Extrasolar moon

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An artistic impression of a hypothetical Earth-like moon orbiting a Saturn- like gas giant

An extrasolar moon, or exomoon for short , is a natural satellite that orbits a planet outside the solar system . It was assumed early on that there are not only exoplanets , but also extrasolar moons. Various detection methods can be used for detection: transit observations, the gravitational lensing effect or gaps in ring systems. So far, no extrasolar moon has been discovered with certainty. Speculations extend to considerations about habitability.

Possible occurrence

While almost 4000 exoplanets have already been detected, so far (as of July 2018) only a few possible candidates for an exomoon have been found, which, with one exception, are to be regarded as speculative. However, a look into the solar system, where (besides the earth) all four gas planets have massive moons, suggests that such moons, or even heavier ones, exist outside the solar system. Since the launch of the Kepler space telescope , detection has become possible, even if (as of July 2018) it has not yet been reliably detected.

Possible methods of discovery

Evidence through transit observations

In 1999, the then searching in France astronomer Paola Sartoretti and Jean Schneider suggested by varying the Exomonde Transit timing (English: transit timing variation TTV) to find. This effect results from the rolling motion of the planet, which is caused by the gravity of the moon in its orbit around the planet. More precisely, both planet and moon orbit the common center of gravity to a good approximation, neglecting other bodies. And so the deflection of the planet in front of the star observed from Earth varies for the assumption of strictly periodic transits. This TTV effect, it could be shown mathematically, allows conclusions to be drawn about the relationship between the mass of the moon and its distance from the planet. The solution of the equation is degenerate in the two parameters, that is, they cannot be determined independently of one another.

In a series of publications, the British astrophysicist David Kipping was able to prove that another effect of planetary transit enables the abolition of degeneracy. This second effect consists in the variation of the transit duration (English: Transit Duration Variation, TDV). On the one hand, it is caused by the varying tangential speed components of the planet: During each transit, the planet crosses the star disc at a different speed due to its orbit around the center of mass in the planet-moon system. On the other hand, an inclination of the planet-moon orbit towards the circumstellar orbit of the planet-moon system can ensure that the planet crosses the star disk at varying "heights". The way across the disk is therefore of different lengths for different transits and so it takes alternately sometimes shorter, sometimes longer.

Using a combination of TTV and TDV, the Kepler telescope should be able to detect moons down to a lower limit of one fifth of the mass of the earth.

Another detection method is the observation of the lunar transit itself. Only such a measurement allows the determination of the moon's radius (at least its ratio to the star's radius), which is of considerable importance for the confirmation and characterization of the moon.

The astronomical research project “The Hunt for Exomoons with Kepler” (HEK) at the Center for Astrophysics at Harvard is looking for signatures of exomondes in the Kepler data . According to a suggestion by Mary Anne Peters and Edwin Turner, exomonde could be subject to strong tidal heating and thus be directly observable with future technology.

Kepler-1625b

For already in 2017 considered possible Exomond to Kepler-1625b in October 2018 new analyzes were Kepler data and new observations of the Hubble Space Telescope released that actually one about neptune great companion this (about jupiter large, but possibly several Jupiter masses heavy) Suggest exoplanets (though not definitely prove).

Gravitational lensing effect

Artist's impression of the two possibilities: planet and moon (left) or brown dwarf and planet (right)

In April 2014, NASA announced that it had found a possible exomoon candidate using gravitational lensing. The observation data of the MOA-2011-BLG-262 system are consistent with a free-flying exoplanet , which is surrounded by an exomoon (which is likely to be slightly smaller than Earth) at a distance of about 0.13 AU (about 20 million kilometers) is circled. However, the data can also be explained with a system consisting of a brown dwarf and a Jupiter-like gas planet, so that there is no definitive discovery of an exomoon.

Gaps in ring systems

In a paper published in January 2015, a 56-day series of attenuations of the light of the young (approx. 16 million year old) star 1SWASP J140747.93-394542.6 observed in 2007 as a transitory transition of the ring system of a (not directly observed) substellar Object (exoplanet or brown dwarf) "J1407b" interpreted. This ring system is assigned a radius of approx. 90 million km (about 200 times the radius of Saturn's rings ), with 37 individual rings being recognizable, with a clear gap at a distance of about 60 million km from the planet. This, in turn, can be explained - analogously to the explanation of gaps in protoplanetary disks by planets - with a moon in the making with a mass of up to 0.8  earth masses .

