Exomoon

An extrasolar moon, short Exomond, is a natural satellite that orbits a planet outside our solar system.

While so far over 1000 exoplanets have been confirmed, the find an extrasolar moon is still pending. The view of the solar system where, for example, the giant planets Jupiter and Saturn (and Earth ) have massive moons, suggesting that such or even more serious moons exist outside the solar system. Since the launch of the space telescope Kepler their detection has become possible, although not yet succeeded.

Detection and Characterization

In 1999, the then searching in France astronomers Sartoretti Paola and Jean Schneider suggested Exomonde on the variation of the transit time ( English: " Transit Timing Variation ," TTV ) to find. This effect is due to the rolling movement of the planet which is caused by the gravitational pull of the moon on its orbit around the planet. Specifically, both circle - Planet and Moon - neglecting other body in good approximation the common center of mass. And so varied, the observed from Earth deflection of the planet from the star for the adoption of strictly periodic transit. This TTV effect, as was demonstrated mathematically, can conclusions on the ratio of the mass of the moon to its distance from the planet. The solution of the equation is degenerate in the two parameters, that is, they can not be determined independently.

In a series of publications of the British astrophysicist David Kipping could prove that another effect of the planetary transits allows the lifting of the degeneracy. This second effect is the variation of the transit time (English: "Transit Duration Variation", TDV ). Firstly, it is caused by the varying tangential velocity component of the planet during each transit crosses the planet due to its orbit around the center of mass in the planet - moon system, the stellar disk at a different speed. On the other hand, a tilt of the planet Moon orbits against the circumstellar orbit of the planet - moon system to ensure that the planet crosses the stellar disk in varying " amount ". The path on the disk is thus different lengths for various transit and so takes that alternately sometimes shorter, sometimes longer.

The Kepler telescope is able to detect up to a lower limit of one-fifth the mass of the Earth from a combination of TTV and TDV moons.

Another detection method is the observation of the moon transits itself only such a measurement allows the determination of the lunar radius ( at least his relationship with the stellar radius ), which is for the confirmation and characterization of the moon is of considerable importance.

The astronomical research project " The Hunt for exomoons with Kepler " (HEK ) at the Center for Astrophysics at Harvard searched for signatures of Exomonden in the Kepler data. At the suggestion of Mary Anne Peters and Edwin Turner could Exomonde strong tidal heating subject and be directly observable with future technology.

Assumed characteristics

As a discovery so far is still pending, one can only speculate about the properties of Exomonden. But one suspects a large variety of different types of moon outside our solar system, as well as our known moons are very different. So it would be possible, for example, that to extrasolar gas giant moons circling that have a Earth-like size.

Habitability

An Earth-sized Exomond could have Earth-like characteristics when it is in the so-called habitable zone of the star's home along with his mother planet. A first publication on the possible occurrence of liquid water on moons, which consider astronomers and biologists as a prerequisite for the emergence of life, offered in 1987, Ray T. Reynolds and two colleagues. They suggested this tidal heating within moons as an essential source of energy before and calculated by the example of Jupiter 's moon Europa, as the tide ice sheet cold moons in the outer region of a star system can at least melt underground. It was not until 10 years later, then the first gas planets were already found outside the solar system, to U.S. scientists of habitability of moons used again. In her article found Darren Williams and his co-authors, that a moon at least 10 % to 20 % of Earth's mass must be hard, that he maintained over billions of years, plate tectonics and a strong magnetic field for deflection of high-energy radiation and a massive atmosphere itself can bind. It should be noted that the two heaviest solar system moons, Ganymede and Titan, only about 2.5 % and 2.3 % Earth masses are heavy.

In a series of publications of the German astrophysicist René Heller and his American colleague Rory Barnes hit in 2012 against a mathematical model, which evaluates the ability of liquid surface water on moons basis of several physical influences. First, the stellar radiation, the reflected light of the planet, the thermal radiation of the planet, as well as the tidal heating will be added in the moon. The sum Fsglob of global energy flow on the moon is then compared with the critical energy flow FRG, the moon a catastrophic greenhouse effect (English: "Runaway Greenhouse " ) would subject ( in the course of the moon would lose in its upper atmosphere the hydrogen into space and its oceans thus dry out ). If the sum of all the average global energy flows Fsglob less than the critical energy flow FRG and the moon is including mother planet in the stellar habitable zone, then the moon is considered to be habitable.

If the evidence of an extrasolar moon around a planet in the stellar habitable zone succeed, the James Webb telescope could whose launch is scheduled for 2018, evoked the presence or absence of for life, provide spectral signatures in the lunar atmosphere.

Satellite brown dwarfs

Any moons around brown dwarfs blur the boundaries between the moon and the planet. According to their definition, moons move around planets, and planets move around stars. But a brown dwarf is neither a planet nor a star.