TOI-178

TOI-178 is a planetary system in the constellation Sculptor,[2] which appears to have at least five, and possibly six planets in a chain of Laplace resonances. That is one of the longest chains yet discovered in a system of planets. The system also has unusual variations in the densities among the planets.[3][4][1]

TOI-178
Observation data
Epoch J2000      Equinox J2000
Constellation Sculptor
Right ascension 00h 29m 12.30s[1]
Declination −30° 27′ 13.46″[1]
Apparent magnitude (V) 11.95[1]
Characteristics
Evolutionary stage Main sequence
Spectral type K[1]
Astrometry
Radial velocity (Rv)57.4±0.5[1] km/s
Proper motion (μ) RA: 149.95±0.07[1] mas/yr
Dec.: −87.25±0.04[1] mas/yr
Parallax (π)15.92 ± 0.05[1] mas
Distance204.9 ± 0.6 ly
(62.8 ± 0.2 pc)
Details
Mass0.650+0.027
−0.029[1] M☉
Radius0.651±0.011[1] R☉
Luminosity (bolometric)0.132±0.010[1] L☉
Surface gravity (log g)4.45±0.15[1] cgs
Temperature4316±70[1] K
Metallicity [Fe/H]−0.23±0.05[1] dex
Rotational velocity (v sin i)1.5±0.3[1] km/s
Other designations
2MASS J00291228-3027133, Gaia DR2 2318295979126499200, TIC 251848941, TYC 6991-00475-1
Database references
SIMBADdata

The system is about 205 light-years away, which is relatively close, implying that such systems may be relatively common.[4][3] The brightness of the star, TOI-178a, facilitates followup observations, which make it an ideal system in which to expand our understanding of planet formation and evolution.[1]

The planetary system was confirmed by data provided by five different planet search projects. After TESS provided first hints at a system with an interesting resonant chain, additional observations to refine the measurement and confirm the finding were provided by CHEOPS, ESPRESSO, NGTS and SPECULOOS. Over the coming years, observations of transit-timing variations in the transits of the various planets, which are expected to range from minutes to tens of minutes, should help pin down the planetary masses and uncover the eccentricities of the various orbits.[1]

Orbital resonance

Of the six planets, named TOI-178b through TOI-178g as per IAU convention, the outer five are locked in a chain of Laplace resonances. The periods of the planets, in days, revolving around the star are b = 1.91, c = 3.24, d = 6.56, e = 9.96, f = 15.23, and g = 20.71. While this is not a perfect integer ratio, there exists a frame of reference that rotates by roughly 1.37° day−1, in which successive conjunctions of the planets form a repeating pattern.[1] For an observer rotating within this frame of reference, the planets c through g form a chain of resonance that can be expressed as 2:4:6:9:12 in ratios of periods, or as 18:9:6:4:3 in ratios of orbits, which means that for every eighteen revolutions of the planet c, the planet d completes nine, the planet e six, the planet f four, and the planet g three.

In addition, the planet b orbits close to where it would also be a part of the same resonant chain. In a slightly bigger orbit of period of ~1.95 days, it would form a 3:5 resonance with the planet c in the same corotating frame of reference as the other five. It is possible that the entire system originally formed in one long resonant chain, but later the innermost planet was pulled out of it, perhaps by tidal interactions.[1]

The TOI-178 planetary system[1]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b 1.50+0.39
−0.44 M⊕		 0.02607±0.00078 1.914558±0.000018 — 88.8+0.8
−1.3°		 1.152+0.073
−0.070 R⊕
c 4.77+0.55
−0.68 M⊕		 0.0370±0.0011 3.238450+0.000020
−0.000019		 — 88.4+1.1
−1.6°		 1.669+0.114
−0.099 R⊕
d 3.01+0.80
−1.03 M⊕		 0.0592±0.0018 6.557700±0.000016 — 88.58+0.20
−0.18°		 2.572+0.075
−0.078 R⊕
e 3.86+1.25
−0.94 M⊕		 0.0783+0.0023
−0.0024		 9.961881±0.000042 — 88.71+0.16
−0.13°		 2.207+0.088
−0.090 R⊕
f 7.72+1.67
−1.52 M⊕		 0.1039±0.0031 15.231915+0.000115
−0.000095		 — 88.723+0.071
−0.069°		 2.287+0.108
−0.110 R⊕
g 3.94+1.31
−1.62 M⊕		 0.1275+0.0038
−0.0039		 20.70950+0.00014
−0.00011		 — 88.823+0.045
−0.047°		 2.87+0.14
−0.13 R⊕

See also
  • Kepler-80 - another system with 5 planets in an orbital resonance

References
  1. Leleu, A.; Alibert, Y.; Hara, N. C.; Hooton, M. J.; Wilson, T. G.; Robutel, P.; Delisle, J. -B.; Laskar, J.; Hoyer, S.; Lovis, C.; Bryant, E. M.; Ducrot, E.; Cabrera, J.; Delrez, L.; Acton, J. S.; Adibekyan, V.; Allart, R.; Prieto, Allende; Alonso, R.; Alves, D.; et al. (2021-01-20). "Six transiting planets and a chain of Laplace resonances in TOI-178". Astronomy & Astrophysics. arXiv:2101.09260. doi:10.1051/0004-6361/202039767. ISSN 0004-6361. S2CID 231693292.
  2. Roman, Nancy G. (1987). "Identification of a Constellation From a Position". Publications of the Astronomical Society of the Pacific. 99 (617): 695–699. Bibcode:1987PASP...99..695R. doi:10.1086/132034.. Requête spécifique à TOI-178 sur VizieR.
  3. Plait, Phil (2021-01-25). "A six-planet system dances in time to the tune of gravity". SYFY WIRE. Retrieved 2021-01-27.
  4. "Nearby Orange Dwarf Hosts Unique System of Six Planets | Astronomy | Sci-News.com". Breaking Science News | Sci-News.com. Retrieved 2021-01-27.

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