Extrasolar moon
Encyclopedia
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 system
s. 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.
While traditional usage implies that moon
s orbit a planet
, the discovery of planet-sized satellites around brown dwarf
s blurs the distinction between planets and moons, due to the low mass
of such failed star
s. To resolve this confusion, the International Astronomical Union
declared, "Objects with true mass
es 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.
'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.
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:
-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.
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.
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.
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
that orbits an extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
or other extrasolar body
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
. Although no extrasolar moons have yet been observed, it can be inferred
Inference
Inference is the act or process of deriving logical conclusions from premises known or assumed to be true. The conclusion drawn is also called an idiomatic. The laws of valid inference are studied in the field of logic.Human inference Inference is the act or process of deriving logical conclusions...
from the empirical
Empirical
The word empirical denotes information gained by means of observation or experimentation. Empirical data are data produced by an experiment or observation....
study of natural satellites in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
that they are likely to be common elements of planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...
s. 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
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
s orbit a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
, the discovery of planet-sized satellites around brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...
s blurs the distinction between planets and moons, due to the low mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of such failed star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s. To resolve this confusion, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
declared, "Objects with true mass
True mass
The term true mass is synonymous with the term mass, but is used in astronomy to differentiate the measured mass of a planet from the lower limit of mass usually obtained from radial velocity techniques...
es below the limiting mass for thermonuclear fusion of deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
, that orbit stars or stellar remnants, are planets." The limiting mass for thermonuclear fusion is currently calculated at 13 Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
masses for objects of solar metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
. The IAU definition indicates that objects smaller than this are considered planets, no matter how they formed.
Characteristics
Extrasolar moons have yet to be detected, hence their properties are unknown at present. However, they likely will vary, as do the solar systemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
's moons. For extrasolar giant planets orbiting within their stellar habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...
, there is a prospect that a terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...
sized satellite may be capable of supporting life.
Proposed detection methods
Doppler spectroscopy
Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....
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 spectroscopyDoppler spectroscopyDoppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....
of host planet - Pulsar timing
Transit timing effects
In 2009, University College LondonUniversity College London
University College London is a public research university located in London, United Kingdom and the oldest and largest constituent college of the federal University of 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
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...
exomoons could be detected by the Kepler space telescope
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...
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 bHD 189733 b
HD 189733 b is an extrasolar planet approximately 63 light-years away in the constellation of Vulpecula . The planet was discovered orbiting the star HD 189733 on October 5, 2005, when astronomers in France observed the planet transiting across the face of the star. The planet is classified as a...
and HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....
. The quality of the retrieved spectra is significantly more affected by noise than the stellar spectrum. As a result, the spectral resolution
Spectral resolution
The spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum...
, 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, SackettPenny Sackett
Penny Diane Sackett is an American-born Australian astronomer and former director of the Research School of Astronomy and Astrophysics at the Australian National University...
and Mardling of the Monash University
Monash University
Monash University is a public university based in Melbourne, Victoria. It was founded in 1958 and is the second oldest university in the state. Monash is a member of Australia's Group of Eight and the ASAIHL....
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
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.
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)