TOI-421

{{Short description|Planetary system in the constellation Lepus}}

{{starbox begin}} {{starbox observe | epoch = J2000 | constell = Lepus<ref name=constellation/> | ra = {{RA|05|27|24.8259}}<ref name="gaia" /> | dec = {{DEC|-14|16|37.046}}<ref name="gaia" /> | appmag_v = 9.93<ref name="tycho2" /> }} {{starbox character | type = Main sequence<ref name="gaia" /> | class = G9V<ref name="Carleo2020" /> }} {{starbox astrometry | radial_v = {{val|79.41|0.16}}<ref name="gaia" /> | prop_mo_ra = {{val|-35.743|(11)}} | prop_mo_dec = {{val|50.387|(11)}} | pm_footnote = <ref name="gaia" /> | parallax = 13.3337 | p_error = 0.0117 | parallax_footnote = <ref name="gaia" /> }} {{starbox detail | source = <ref name="Krenn2024" /> | mass = {{val|0.833|0.048|0.054}} | radius = {{val|0.866|0.006}}  | gravity = {{val|4.48|0.03}} | temperature = {{val|5291|64}} | metal_fe = {{val|-0.044|0.04}} | rotation = {{val|39.6|1.6|ul=d}} | rotational_velocity = {{val|1.8|1.0}}<ref name="Carleo2020" /> | age_gyr = {{val|10.9|2.9|5.2}} }} {{starbox catalog | names = {{odlist |BD=-14 1137 |GSC=05344-01206 |2MASS=J05272482-1416370 |PPM=711571 |TIC=94986319 |TOI=421 |TYC=5344-1206-1 |Gaia DR2=2984582227215748864 |Gaia DR3=2984582227215748864}} }} {{starbox reference | Simbad = BD-14+1137 | NSTED = TOI-421 }} {{starbox end}}

'''TOI-421''' (also known as '''BD−14 1137''')<ref name="simbad" /> is a G-type main-sequence star located in the constellation Lepus. The star hosts two transiting exoplanets discovered in 2020 and forms a visual binary system with a nearby red dwarf star '''BD−14 1137 B'''. Both planets are hot Neptunes well-suited for atmospheric characterization. Due to the star's old age, knowing the composition of their atmosphere can provide valuable insights for validation of atmospheric evolution models.

== Stellar characteristics ==

TOI-421 is a Solar-type star, of similar metallicity though somewhat smaller the Sun. According to the evolutionary models, it is thought to be an old star, age of approximately 10 billion years.<ref name="Carleo2020" /><ref name="Krenn2024" /> Observations of variability related to its ~42-day rotation indicate a low level of stellar activity with only few surface spots, dominated by plages.<ref name="Carleo2020" />

The red dwarf companion is positioned {{val|29.4|u=arcsec}} NW, corresponding to actual separation of approximately {{val|2200|ul=AU}}.<ref name="Carleo2020" />

== Planetary system ==

A discovery of two planets transiting the star was announced in 2020, based on TESS observations between {{date|2018-11-15}} and {{date|2019-01-07}}, and confirmed with ground-based observations.<ref name="Carleo2020" /> The TESS light curve features two series of transit signals identified with the two planets; a deeper one every ~16.1 days and a shallower one every ~5.2 days.

The orbital parameters were refined in a 2024 study that included additional transit observations by TESS and CHEOPS as well as radial velocity measurements for determination of mass of the planets, showing that both planets have moderately eccentric orbits. The study did not find evidence of transit-timing variations, which could hint at existence of additional non-transiting planets.<ref name="Krenn2024" />

{{Orbitbox planet begin | name = TOI-421 | table_ref = <ref name="Krenn2024" /><ref name="Carleo2020" /> }} {{Orbitbox planet | exoplanet = b | mass_earth = {{val|6.7|0.6}} | radius_earth = {{val|2.64|0.08}} | semimajor = {{val|0.0554|0.0010}} | period = {{val|5.197576|0.000005}} | eccentricity = {{val|0.13|0.05}} | inclination = {{val|85.68|0.36|0.46}} }} {{Orbitbox planet | exoplanet = c | mass_earth = {{val|14.1|1.4}} | radius_earth = {{val|5.09|0.07}} | semimajor = {{val|0.1170|0.0018}} | period = {{val|16.067541|0.000004}} | eccentricity = {{val|0.19|0.04}} | inclination = {{val|88.353|0.078|0.084}} }} {{Orbitbox end}}

