Exomoon

An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.[1]

It is inferred from the empirical study of natural satellites in the Solar System that they are likely to be common elements of planetary systems. The majority of detected exoplanets are giant planets. In the Solar System, the giant planets have large collections of natural satellites (see Moons of Jupiter, Moons of Saturn, Moons of Uranus and Moons of Neptune). Therefore, it is reasonable to assume that exomoons are equally common.

Though exomoons are difficult to detect and confirm using current techniques,[2] observations from missions such as Kepler have observed a number of candidates, including some that may be habitats for extraterrestrial life and one that may be a rogue planet.[1] To date there are no confirmed exomoon detections.[3] Nevertheless, in September 2019, astronomers reported that the observed dimmings of Tabby's Star may have been produced by fragments resulting from the disruption of an orphaned exomoon.[4][5][6]

Definition of satellites around brown dwarfs

Although traditional usage implies moons orbit a planet, the discovery of planet-sized satellites around brown dwarfs blurs the distinction between planets and moons, due to the low mass of such failed stars. To resolve this confusion, the International Astronomical Union declared, "Objects with true masses below the limiting mass for thermonuclear fusion of deuterium, that orbit stars or stellar remnants, are planets."[7]

Characteristics

Characteristics of any extrasolar satellite are likely to vary, as do the Solar System's moons. For extrasolar giant planets orbiting within their stellar habitable zone, there is a prospect a terrestrial planet-sized satellite may be capable of supporting life.[8][9]

In August 2019, astronomers reported that an exomoon in the WASP-49b exoplanet system may be volcanically active.[10]

Orbital inclination

For impact-generated moons of terrestrial planets not too far from their star, with a large planet–moon distance, it is expected that the orbital planes of moons will tend to be aligned with the planet's orbit around the star due to tides from the star, but if the planet–moon distance is small it may be inclined. For gas giants, the orbits of moons will tend to be aligned with the giant planet's equator because these formed in circumplanetary disks.[11]

Lack of moons around planets close to their stars

Planets close to their stars on circular orbits will tend to despin and become tidally locked. As the planet's rotation slows down the radius of a synchronous orbit of the planet moves outwards from the planet. For planets tidally locked to their stars, the distance from the planet at which the moon will be in a synchronous orbit around the planet is outside the Hill sphere of the planet. The Hill sphere of the planet is the region where its gravity dominates that of the star so it can hold on to its moons. Moons inside the synchronous orbit radius of a planet will spiral into the planet. Therefore, if the synchronous orbit is outside the Hill sphere, then all moons will spiral into the planet. If the synchronous orbit is not three-body stable then moons outside this radius will escape orbit before they reach the synchronous orbit.[11]

A study on tidal-induced migration offered a feasible explanation for this lack of exomoons. It showed the physical evolution of host planets (i.e. interior structure and size) plays a major role in their final fate: synchronous orbits can become transient states and moons are prone to be stalled in semi-asymptotic semimajor axes, or even ejected from the system, where other effects can appear. In turn, this would have a great impact on the detection of extrasolar satellites.[12]

Proposed detection methods
Artist's impression of a hypothetical Earth-like moon around a Saturn-like exoplanet

The existence of exomoons around many exoplanets is theorized.[8] Despite the great successes of planet hunters with Doppler spectroscopy of the host star,[13] exomoons cannot be found with this technique. This is because the resultant shifted stellar spectra due to the presence of a planet plus additional satellites would behave identically to a single point-mass moving in orbit of the host star. In recognition of this, there have been several other methods proposed for detecting exomoons, including:

Direct imaging

Direct imaging of an exoplanet is extremely challenging due to the large difference in brightness between the star and exoplanet as well as the small size and irradiance of the planet. These problems are greater for exomoons in most cases. However, it has been theorized that tidally heated exomoons could shine as brightly as some exoplanets. Tidal forces can heat up an exomoon because energy is dissipated by differential forces on it. Io, a tidally heated moon orbiting Jupiter, has volcanoes powered by tidal forces. If a tidally heated exomoon is sufficiently tidally heated and is distant enough from its star for the moon's light not to be drowned out, it would be possible for future telescopes (such as the James Webb Space Telescope) to image it.[14]

Doppler spectroscopy of host planet

Doppler spectroscopy is an indirect detection method that measures the velocity shift and result stellar spectrum shift associated with an orbiting planet.[15] This method is also known as the Radial Velocity method. It is most successful for main sequence stars The spectra of exoplanets have been successfully partially retrieved for several cases, including HD 189733 b and HD 209458 b. The quality of the retrieved spectra is significantly more affected by noise than the stellar spectrum. As a result, the spectral resolution, and number of retrieved spectral features, is much lower than the level required to perform doppler spectroscopy of the exoplanet.

