K2-138, also designated EPIC 245950175 or EE-1, is a large early K-type main sequence star with a system of at least 6 planets discovered by

citizen scientists The term citizen science (synonymous to terms like community science, crowd science, crowd-sourced science, civic science, participatory monitoring, or volunteer monitoring) is research conducted with participation from the general public, or am ...

. Four were found in the first two days of the Exoplanet Explorers project on

Zooniverse Zooniverse is a citizen science web portal owned and operated by the Citizen Science Alliance. It is home to some of the Internet's largest, most popular and most successful Citizen science, citizen science projects. The organization grew from ...

in early April 2017, while two more were revealed in further analysis. The system is about away in the constellation Aquarius, within K2 Campaign 12.

Planetary system

K2-138 is notable for its large number 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 was a possibility that the two transits for this candidate were from two individual long-period planets. K2-138g was confirmed by follow-up studies in 2019 and 2021. All six planets are within the

super-Earth A super-Earth is a type of exoplanet with a mass higher than Earth, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17.1 times Earth's, respectively. The term "super-Earth" refers only to t ...

and

mini-Neptune A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen-helium atmosphere, probably with deep layers of ice, rock or liquid oceans (made o ...

categories, with radii between about 1.6 to 3.3 . The outer five, including K2-138g, are likely small gaseous worlds with no solid surface. However, the smaller K2-138b could be rocky. The masses of the planets were initially unknown, as the data for K2-138 did not have a high enough signal-to-noise ratio for transit-timing variation (TTV) analysis. However, the

Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003, that was deactivated when operations ended on 30 January 2020. Spitzer was the third space telescope dedicate ...

could be able to accurately detect TTVs and lead to the masses of the planets being calculated. Planets b through f are predicted to cause TTVs on the order of 2.5 to 7.1 minutes, for predicted masses between 4 and 7 . The five validated planets of K2-138 are very close to the parent star and form an unbroken chain of near-3:2

resonances Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

. Their orbital periods range from 2.35 to 12.76 days, with the 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

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and

Kepler-80 Kepler-80, also known as KOI-500, is a red dwarf star of the spectral type M0V. This stellar classification places Kepler-80 among the very common, cool, class M stars that are still within their main evolutionary stage, known as the main ...

. Like the former, K2-138 could show the end result of slow, inward disk migration.

Spitzer Spitzer is a surname. Notable people with the surname include: * Andre Spitzer (1945–1972), Israeli fencing coach and victim of the Munich massacre * Bernard Spitzer (1924–2014), American real estate developer and philanthropist, father of E ...

observations of K2-138g were announced on the AAS Meeting #233. The iPoster shows an updated radius of K2-138g of 3.7 , making it the largest planet in the system. This result was preliminary until being confirmed in February 2021. The planet g could be in a first-order three-planet resonance (3P-MMR) with planet e and f. This would make the system a six planet resonant chain. A team of astronomers collected 215 spectra over 79 nights with the instrument

HARPS The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision Echelle grating, echelle planet-finding spectrograph installed in 2002 on the ESO 3.6 m Telescope, ESO's 3.6m telescope at La Silla Observatory in Chile. The First l ...

mounted on the

ESO 3.6 m Telescope image:ESO3 6 Telescope.jpg, ESO 3.6 m Telescope The ESO 3.6 m Telescope is an optical reflecting telescope run by the European Southern Observatory at La Silla Observatory, Chile since 1977, with a clear aperture of about and area. The telesco ...

. With a Bayesian analysis of the K2 photometry and HARPS radial-velocities ( RVs) the team were able to constrain the mass of planet b to e. The bulk

densities Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be use ...

of the planets range from Earth-like density for planet b to Neptune-like density for planet e. The determination of masses and densities constrains the composition of the planets. They have likely rocky cores and a substantial atmospheric layer, composed of volatile molecules. For planets f and g this team was able to constrain the upper limit of the mass to 8.7 and 25.5 earth masses. A paper by Acuña et al. studied the

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

content of the K2-138 system, assuming a volatile layer constituted of water in steam and supercritical phases. This paper found that the planet b has an upper water-mass-fraction of 0.7% and is a volatile-poor planet. Planet b could have formed with a thick water atmosphere that was blown away by XUV-radiation coming from the host star, a process which is called

photoevaporation Photoevaporation is the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. The term is typically used in an astrophysical context where ultraviolet radiation from hot stars acts on clouds of mate ...

. Planet f is possibly the most water-rich planet in the system, with an estimated upper water-mass-fraction of 66%. The radius of planet g is larger than a planet with a water-rich composition and the researchers conclude that planet g likely has an atmosphere rich in

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

and

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

and in this case the upper volatile-mass fraction would be only 5%. All planets of the system likely have a less massive core compared to earth. K2-138 was selected as a target by

ESA The European Space Agency (ESA) is a 23-member international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 in the context of European ...

in the first Announcement of Opportunity (AO-1) Program of the CHEOPS mission, which was launched in December 2019. For 87.6 orbits the spacecraft will record the transits to measure TTVs of the planets. K2-138 could become a benchmark system to compare RV and TTV masses. The system is also a good candidate to search for co-orbital bodies, which are predicted to exist and to be stable in resonant chain systems like K2-138. CHEOPS observations did not detect planet b, which could be the result of a large error in the

ephemeris In astronomy and celestial navigation, an ephemeris (; ; , ) is a book with tables that gives the trajectory of naturally occurring astronomical objects and artificial satellites in the sky, i.e., the position (and possibly velocity) over tim ...

due to

precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...

of the planet. Planets in K2-138 have detected TTVs in the order of 10 minutes, only planet d shows TTVs up to 60 minutes. The number of transits observed were not enough to perform a full TTV analysis. The masses are consistent with previous RV masses, except for planet d, which showed a low mass and high eccentricity in the first-order TTV analysis. A better sample of TTVs is needed to resolve this issue. PIA23003 K2-138 System Diagram

Note

References

{{DEFAULTSORT:K2-138 Aquarius (constellation) K-type main-sequence stars J23154776-1050590 Planetary systems with six confirmed planets

Planetary system

See also

Note

References