Exoluna

Many exoplanets have exolunas, but finding and exploring them is a difficult task. Despite the great success of exoplanet searches, exoluns are difficult to detect by all existing methods of such searches. So, according to the shift of lines in the spectrum of the host star, a planet with satellites cannot be distinguished from a lonely one. However, there are several other ways to search for exoluns, but they are ineffective:

• Direct observation
• Transit method
• Exoplanet spectroscopy
• Pulsar timing
• Transition Timing Effects

Direct observation

Direct observation of even the exoplanets, not to mention the exoloon, is complicated by the large difference in the luminosity of any planet and the parent star. Nevertheless, direct observations of exoluns heated by tidal heating are possible even with existing technologies ^[1] .

Transit Method

When an exoplanet passes in front of its star, the apparent brightness of the star decreases slightly. The magnitude of this effect is proportional to the square of the radius of the planet. The smallest object discovered by this method is Gliese 436 b - the size of Neptune . Even the planned space telescopes will not be able to detect exolunas the size of the satellites of our solar system .

As of 2013, the Kepler orbital telescope , which tracks about 150,000 stars, is the most suitable instrument for searching for exoluns. There are a number of works devoted to the search for exoluns with its help ^[2] . In 2009, it was predicted that Kepler would be able to detect satellites weighing from 0.2 terrestrial (10 times more massive than the most massive satellites of the solar system) ^[3] . But according to the work of 2013, in red dwarf systems of planets weighing up to 25 terrestrial, even satellites weighing 8-10 terrestrial can be detected only in 25-50% of cases ^[2] .

Exoplanet Spectroscopy

Several successful cases are known for studying the spectra of exoplanets, including HD 189733 A b and HD 209458 b . But the quality of the spectral data for planets is much worse than for stars, and it is now impossible to isolate the component of the spectrum introduced by the satellite.

Pulsar Timing

In 2008, Lewis, Sackett, and Mardling from the University of Monaco proposed using pulsar timing to search for satellites of pulsar planets . The authors applied this method to PSR B1620-26 b and found that if a stable satellite revolves around this planet, then it can be detected if the distance between the planet and the satellite is 1/15 of the distance between the planet and the pulsar, and the ratio of the mass of the moon to the planet there will be 5% or more.

Transition Timing Effects

In 2008, astronomer David Kipping published an article on how to combine numerous observations of changes in the mid-transit time with changes in transit time, which will determine the unique signature of the exoloon. Moreover, the work demonstrates how the mass of the exoloon and its distance to the planet can be determined using these two effects. The author tested this method on Gliese 436 b and showed that the effect of timing the earth's satellite for this planet can be found within 20 seconds.

Due to the difficulty of searching and observing exoluns, their properties remain little known. They should vary widely, as well as the properties of the satellites of our solar system.

The International Astronomical Union has not yet established an exolun nomenclature system, since too few are known. Such a system, perhaps, will use either Arabic or Roman numerals, with a number increase in the order of discovery of the satellites or the distance of the satellite to the home planet. For example, if the satellites open around 51 Pegasus b , they will be called either: “51 Pegasus b 1”, “51 Pegasus b 2” and so on, or: “51 Pegasus b I”, “51 Pegasus b II” and so Further.

An example of an exoloon model is Pandora from the movie Avatar , a satellite of the gas giant. The film accurately reproduces the features of the starry sky, periods of the day, as well as volcanic and electrical phenomena that are possible on such an exoloon.

Modeling the mass of satellites

There is a model ^[4] that allows one to estimate the total mass of satellites depending on the mass of the planet around which they revolve, their maximum number and orbit parameters . The model is based on an empirically established dependence of the mass of satellites of the giants of the solar system on the mass of the planets themselves. On average, the mass of satellites is about 0.0001 mass of the planet, regardless of the number of satellites and the distribution of mass among the satellites.

Dr. Robin Kanap and Dr. William R. Ward, along with a group of scientists from the Southwestern Research Institute of the United States, put forward a hypothesis that during the formation of the planet attract gas (mainly hydrogen ) and dust from the circumstellar space. At the final stage of planet formation, there is an influx of matter into the near-planet orbit, and later satellites are formed there. Growing satellites cause spiral waves in the gas-dust disk, which, interacting gravitationally with satellites, reduce their orbit until the satellite collides with the planet.

This effect increases with increasing satellite mass. The final mass of the satellite depends on two processes. Firstly, from a substance constantly arriving from a gas-dust disk. Secondly, from increasing gravity, leading to a decrease in the satellite’s orbit. Thus, the satellites in the model are the last generation of satellites that did not collide with the planet after the substance of the disk was exhausted and the growth of satellites and their interaction with the disk ended.

Calculations and computer simulations showed that during this process the final ratio of the mass of all remaining satellites to the mass of the planet is 10 ^{−4 of the} mass of the planet in a wide range of initial conditions ^[5] .

An example of a model for the planet.

The results introduce additional restrictions on the masses of gas giants of other stars for the possibility of the existence of earthly life on their satellites. One of them is that for this type of life you need a fairly dense atmosphere , similar to Earth . The satellite must have sufficient mass and, as a result, sufficient gravity on the surface so that the atmosphere does not escape into outer space. For example, for a satellite to have a mass of the Earth , a gas giant must have a mass of at least 31 masses of Jupiter (and with a number of additional low-mass satellites, similar to the moons of Jupiter and Saturn , 32-33) being essentially a medium-sized brown dwarf .

• In 2012, a hypothesis was published about the existence of the WASP-12 b 1 exoluna in the planet WASP-12 b . This conclusion is based on the characteristics of the star’s light curve during the passage of a planet through its disk ( transit detection method ). The estimated satellite size is 1/3 of the size of its planet or 6.4 of the size of the Earth ^{[6] [7]} .
• In 2014, the discovery using microlensing of the object MOA-2011-BLG-262L b was published - either a satellite from a freely floating giant planet, or a planet from a star - a red or brown dwarf ^{[8] [9]} .
• In 2017, it was announced that, in transit, the Kepler-1625 b I exoluna ^{[10] would} orbit around the planet Kepler-1625 b in the yellow dwarf Kepler-1625 system . In October 2017, it is planned to directly observe it with the Hubble Space Telescope , since its transit is expected at this time ^[11] .
• The presence in the system of the yellow dwarf WASP-49 of the planet WASP-49 b of gaseous sodium (Na I) at a distance of ~ 1.5-2 planet radii may indicate the presence in the orbit of this hot jupiter of a volcanically active exoloon the size of Jupiter’s moon Io ^[12] .

• Exoloons can already be seen with today's telescopes ( archive )
• Do exoluns often measure the size of the earth?
• 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)
• A periodic system of satellites of giant planets (rus.) Is proposed (Modeling the mass of satellites, Canup, R., Ward, W. (2006))
• A periodic satellite system (Rus.) Is proposed (Modeling the mass of satellites, Canup, R., Ward, W. (2006))
• The Hunt for Exomoons with Kepler (HEK): I. Description of a New Observational Project