K2-138
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Aquarius |
| Right ascension | 23h 15m 47.77s[1] |
| Declination | −10° 50′ 58.91″[1] |
| Apparent magnitude (V) | 12.21[1] |
| Characteristics | |
| Evolutionary stage | Main sequence |
| Spectral type | K1V[1] |
| Astrometry | |
| Radial velocity (Rv) | 2.7 ± 1.5[1] km/s |
| Distance | 597 ± 55 ly (183 ± 17[1] pc) |
| Details | |
| Mass | 0.93 ± 0.06[1] M☉ |
| Radius | 0.86 ± 0.08[1] R☉ |
| Luminosity | ~0.554[2] L☉ |
| Surface gravity (log g) | 4.59 ± 0.07[1] cgs |
| Temperature | 5738 ± 60[1] K |
| Metallicity [Fe/H] | +0.16 ± 0.04[1] dex |
| Other designations | |
EPIC 245950175, 2MASS J23154776-1050590, EE-1 | |
| Database references | |
| SIMBAD | data |
K2-138, also designated EPIC 245950175 and unofficially named EE-1,[3] is a large early K-type main sequence star with a system of up to 6 planets discovered by citizen scientists. Four were found in the first two days of the Exoplanet Explorers project on Zooniverse in early April 2017, while up to two more were revealed in further analysis. The system is almost 600 light years away in the constellation Aquarius, within K2 Campaign 12.
Planetary System
K2-138 is notable for its large amount of planets, all found through the efforts of citizen scientists. They are designated K2-138b, c, d, e, f, and g in order from their host star. The first five were validated by Christiansen et al., while K2-138g was noted as being a likely candidate. However, since there were only two transits of it, K2-138g could not be validated. There is a possibility that the two transits for this candidate are from two individual long-period planets.[1]
All six planets are within the Super-Earth and Mini-Neptune categories, with radii between about 1.6 R⊕ to 3.3 R⊕. The outer five, including the unconfirmed K2-138g, are likely small gaseous worlds with no solid surface. However, the smaller K2-138b could be rocky. The masses of the planets remain unknown, as the data for K2-138 does not have a high enough signal-to-noise ratio for transit-timing variation (TTV) analysis. However, the Spitzer space telescope could be able to accurately detect TTVs and lead to the masses of the planets being calculated. Planets b through f are predicted to caused TTVs on the order of 2.5 to 7.1 minutes, for predicted masses between 4 M⊕ and 7 M⊕.[1]
The five validated planets of K2-138 are very close to the parent star and from an unbroken chain of near-3:2 resonances. Their orbital periods range from 2.35 to 12.76 days, with the unconfirmed sixth planet orbiting much further out with a period of about 41 days. K2-138b, c, d, e, and f are locked in several chains of three-body resonances, a feat shared by only a handful of systems, including TRAPPIST-1 and Kepler-80. Like the former, K2-138 could show the end result of slow, inward disk migration.[1]
| Companion (in order from star) |
Mass | Semimajor axis (AU) |
Orbital period (days) |
Eccentricity | Inclination | Radius |
|---|---|---|---|---|---|---|
| b | — | 0.03380±0.00024 | 2.35322±0.00036 | <0.403 | 86.9+2.2 −4.6° |
1.57+0.28 −0.17 R⊕ |
| c | — | 0.04454±0.00032 | 3.55987+0.00023 −0.00022 |
<0.296 | 87.5+1.8 −3.3° |
2.52+0.34 −0.16 R⊕ |
| d | — | 0.05883±0.00042 | 5.40478+0.00048 −0.00046 |
<0.348 | 87.9+1.5 −2.8° |
2.66+0.39 −0.18 R⊕ |
| e | — | 0.07807±0.00056 | 8.26144+0.00045 −0.00044 |
<0.315 | 88.70+0.93 −1.66° |
3.29+0.35 −0.18 R⊕ |
| f | — | 0.10430+0.00074 −0.00075 |
12.75759±0.00092 | <0.364 | 89.03+0.70 −1.22° |
2.81+0.36 −0.19 R⊕ |
| g (unconfirmed) | — | — | 41.97 | — | — | ~2.8 R⊕ |
See also
References
- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Christiansen, Jessie L.; Crossfield, Ian J. M.; Barentsen, Geert; Lintott, Chris J.; Barclay, Thomas; Simmons, Brooke D.; Petigura, Erik; Schlieder, Joshua E.; Dressing, Courtney D.; Vanderburg, Andrew; Ciardi, David R.; Allen, Campbell; McMaster, Adam; Miller, Grant; Veldthuis, Martin; Allen, Sarah; Wolfenbarger, Zach; Cox, Brian; Zemiro, Julia; Howard, Andrew W.; Livingston, John; Sinukoff, Evan; Catron, Timothy; Grey, Andrew; Kusch, Joshua J. E.; Terentev, Ivan; Vales, Martin; Kristiansen, Martti H. (11 January 2018). "The K2-138 System: A Near-Resonant Chain of Five Sub-Neptune Planets Discovered by Citizen Scientists". arXiv:1801.03874 [astro-ph].
- ↑ http://phl.upr.edu/projects/habitable-exoplanets-catalog/calculators
- ↑ https://www.zooniverse.org/projects/ianc2/exoplanet-explorers/about/results