Detection of radio wave emissions from the magnetosphere of host planet

During its orbit, Io's ionosphere interacts with Jupiter's magnetosphere, to create a frictional current that causes radio wave emissions. These are called "Io-controlled decametric emissions" and the researchers believe finding similar emissions near known exoplanets could be key to predicting where other moons exist.[16]

Microlensing

In 2002, Cheongho Han & Wonyong Han proposed microlensing be used to detect exomoons.[17] The authors found detecting satellite signals in lensing light curves will be very difficult because the signals are seriously smeared out by the severe finite-source effect even for events involved with source stars with small angular radii.

Pulsar timing

In 2008, Lewis, Sackett, and Mardling[18] of the Monash University, Australia, proposed using pulsar timing to detect the moons of pulsar planets. The authors applied their method to the case of PSR B1620-26 b and found that a stable moon orbiting this planet could be detected, if the moon had a separation of about one fiftieth of that of the orbit of the planet around the pulsar, and a mass ratio to the planet of 5% or larger.

Transit timing effects

In 2007, physicists A. Simon, K. Szatmáry, and Gy. M. Szabó published a reserch note titled 'Determination of the size, mass, and density of “exomoons” from photometric transit timing variationss''.[19]

In 2009, University College London-based astronomer David Kipping published a paper[2][20] outlining how by combining multiple observations of variations in the time of mid-transit (TTV, caused by the planet leading or trailing the planet–moon system's barycenter when the pair are oriented roughly perpendicular to the line of sight) with variations of the transit duration (TDV, caused by the planet moving along the direction path of transit relative to the planet–moon system's barycenter when the moon–planet axis lies roughly along the line of sight) a unique exomoon signature is produced. Furthermore, the work demonstrated how both the mass of the exomoon and its orbital distance from the planet could be determined using the two effects.

In a later study, Kipping concluded that habitable zone exomoons could be detected by the Kepler Space Telescope[21] using the TTV and TDV effects.

Transit method

When an exoplanet passes in front of the host star, a small dip in the light received from the star may be observed. The transit method is currently the most successful and responsive method for detecting exoplanets. This effect, also known as occultation, is proportional to the square of the planet's radius. If a planet and a moon passed in front of a host star, both objects should produce a dip in the observed light.[22] A planet–moon eclipse may also occur[23] during the transit, but such events have an inherently low probability.

Orbital sampling effects

If a glass bottle is held up to the light it is easier to see through the middle of the glass than it is near the edges. Similarly a sequence of samples of a moon's position will be more bunched up at the edges of the moon's orbit of a planet than in the middle. If a moon orbits a planet that transits its star then the moon will also transit the star and this bunching up at the edges may be detectable in the transit light curves if a sufficient number of measurements are made. The larger the star the greater the number of measurements are needed to create observable bunching. The Kepler spacecraft data may contain enough data to detect moons around red dwarfs using orbital sampling effects but won't have enough data for Sun-like stars.[24][25]

Candidates
Artist's impression of the MOA-2011-BLG-262 system

It has been surmised that the star 1SWASP J140747.93-394542.6, in the constellation Centaurus, might have a planet with a moon.[26] The confirmed extrasolar planet WASP-12b may also possess a moon.[27]

Artist's impression of exomoon Kepler-1625b I orbiting its planet.[28]

In December 2013, a candidate exomoon of a free-floating planet MOA-2011-BLG-262, was announced, but due to degeneracies in the modelling of the microlensing event, the observations can also be explained as a Neptune-mass planet orbiting a low-mass red dwarf, a scenario the authors consider to be more likely.[29][30][31] This candidate also featured in the news a few months later in April 2014.

In October 2018, researchers using the Hubble Space Telescope published observations of the candidate exomoon Kepler-1625b I, which suggest that the host planet is likely several Jupiter masses, while the exomoon may have a mass and radius similar to Neptune. The study concluded that the exomoon hypothesis is the simplest and best explanation for the available observations, though warned that it is difficult to assign a precise probability to its existence and nature.[32][33] However, a reanalysis of the data published in April 2019 concluded that the data was fit better by a planet-only model. According to this study, the discrepancy was an artifact of the data reduction, and Kepler-1625b I likely does not exist.[34]

On 23 June 2020, Chris Fox and Paul Wiegert reported six exomoon candidates from transit timing variations. If confirmed, they would become the first exomoons discovered.[35] However, an independent study by David Kipping in August 2020 found no evidence for the existence of these exomoons, while constraining the possible masses of any exomoons that may orbit the exoplanets in question. The same study finds that Kepler-1625b I remains an exomoon candidate.[36]

