TOI-6894
{{short description|Star in the contsellation Leo}}
{{Starbox begin}}
{{Starbox observe
| epoch = J2000
| constell = Leo
| dec = {{DEC|+12|27|30.5817}}
}}
{{Starbox character
| type = Red dwarf
| appmag_1_passband = J
| appmag_1 = {{Val|13.169|0.023}}
| appmag_2_passband = H
| appmag_2 = {{Val|12.486|0.022}}
| appmag_3_passband = K
| appmag_3 = {{Val|12.207|0.021}}
| appmag_4_passband = TESS
| appmag_4 = {{val|14.905|0.008}}
}}
{{Starbox astrometry
| radial_v =
| prop_mo_ra = {{Val|-146.897}}
| prop_mo_dec = +22.227
| parallax = 13.6842
| p_error = 0.0532
| absmag_v =
}}
{{Starbox detail
| mass = {{Val|0.207|0.011}}
| radius = {{Val|0.2276|0.0057}}
| luminosity = {{Val|0.00375|0.00033}}
| temperature = {{Val|3007|58|fmt=commas}}
| gravity = {{Val|5.039|0.011}}
| metal_fe = {{val|0.142|0.087}}
| age_gyr =
}}
{{Starbox catalog
| names = {{odlist | TIC = 67512645 | 2MASS=J11335277+1227034}}
}}
{{Starbox reference
| Simbad=TIC+67512645
}}
{{Starbox end}}
TOI-6894 is a star in the constellation Leo. Its apparent magnitude of 18.22 is below naked eye vision. Based on parallax measurements, it lies at a distance of {{convert|238|ly|pc|abbr=off|lk=on}}. The star was first described in 2003 by the 2MASS survey, and was analysed by the TESS satellite in search for transiting exoplanets.
This is a red dwarf of spectral class M5.0. It has 0.207 times the Sun's mass, 0.2276 times the Sun's radius, and a mere 0.38% of the solar luminosity. Its effective temperature of {{Val|3000|fmt=commas|ul=K}} give it a typical hue. The iron-to-hydrogen abundance ratio, a metallicity indicator, is 40% more than that of the Sun. The discovery of TOI-6894 b makes the star the last massive star to host gas planet.
Planetary system
In 2019 Transiting Exoplanet Survey Satellite (TESS) discovered periodic dips on TOI-6894's brightness, which would be a signature of an exoplanet that periodically transits its host star as seen from Earth. Confirmation was enabled by a team of scientists lead by Edward M. Bryant, which used photometry and Doppler spectroscopy to confidently confirm that the dips were caused by an exoplanet. Named {{nobr|TOI-6894 b}}, where TOI is "TESS Object of Interest", this exoplanet has an orbital period of about 3.37 days and an average separation of {{convert|0.026|AU|km}}. This somewhat close separation mean it is receiving 5.5 times the radiation Earth receives from the Sun. The equilibrium temperature is {{convert|418|K|C F|lk=in}}, assuming an albedo of 0.1, which is unusual for hot Jupiters.
The planet has a large radius of 9.6 Earth radii ({{earth radius}}), or about {{Jupiter radius|0.855}}, contrasting with its small host star. This make planetary transits deep, with 17% of the stellar surface blocked at maximum. Its mass, measured from the gravitational pull it exerts on the star, is 53 times that of Earth mass ({{Earth mass|link=y}}) (or {{Jupiter mass|0.168|link=y}}), and given the relatively high metallicity of host star, it is estimated that elements heavier than hydrogen and helium, what astronomers term metals, make up {{val|12|2|u=Earth mass}} (or 22% of the total).
TOI-6894 b is unusual on that it has a relatively high mass while orbiting a low-mass star. Current knowledge predicts that smaller stars, such as TOI-6894, should also have a small amount of material in their protoplanetary disks. It is believed that stars with masses lower than {{solar mass|0.3}} should not form gas giants, yet several examples of such planets, as LHS 3154 b, TZ Arietis b and GJ 3512 b have been found, posing challenges to planetary formation theories.
The core accretion model suggest that gas giants form from the accretion of gas around a sufficiently massive core, which forms quickly enough to gain a large amount of gases. In this system, the relatively low amount of solid material in the primordial disk makes the formation of a sufficiently massive core difficult. Another plausible scenario is that TOI-6894 b's massive core accreted heavy elements before accreting gas. The predicted amount of solid material in disks around observed low-mass intermediate-age protostars is still lower than that the {{Earth mass|12}} in {{nobr|TOI-6894 b}}, but those values are potentially severly underestimated, and the properties of most observed protoplanetary disks are poorly constrained. Additionally, it has been theorized that such massive planets can form during the very early stages of star formation, where the dust amount is high enough, and combined with the rarity of planets like {{nobr|TOI-6894 b}}, it is still possible that it formed via the those pathways.
Alternatively, the planet may have formed in a wider orbit around a disk that was gravitationally unstable and then condensed into a planet. At some point, it migrated to its current position.
TOI-6894 b appears promising for characterization of its atmosphere, which can give further insights to its formation history. Based on its amount of radiation received by its host star, it is expected that the atmosphere is dominated by methane-based chemistry.
{{PlanetboxOrbit begin
| name = TOI-6894}}
{{Orbitbox planet
| exoplanet = {{nsl|TOI-6894 b|b}}
| mass = {{Val|0.168|0.022}}
| semimajor = {{val|0.02604|0.00045}}
| period = {{Val|3.37077196|(59)}}
| eccentricity = {{Val|0.029|0.030}}
| inclination = {{Val|89.58|0.10|0.07}}
| radius = {{Val|0.855|0.022}}
}}
{{Orbitbox end}}
References
{{reflist|refs=
{{cite constellation|TIC 67512645}}
{{Cite DR3|3917278287286247808}}
{{cite Exoplanet Archive|TOI-6894}}
{{cite SIMBAD|title=TIC 67512645}}
}}
{{Stars of Leo}}
{{sky|11|33|52.75|+|12|27|30.6}}
Category:M-type main-sequence stars