Exomoon

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Artist's impression of candidate exomoon Kepler-1625b I orbiting its planet.[1]

An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.[2]

Exomoons are difficult to detect and confirm using current techniques,

habitats for extraterrestrial life.[2]

Definition and designation

Although traditional usage implies moons orbit a planet, the discovery of brown dwarfs with planet-sized satellites blurs the distinction between planets and moons, due to the low mass of brown dwarfs. This confusion is resolved by the International Astronomical Union (IAU) declaration that "Objects with true masses below the limiting mass for thermonuclear fusion of deuterium that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+√621) are planets."[12]

The IAU definition does not address the naming convention for the satellites of free-floating objects that are less massive than brown dwarfs and below the deuterium limit (the objects are typically referred to as free-floating planets, rogue planets, low-mass brown dwarfs or isolated planetary-mass objects). The satellites of these objects are typically referred to as exomoons in the literature.[7][8][13]

Exomoons take their designation from that of their

submoon
).

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.[14][15][clarification needed
]

In August 2019, astronomers reported that an exomoon in the WASP-49b exoplanet system may be volcanically active.[16]

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.[17]

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.[17]

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.[18]

Detection methods

The existence of exomoons around many exoplanets is theorized.[14] Despite the great successes of planet hunters with Doppler spectroscopy of the host star,[19] 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 a telescope such as the James Webb Space Telescope to image it.[20]

Doppler spectroscopy of host planet

Doppler spectroscopy is an indirect detection method that measures the velocity shift and resulting stellar spectrum shift associated with an orbiting planet.[21] 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.

Radio wave emissions from the host planet's magnetosphere

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.[22]

Microlensing

In 2002, Cheongho Han & Wonyong Han proposed

microlensing be used to detect exomoons.[23]
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,

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 research note titled 'Determination of the size, mass, and density of “exomoons” from photometric transit timing variations'.[25]

In 2009, David Kipping published a paper

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

Kepler Space Telescope[27]
using the TTV and TDV effects.

Transit method (star-planet-moon systems)

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 pass in front of a host star, both objects should produce a dip in the observed light.[28] A planet–moon eclipse may also occur[29] during the transit, but such events have an inherently low probability.

Transit method (planet-moon systems)

If the host planet is directly imaged, then transits of an exomoon may be observable. When an exomoon passes in front of the host planet, a small dip in the light received from the directly-imaged planet may be detected.[29] Exomoons of directly imaged exoplanets and free-floating planets are predicted to have a high transit probability and occurrence rate. Moons as small as Io or Titan should be detectable with the James Webb Space Telescope using this method, but this search method requires a substantial amount of observation time.[13]

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

Kepler telescope 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.[30][31]

Indirect detection around white dwarfs

The atmosphere of

Mimas, should be enriched in Be, B, and Li.[34]

Candidates

Main article: List of exomoon candidates

Detection projects

There are several missions underway now using some of the methods described above, which will find many more candidate exomoons and be able to confirm or disprove some candidates. PLATO, for example, is expected to launch in 2026.

As part of the Kepler mission, the Hunt for Exomoons with Kepler (HEK) project was intended to detect exomoons, and generated some of the candidates still discussed today.[35][36]

Habitability

Main article: Habitability of natural satellites
Artist's impression of a hypothetical Earth-like moon around a Saturn-like exoplanet

The habitability of exomoons has been considered in at least two studies published in peer-reviewed journals. René Heller & Rory Barnes[37] 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 to 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[38] 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 indefinitely. Models for moon formation suggest the formation of even more massive moons than Ganymede is common around many of the super-Jovian exoplanets.[39]

Earth-sized exoplanets in the habitable zone around

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.[40]

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.

The possible exomoon candidate transiting

Myr old. If confirmed, the exomoon may be similar to primordial earth and characterization of its atmosphere with the James Webb Space Telescope could perhaps place limits on the time scale for the formation of life.[13]

See also

References

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External links

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