Assumed characteristics

Since a discovery has not yet been made, one can only speculate about the properties of exomonde. One suspects a great variety of different moon types outside of the solar system, since the known moons are also very different. So it would be B. possible that around extrasolar gas giants moons that have an earth-like size orbit.

Habitability

An earth-sized exomoon could have earth-like characteristics if it is located together with its mother planet in the so-called habitable zone of the home star. A first publication on the possible occurrence of liquid water on moons, which astronomers and biologists consider to be a prerequisite for the development of life, was published in 1987 by Ray T. Reynolds and two colleagues. They proposed tidal heating within moons as an essential source of energy and, using the example of Jupiter's moon Europa , calculated how tides can melt the ice sheets of cold moons in the outer area of ​​a star system, at least underground. It was not until 10 years later, when the first gas planets had already been found outside the solar system, that US scientists turned back to the habitability of moons. In their article, Darren Williams and his co-authors found that a moon must be at least 10% to 20% of the earth's mass in order to be able to sustain plate tectonics and a strong magnetic field to deflect high-energy radiation and to bind a massive atmosphere for billions of years . It should be noted that the two heaviest moons in the solar system, Ganymede and Titan , have only about 2.5% and 2.3% Earth masses, respectively.

In a series of publications in 2012, the German astrophysicist René Heller and his American colleague Rory Barnes proposed a mathematical model that assesses the possibility of liquid surface water on moons on the basis of several physical influences. First, the stellar radiation, the reflected light of the planet, the thermal radiation of the planet and the tidal heating in the moon are added. The sum F s glob of global energy flow on the moon is then the critical energy flux F RG compared, the moon a galloping greenhouse effect (English runaway greenhouse effect would be) subject (which would in the course of the moon in the upper atmosphere, the hydrogen into space lose and dry out its oceans). If the sum of all average global energy flows F s glob is less than the critical energy flow F RG and if the moon and mother planet are in the stellar habitable zone, the moon is considered habitable.

If an extrasolar moon around a planet in the stellar habitable zone can be detected, the James Webb telescope , which is scheduled to start in 2021 , could reveal the presence or absence of life-induced spectral signatures in the lunar atmosphere.