=== TOI-421 b === [[File:NASA's James Webb Space Telescope Primed to Lift the Haze Surrounding Sub-Neptunes (51685676732).jpg|thumb|An artist’s concept of TOI-421 b, a sub-Neptune.]] The planet b is a hot mini-Neptune with a moderate density of {{val|0.37|0.05|u={{rho+}}}} ({{val|2.04|0.28|u=g.cm-3}})<ref name="Krenn2024" />{{efn|name=rho_Earth|Density converted from {{rho+}} to g⋅cm<sup>−3</sup> by using the value for Earth density {{rho+}} {{=}} {{val|5.513|u=g.cm-3}}.}} and an equilibrium temperature of {{val|922|14|u=K}}.<ref name="Krenn2024" /> Due to its high equilibrium temperature and small mass it is expected to experience significant atmospheric escape.<ref name="Carleo2020" /><ref name="Krenn2024" /> However, the observed density is not consistent with a thin, hydrogen-depleted atmosphere expected from evolutionary models for a planet forming this close to the star. Planetary migration is a plausible explanation, meaning that the planet could have formed further away from the star and only more recently moved to current location.<ref name="Krenn2024" />

TOI-421 b was selected as a James Webb Space Telescope target early on as a planet similar in size and density to the well-studied sub-Neptune GJ 1214 b, but thought to be too hot for formation of atmospheric haze, which would allow for bulk chemical characterization of atmosphere.<ref name="Webb2021" /> In 2025 it became the first sub-Neptune around a solar-type star to have its atmosphere characterized with JWST. The observed infrared spectrum is consistent with a hydrogen/helium dominated atmosphere and detected presence of water at abundances of {{val|e=-3}}{{nbnd}}{{val|e=-4}} and hints of sulfur dioxide and carbon monoxide, but no carbon dioxide or methane. The observed spectrum also excludes the possibility of presence of high-altitude haze. This is in contrast to the observed composition of sub-Neptunes around red dwarf stars, which typically show a much higher fraction of heavier gas species.<ref name="Davenport2025" />

=== TOI-421 c ===

The planet c is a hot Neptune with a low density of {{val|0.107|0.012|u={{rho+}}}} ({{val|0.590|0.066|u=g.cm-3}}),<ref name="Krenn2024" />{{efn|name=rho_Earth}} belonging to the super-puff class.<ref name="Carleo2020" /> Its equilibrium temperature is {{val|635|9|u=K}}.<ref name="Krenn2024" /> Like the planet b, it is also expected to exhibit significant atmospheric escape. However, the low density implies that the planet has retained an extensive atmosphere.

== See also ==  * HD 152843 * K2-24

== Notes == {{notelist}}

== References ==

<ref name="simbad">{{cite simbad |title=BD-14 1137 |access-date=2025-01-09}}</ref> <ref name=constellation>{{cite constellation|TOI-421}}</ref> <ref name="gaia">{{cite Gaia DR3|2984582227215748864}}</ref> <ref name="tycho2">{{cite journal |last1=Høg |first1=E. |last2=Fabricius |first2=C. |last3=Makarov |first3=V. V. |last4=Urban |first4=S. |last5=Corbin |first5=T. |last6=Wycoff |first6=G. |last7=Bastian |first7=U. |last8=Schwekendiek |first8=P. |last9=Wicenec |first9=A. |title=The Tycho-2 catalogue of the 2.5 million brightest stars |journal=Astronomy and Astrophysics |date=March 2000 |volume=355 |pages=L27–L30 |issn=0004-6361 |bibcode=2000A&A...355L..27H}}</ref>

