Details

Autor(en) / Beteiligte

• Mróz, Przemek
• Udalski, Andrzej
• Skowron, Jan
• Poleski, Radosław
• Kozłowski, Szymon
• Szymański, Michał K.
• Soszyński, Igor
• Wyrzykowski, Łukasz
• Pietrukowicz, Paweł
• Ulaczyk, Krzysztof
• Skowron, Dorota
• Pawlak, Michał

Titel

No large population of unbound or wide-orbit Jupiter-mass planets

Ist Teil von

• Nature (London), 2017-08, Vol.548 (7666), p.183-186

Ort / Verlag

London: Nature Publishing Group UK

Erscheinungsjahr

2017

Beschreibungen/Notizen

• In an analysis of a large sample of microlensing events, a few suggest the existence of Earth-mass free-floating planets, but only the expected number of Jupiter-mass free-floating objects were detected. No large population of unbound or wide-orbit Jupiter-mass planets Theories of planet formation predict that there should be a population of free-floating planets with masses ranging from less than to several times the mass of Earth. However, the theories do not predict a substantial population of unbound Jupiter-mass planets, the existence of which was surprisingly inferred from the results of a previous analysis of microlensing events. Przemek Mróz et al . have now analysed a much larger sample of microlensing events and place an upper limit, at two standard deviations, on the population of free-floating Jupiter-mass planets that is almost a factor of ten lower than the previous claim, thereby essentially ruling out the existence of this particular population. They do, however, see a few very short events, which, on the basis of the theories of planet formation, indicate the existence of free-floating Earth-mass planets. Planet formation theories predict that some planets may be ejected from their parent systems as result of dynamical interactions and other processes 1 , 2 , 3 . Unbound planets can also be formed through gravitational collapse, in a way similar to that in which stars form 4 . A handful of free-floating planetary-mass objects have been discovered by infrared surveys of young stellar clusters and star-forming regions 5 , 6 as well as wide-field surveys 7 , but these studies are incomplete 8 , 9 , 10 for objects below five Jupiter masses. Gravitational microlensing is the only method capable of exploring the entire population of free-floating planets down to Mars-mass objects, because the microlensing signal does not depend on the brightness of the lensing object. A characteristic timescale of microlensing events depends on the mass of the lens: the less massive the lens, the shorter the microlensing event. A previous analysis 11 of 474 microlensing events found an excess of ten very short events (1–2 days)—more than known stellar populations would suggest—indicating the existence of a large population of unbound or wide-orbit Jupiter-mass planets (reported to be almost twice as common as main-sequence stars). These results, however, do not match predictions of planet-formation theories 3 , 12 and surveys of young clusters 8 , 9 , 10 . Here we analyse a sample of microlensing events six times larger than that of ref. 11 discovered during the years 2010–15. Although our survey has very high sensitivity (detection efficiency) to short-timescale (1–2 days) microlensing events, we found no excess of events with timescales in this range, with a 95 per cent upper limit on the frequency of Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star. We detected a few possible ultrashort-timescale events (with timescales of less than half a day), which may indicate the existence of Earth-mass and super-Earth-mass free-floating planets, as predicted by planet-formation theories 3 , 12 .