Candidates

Web links

Commons : Extrasolar Moons  - Collection of images, videos and audio files

Individual evidence

  1. ↑ a b Faraway Moon or Faint Star? Possible Exomoon Found. NASA / JPL , April 10, 2014, accessed October 9, 2014 .
  2. ^ A b David M. Kipping, Stephen J. Fossey, Giammarco Campanella: On the detectability of habitable exomoons with Kepler-class photometry . In: Publications of the Astronomical Society of the Pacific . 109, 2009, pp. 1278-1284. bibcode : 2009MNRAS.400..398K . doi : 10.1111 / j.1365-2966.2009.15472.x .
  3. ^ Paola Sartoretti, Jean Schneider: On the detection of satellites of extrasolar planets with the method of transits . In: Astronomy and Astrophysics Supplement . 134, 1999, pp. 553-560. bibcode : 1999A & AS..134..553S . doi : 10.1051 / aas: 1999148 .
  4. ↑ David M. Kipping: Transit timing effects due to an exomoon . In: Monthly Notices of the Royal Astronomical Society . 392, 2009, pp. 181-189. bibcode : 2009MNRAS.392..181K . doi : 10.1111 / j.1365-2966.2008.13999.x .
  5. ↑ David M. Kipping: Transit timing effects due to an exomoon - II . In: Monthly Notices of the Royal Astronomical Society . 396, 2009, pp. 1797-1804. bibcode : 2009MNRAS.396.1797K . doi : 10.1111 / j.1365-2966.2009.14869.x .
  6. ↑ Luis Ricardo M. Tusnski, Adriana Valio: Transit Model of Planets with Moon and ring system . In: The Astrophysical Journal . 743, 2011, S. article id. 97, 9 pp. bibcode : 2011ApJ ... 743 ... 97T . doi : 10.1088 / 0004-637X / 743/1/97 .
  7. ^ András Pál: Light-curve modeling for mutual transits . In: Monthly Notices of the Royal Astronomical Society . 420, 2012, pp. 1630-1635. bibcode : 2012MNRAS.420.1630P . doi : 10.1111 / j.1365-2966.2011.20151.x .
  8. ^ David M. Kipping, Gáspár Á. Bakos, Lars A. Buchhave, David Nesvorný, Allan R. Schmitt: The Hunt for Exomoons with Kepler (HEK). I. Description of a New Observational Project . In: The Astrophysical Journal . 750, 2012, p. Id. 115, 19 pp. bibcode : 2012ApJ ... 750..115K . doi : 10.1088 / 0004-637X / 750/2/115 .
  9. ^ David M. Kipping, Joel Hartman, Lars A. Buchhave, Allan R. Schmitt, Gáspár Á. Bakos, David Nesvorný: The Hunt for Exomoons with Kepler (HEK). II. Analysis of Seven Viable Satellite Hosting Planet Candidates . In: The Astrophysical Journal . (submitted), 2013. bibcode : 2013arXiv1301.1853K .
  10. ↑ The Hunt for exomoons with Kepler (HEK). (No longer available online.) July 26, 2017, archived from the original on December 13, 2017 ; accessed on April 9, 2018 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / hek.astro.columbia.edu
  11. ↑ New Exomoon Project Will Use Kepler Data. At: centauri-dreams.org.
  12. ^ Forget Exoplanets - The Hunt for Exomoons Is Heating Up. At: time.com. Retrieved January 14, 2012.
  13. ^ Mary Anne Peters, Edwin L. Turner: On the Direct Imaging of Tidally Heated Exomoons . In: The Astrophysical Journal . 2013. arxiv : 1209.4418 . bibcode : 2012arXiv1209.4418P .
  14. ↑ Alex Teachey, David M. Kipping: Evidence for a large exomoon orbiting Kepler-1625b. Science Advances , October 3, 2018, accessed October 4, 2018 . doi : 10.1126 / sciadv.aav1784
  15. ↑ Did astronomers discover the first exomoon? - Gas giant Kepler 1625b most likely has a Neptune-sized satellite. scinexx , October 4, 2018, accessed October 4, 2018 .
  16. ↑ DP Bennett et al: A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge . December 13, 2013, arxiv : 1312.3951 .
  17. ^ Matthew A. Kenworthy, Eric E. Mamajek: Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons? January 22, 2015, arxiv : 1501.05652 (English).
  18. ↑ Exoplanet with gigantic ring system discovered. scinexx .de, January 27, 2015, accessed on January 27, 2015 .
  19. ^ Ray T. Reynolds, Christopher P. McKay, James F. Kasting: Europa, tidally heated oceans, and habitable zones around giant planets . In: Advances in Space Research . 7, 1987, pp. 125-132. bibcode : 1987AdSpR ... 7..125R . doi : 10.1016 / 0273-1177 (87) 90364-4 .
  20. ↑ Darren M. Williams, James F. Kasting, Richard A. Wade: Habitable moons around extrasolar giant planets . In: Nature . 385, 1997, pp. 234-236. bibcode : 1997Natur.385..234W . doi : 10.1038 / 385234a0 .
  21. ↑ René Heller: Exomoon habitability constrained by energy flux and orbital stability . In: Astronomy & Astrophysics . 545, 2012, p. Id. L8, 4 pp. bibcode : 2012A & A ... 545L ... 8H . doi : 10.1051 / 0004-6361 / 201220003 .
  22. ↑ René Heller, Rory Barnes: Exomoon habitability constrained by illumination and tidal heating . In: Mary Ann Liebert, Inc. (Ed.): Astrobiology . 13, No. 1, January 2013, pp. 18-46. arxiv : 1209.5323 . bibcode : 2012arXiv1209.5323H . doi : 10.1089 / ast.2012.0859 .
  23. ↑ René Heller, Rory Barnes: Constraints on the habitability of extrasolar moons . In: Proceedings to the XXVIII IAU General Assembly (2012, Beijing) . 2012. arxiv : 1210.5172 . bibcode : 2012arXiv1210.5172H .
  24. ↑ René Heller, Rory Barnes: Hot Moons and Cool Stars . In: Proceedings to the ROPACS meeting “Hot Planets and Cool Stars” (Nov. 2012, Garching) . 2012. arxiv : 1301.0235 . bibcode : 2013arXiv1301.0235H .
  25. ↑ Lisa Kaltenegger : Characterizing Habitable Exomoons . In: The Astrophysical Journal Letters . 712, No. 2, April 2010, pp. L125-L130. arxiv : 0912.3484 . bibcode : 2010ApJ ... 712L.125K . doi : 10.1088 / 2041-8205 / 712/2 / L125 .