Extrasolar moon

An extrasolar moon, or exomoon, is a natural satellite that orbits an extrasolar planet or other extrasolar body. Although no extrasolar moons have yet been observed, it can be 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 gas giants and in our Solar System, the gas giants have numerous and large collections of natural satellites (see Moons of Jupiter and Moons of Saturn). However, detecting them would be extremely difficult with current techniques.

Definition of satellites around brown dwarfs

Artist's impression of the view from a hypothetical moon around an exoplanet orbiting a triple star system.

While traditional usage implies that 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." The limiting mass for thermonuclear fusion is currently calculated at 13 Jupiter masses for objects of solar metallicity. The IAU definition indicates that objects smaller than this are considered planets, no matter how they formed. Nevertheless, though, many reputable sources classify objects above 13 Jupiter masses as planets, such as the Extrasolar Planets Encyclopedia, which aims to include objects up to 25 Jupiter masses.

Characteristics

Extrasolar moons have yet to be detected, hence their properties are unknown at present. However, they likely will vary, as do the solar system's moons. For extrasolar giant planets orbiting within their stellar habitable zone, there is a prospect that a terrestrial planet-sized satellite may be capable of supporting life.

Proposed detection methods

Artist's impression of a hypothetical Earth-like moon around a Saturn-like exoplanet.

There are no known extrasolar moons, but their existence is theorized around many exoplanets. 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:

• Transit timing effects
• Direct imaging
• Transit method
• Microlensing
• Doppler spectroscopy of host planet
• Pulsar timing

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 center of gravity when the moon and planet 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 center of gravity 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, the author and two colleagues have concluded that habitable zone exomoons could be detected by the Kepler space telescope using the TTV and TDV effects.

Direct imaging

Direct imaging even an exoplanet is extremely challenging due to the large difference in brightness between the objects and the small angular size of the planet. These problems are exacerbated for small exomoons.

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.

Microlensing

In 2002, Cheongho Han & Wonyong Han proposed that microlensing could be used to detect the moons of extrasolar planets. The authors found that '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.

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.

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.

See also

• Natural satellite
• Habitable moon

External links

• Shadow Moons: The Unknown Sub-Worlds that Might Harbor Life
• Likely First Photo of Planet Beyond the Solar System
• Working Group on Extrasolar Planets - Definition of a "Planet" Position statement on the definition of a planet. (IAU)