HD 21749

HD 21749
HD 21749
ClassificazioneNana arancione
Classe spettraleK4.5V[1]
Distanza dal Sole53 al
CostellazioneReticolo
Coordinate
Ascensione retta03h 26m 59,22s
Declinazione-63° 29′ 56,76″
Dati fisici
Raggio medio0,695[2] R
Massa
0,73±0,07[2] M
Temperatura
superficiale
  • 4640±100 K[2] (media)
Dati osservativi
Magnitudine app.8,14
Parallasse61,25 mas
Moto proprioAR: 355,084 mas/anno
Dec−247,460 mas/anno
Velocità radiale59,46 km/s
Nomenclature alternative
Gliese 143, HIP 16069, TOI-186

HD 21749 è una stella nana arancione nella costellazione del Reticolo, situata a circa 53 al dal sistema solare. Il 7 gennaio 2019, è stata annunciata la scoperta di due pianeti extrasolari in orbita attorno alla stella: uno è un mininettuno, con un raggio 3 volte quello terrestre, denominato HD 21749 b, mentre un altro HD 21749 c, molto caldo, sarebbe di dimensioni terrestri. Gli esopianeti sono stati scoperti dal Transiting Exoplanet Survey Satellite (TESS) e sono stati tra i primi pianeti scoperti da TESS dopo Pi Mensae c e Lhs 3884 b.[3]

Caratteristiche

La stella, una nana arancione non troppo dissimile dal Sole, ha una massa del 68% e un raggio del 76% di quelli della nostra stella, mentre la sua temperatura superficiale è di circa 4570 K.[4]

Sistema planetario

Il pianeta confermato è stato classificato come un mininettuno, poiché il suo raggio è il triplo di quello terrestre (quello di Nettuno è il quadruplo circa), tuttavia pare avere una massa 20 volte quella della Terra (Nettuno = 17 M),[3] quindi superiore a quella del gigante gassoso del sistema solare.

Non è ancora ben chiara la natura di questo tipo di pianeti, poiché non ne esistono di simili nel sistema solare,[5] e soprattutto non ne esistono così vicini alla propria stella. Il pianeta b orbita infatti in appena 36 giorni attorno alla propria stella, non molto meno luminosa del Sole, e si presume che la temperatura superficiale sia attorno ai 150 °C.[5] Come per i gioviani caldi, è possibile che il calore e il vento stellare abbiano spazzato via gli elementi più leggeri dell'atmosfera, come l'idrogeno e l'elio. Gli astronomi autori della scoperta pensano che il pianeta non sia roccioso ma potrebbe essere un pianeta oceanico o avere qualche altro tipo di atmosfera, sconosciuta nel sistema solare.[3]

Gli stessi autori dello studio hanno suggerito l'esistenza di un altro pianeta, confermato ad aprile 2019[6] di dimensioni terrestri e che orbita in poco meno di 8 giorni attorno a HD 21749.[3] Il pianeta sarebbe molto caldo, avendo una temperatura di superficie stimata di circa 400 °C.[2]

Prospetto del sistema

La stella HD 21749 ripresa dal telescopio TESS e una rappresentazione artistica del pianeta HD 21749 b.
PianetaTipoMassaRaggioDensitàPeriodo orb.Sem. maggioreScoperta
cRoccioso< 3,5 M1,13±0,1 r< 13,2 g/cm³8 giorni0.1932 UA2019
bMininettuno20±2,7 M2,86±0,2 r4,7±1,7 g/cm³35,61 giorni0,0695 UA2019

Note

  1. ^ HD 21749 -- High proper-motion Star, in SIMBAD.
  2. ^ a b c d Tianjun Gan et al., Revisiting the HD 21749 planetary system with stellar activity modelling, in Monthly Notices of the Royal Astronomical Society, vol. 501, n. 4, marzo 2021, pp. 6042-6061. arΧiv:2012.04873
  3. ^ a b c d Tess fa tris, forse poker, su media.inaf.it, Istituto nazionale di astrofisica, 9 gennaio 2019.
  4. ^ Stellar Overview Page - HD 21749, in NASA Exoplanet Archive, 2019. URL consultato l'8 gennaio 2019.
  5. ^ a b Dennis Overbye, Another Day, Another Exoplanet: NASA’s TESS Keeps Counting More, The New York Times, 7 gennaio 2019.
  6. ^ (EN) Diana Dragomir et al., TESS Delivers Its First Earth-sized Planet and a Warm Sub-Neptune, in The Astrophysical Journal Letters, vol. 875, n. 2, 15 aprile 2019, DOI:10.3847/2041-8213/ab12ed.

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