<ref name="Carleo2020">{{Cite journal |title=The Multiplanet System TOI-421: A Warm Neptune and a Super Puffy Mini-Neptune Transiting a G9 V Star in a Visual Binary |journal=The Astronomical Journal |date=2020 |doi=10.3847/1538-3881/aba124|doi-access=free |last1=Carleo |first1=Ilaria |last2=Gandolfi |first2=Davide |last3=Barragán |first3=Oscar |last4=Livingston |first4=John H. |last5=Persson |first5=Carina M. |last6=Lam |first6=Kristine W. F. |last7=Vidotto |first7=Aline |last8=Lund |first8=Michael B. |last9=d'Angelo |first9=Carolina Villarreal |last10=Collins |first10=Karen A. |last11=Fossati |first11=Luca |last12=Howard |first12=Andrew W. |last13=Kubyshkina |first13=Daria |last14=Brahm |first14=Rafael |last15=Oklopčić |first15=Antonija |last16=Mollière |first16=Paul |last17=Redfield |first17=Seth |last18=Serrano |first18=Luisa Maria |last19=Dai |first19=Fei |last20=Fridlund |first20=Malcolm |last21=Borsa |first21=Francesco |last22=Korth |first22=Judith |last23=Esposito |first23=Massimiliano |last24=Díaz |first24=Matías R. |last25=Nielsen |first25=Louise Dyregaard |last26=Hellier |first26=Coel |last27=Mathur |first27=Savita |last28=Deeg |first28=Hans J. |last29=Hatzes |first29=Artie P. |last30=Benatti |first30=Serena |volume=160 |issue=3 |page=114 |arxiv=2004.10095 |bibcode=2020AJ....160..114C |s2cid=216036363 |display-authors=1 }}</ref>

<ref name="Krenn2024">{{Cite journal |title=Characterisation of the TOI-421 planetary system using CHEOPS, TESS, and archival radial velocity data |journal=Astronomy & Astrophysics |date=2024 |doi=10.1051/0004-6361/202348584 |doi-access=free |last1=Krenn |first1=A. F. |last2=Kubyshkina |first2=D. |last3=Fossati |first3=L. |last4=Egger |first4=J. A. |last5=Bonfanti |first5=A. |last6=Deline |first6=A. |last7=Ehrenreich |first7=D. |last8=Beck |first8=M. |last9=Benz |first9=W. |last10=Cabrera |first10=J. |last11=Wilson |first11=T. G. |last12=Leleu |first12=A. |last13=Sousa |first13=S. G. |last14=Adibekyan |first14=V. |last15=Correia |first15=A. C. M. |last16=Alibert |first16=Y. |last17=Delrez |first17=L. |last18=Lendl |first18=M. |last19=Patel |first19=J. A. |last20=Venturini |first20=J. |last21=Alonso |first21=R. |last22=Anglada |first22=G. |last23=Asquier |first23=J. |last24=Bárczy |first24=T. |last25=Barrado Navascues |first25=D. |last26=Barros |first26=S. C. C. |last27=Baumjohann |first27=W. |last28=Beck |first28=T. |last29=Billot |first29=N. |last30=Bonfils |first30=X. |volume=686 |pages=A301 |arxiv=2404.11074 |bibcode=2024A&A...686A.301K |s2cid=269188184 |display-authors=1 }}</ref>

<ref name="Davenport2025">{{Cite journal |title=TOI-421 b: A Hot Sub-Neptune with a Haze-Free, Low Mean Molecular Weight Atmosphere |journal=The Astrophysical Journal Letters |arxiv=2501.01498 |last1=Davenport |first1=Brian |last2=Kempton |first2=Eliza M. -R. |last3=Nixon |first3=Matthew C. |last4=Ih |first4=Jegug |last5=Deming |first5=Drake |last6=Fu |first6=Guangwei |last7=May |first7=E. M. |last8=Bean |first8=Jacob L. |last9=Gao |first9=Peter |last10=Rogers |first10=Leslie |last11=Malik |first11=Matej |date=2025 |s2cid=275324192 |display-authors=1 }}</ref>

<ref name="Webb2021">{{Cite web |last1=Carruthers |first1=Margaret W. |last2=Pulliam |first2=Christine |date=2021-11-17 |title=Webb Primed to Lift the Haze Surrounding Sub-Neptunes |url=https://webbtelescope.org/contents/news-releases/2021/news-2021-054 |website=Webb |publisher=Space Telescope Science Institute}}</ref>

</references>

Category:Lepus (constellation) 0421 BD-14 1137 Category:G-type main-sequence stars Category:Planetary systems with two confirmed planets Category:Planetary transit variables