Exomoon

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."

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.

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.

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.

Proposed detection methods

The existence of exomoons around many exoplanets is theorized. Despite the great successes of planet hunters with Doppler spectroscopy of the host star, 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.

Doppler spectroscopy of host planet

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.

Microlensing

In 2002, Cheongho Han & Wonyong Han proposed microlensing be used to detect exomoons. 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 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 2009, University College London-based astronomer David Kipping published a paper 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 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. 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. A planet–moon eclipse may also occur during the transit, but such events are 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.

Candidates

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

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. This candidate also featured in the news a few months later in April 2014.

Detection projects

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

Habitability

Habitability of exomoons has been considered in at least two studies published in peer-reviewed journals. René Heller & Rory Barnes 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 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.