List
Host star of the host planet Planet designation Planet mass Planet semimajor axis (AU) Exomoon semimajor axis Exomoon mass (M⊕) Notes
1SWASP J140747.93-394542.6 J1407b[37] 14–26 MJ 2.2–5.6 0.24 AU <0.3 Two possible exomoons residing in small ring gaps around J1407b.
0.25 AU
0.40 AU<0.8Possible exomoon residing in a large ring gap around J1407b.
DH TauriDH Tauri b10.6 MJ33010 AU1 MJCandidate Jupiter-mass satellite from direct imaging. If confirmed, it could also be considered a planet orbiting a brown dwarf.[38]
HD 189733HD 189733 b1.13 MJ0.03116 RP?Found by studying periodic increases and decreases in light given off from HD 189733 b. Outside of planet's Hill sphere.[39]
Kepler-1625Kepler-1625b<11.6 MJ[40]0.9845 RP10Possible Neptune-sized exomoon or double planet, indicated by transit observations.[41][33]
N/AMOA-2011-BLG-262L[42]3.6 MJN/A0.13 AU0.54Found by microlensing; however it is unknown if the system is a sub-Earth-mass exomoon orbiting a free-floating planet, or a Neptune-mass planet orbiting a low-mass red dwarf star.[43]
N/AMOA-2015-BLG-337L9.85 MJN/A0.24 AU33.7Found by microlensing; however it is unknown if the system is a super-Neptune-mass planet orbiting a free-floating planet, or a binary brown dwarf system.[44]
WASP-12WASP-12b[45]1.465 MJ0.02326 RP0.57–6.4Found by studying periodic increases and decreases in light given off from WASP-12b. Outside of planet's Hill sphere.[39]
WASP-49WASP-49b0.37 MJ0.0379??The sodium envelope around WASP-49b could be due to an Io-like exomoon.[46]

Detection projects

As part of the Kepler mission, the Hunt for Exomoons with Kepler (HEK) project is intended to detect exomoons.[47][48]

Habitability

Habitability of exomoons has been considered in at least two studies published in peer-reviewed journals. René Heller & Rory Barnes[49] considered stellar and planetary illumination on moons as well as the effect of eclipses on their orbit-averaged surface illumination. They also considered tidal heating as a threat for their habitability. In Sect. 4 in their paper, they introduce a new concept to define the habitable orbits of moons. Referring to the concept of the circumstellar habitable zone for planets, they define an inner border for a moon to be habitable around a certain planet and call it the circumplanetary "habitable edge". Moons closer to their planet than the habitable edge are uninhabitable. In a second study, René Heller[50] then included the effect of eclipses into this concept as well as constraints from a satellite's orbital stability. He found that, depending on a moon's orbital eccentricity, there is a minimum mass for stars to host habitable moons at around 0.2 solar masses.

Taking as an example the smaller Europa, at less than 1% the mass of the Earth, Lehmer et al. found if it were to end up near to Earth orbit it would only be able to hold onto its atmosphere for a few million years. However, for any larger, Ganymede-sized moons venturing into its solar system's habitable zone, an atmosphere and surface water could be retained pretty much indefinitely. Models for moon formation suggest the formation of even more massive moons than Ganymede is common around many of the super-Jovian exoplanets.[51]

Earth-sized exoplanets in the habitable zone around M-dwarfs are often tidally locked to the host star. This has the effect that one hemisphere always faces the star, while the other remains in darkness. An exomoon in an M-dwarf system does not face this challenge, as it is tidally locked to the planet and it would receive light for both hemispheres. Martínez-Rodríguez et al. studied the possibility of exomoons around planets that orbit M-dwarfs in the habitable zone. While they found 33 exoplanets from earlier studies that lie in the habitable zone, only four could host Moon- to Titan-mass exomoons for timescales longer than 0.8 Gyr (CD–23 1056 b, Ross 1003 b, IL Aquarii b and c). For this mass range the exomoons could probably not hold onto their atmosphere. The researchers increased the mass for the exomoons and found that exomoons with the mass of Mars around IL Aquarii b and c could be stable on timescales above the Hubble time. The CHEOPS mission could detect exomoons around the brightest M-dwarfs or ESPRESSO could detect the Rossiter–McLaughlin effect caused by the exomoons. Both methods require a transiting exoplanet, which is not the case for these four candidates.[52]

Like an exoplanet, an exomoon can potentially become tidally locked to its primary. However, since the exomoon's primary is an exoplanet, it would continue to rotate relative to its star after becoming tidally locked, and thus would still experience a day/night cycle indefinitely.

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See also

References
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