 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.[32][26] The planet also possibly lacks a substantial atmosphere.[4][26][31]

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,[33][26] in addition to possibly having volcanic activity.[34][35] Meanwhile, the nightside of the planet, which never faces its star, may be the opposite, resembling Pluto with glaciers of frozen nitrogen.[36]

HD 63433 d was discovered in 2024, through an analysis of a transit observation made by the Transiting Exoplanet Survey Satellite (TESS).[26] 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.[26][37][4] Further investigation allowed researchers to confirm that this signal was from another orbiting exoplanet, now called HD 63433 d.[26] The discovery was announced on January 10, 2024, in The Astronomical Journal.[32][36]

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.[9][4] 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.[29] 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 (−16.07 km/s).[29]

See also

Notes and references

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    Capistrant, Benjamin K.; Soares-Furtado, Melinda; Vanderburg, Andrew; Jankowski, Alyssa; Mann, Andrew W.; Ross, Gabrielle; Srdoc, Gregor; Hinkel, Natalie R.; Becker, Juliette (2024-01-09), "TESS Hunt for Young and Maturing Exoplanets (THYME) XI: An Earth-sized Planet Orbiting a Nearby, Solar-like Host in the 400Myr Ursa Major Moving Group", The Astronomical Journal, 167 (2): 54, arXiv:2401.04785, Bibcode:2024AJ....167...54C, doi:10.3847/1538-3881/ad1039
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    Notsu, Yuta; Honda, Satoshi; Maehara, Hiroyuki; Notsu, Shota; Namekata, Kosuke; Nogami, Daisaku; Shibata, Kazunari (2017-02-01). "Spectroscopic observations of active solar-analog stars having high X-ray luminosity, as a proxy of superflare stars". Publications of the Astronomical Society of Japan. 69 (1): 12. arXiv:1611.03659. doi:10.1093/pasj/psw116. ISSN 0004-6264.
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    Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
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    Mann, Andrew W.; Johnson, Marshall C.; Vanderburg, Andrew; Kraus, Adam L.; Rizzuto, Aaron C.; Wood, Mackenna L.; Bush, Jonathan L.; Rockcliffe, Keighley; Newton, Elisabeth R.; Latham, David W.; Mamajek, Eric E.; Zhou, George; Quinn, Samuel N.; Thao, Pa Chia; Benatti, Serena (2020-09-24). "TESS Hunt for Young and Maturing Exoplanets (THYME). III. A Two-planet System in the 400 Myr Ursa Major Group". The Astronomical Journal. 160 (4): 179. arXiv:2005.00047. Bibcode:2020AJ....160..179M. doi:10.3847/1538-3881/abae64. ISSN 0004-6256.
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  1. The age of the Sun is around 4.6 billion years.[11]
  2. From a logarithm of 4.52, later divided by 980.665.
  3. The age of the Solar System is 4.532 billion years.
  4. Neptune is 3.883 times larger than Earth.[27]
  5. Assuming albedos of 0.6 and 0 respectively.
  6. Assuming albedos of 0.6 and 0 respectively.
  7. Assuming an albedo of 0, similar to the measured albedo of TRAPPIST-1b.
  1. [N 1]
    Obtained with a right ascension of 07h 49m 55.06s and a declination of 27° 21′ 47.5″[7] on this website.
  2. [N 2]
    The density of Earth is 5.513 g/cm3.[28]
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