Cold, orphaned, how many planets are "free-floating" between the stars? Host @NatureMagazine

Oct 07, 2018, 02:07 AM


(Photo:This Voyager 2 image shows Uranus's moon Puck. At 150 km in diameter, Puck is the largest of Uranus' lesser satellites, behind the five major satellites, all of which have diameters greater than 400 km. Image Credit: NASA )

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Cold, orphaned, how many planets are "free-floating" between the stars? Host @NatureMagazine

Planet formation theories predict that some planets may be ejected from their parent systems as result of dynamical interactions and other processes1, 2, 3. Unbound planets can also be formed through gravitational collapse, in a way similar to that in which stars form4. A handful of free-floating planetary-mass objects have been discovered by infrared surveys of young stellar clusters and star-forming regions5, 6 as well as wide-field surveys7, but these studies are incomplete8, 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 analysis11 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 theories3, 12 and surveys of young clusters8, 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 theories3, 12.

A rogue planet (also termed an interstellar planet, nomad planet, free-floating planet, orphan planet, wandering planet, starless planet, sunless planet, or Planemo) is a planetary-mass object that orbits the galaxy directly. Such objects have either been ejected from the planetary system in which they formed or have never been gravitationally bound to any star or brown dwarf.[1][2][3] The Milky Way alone may have billions of rogue planets.[4]

Some planetary-mass objects are thought to have formed in a similar way to stars, and the IAU has proposed that those objects be called sub-brown dwarfs.[5] A possible example is Cha 110913-773444, which might have been ejected and become a rogue planet, or otherwise formed on its own to become a sub-brown dwarf.[6] The closest free-floating planetary-mass object to Earth yet discovered, WISE 0855−0714, is at 7 light years, though it may be a sub-brown dwarf.[citation needed]

Recent observations of a very young free-floating planetary-mass object, OTS 44, with the Herschel Space Observatory and the Very Large Telescope demonstrate that the processes that characterize the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses. Herschel far-infrared observations show that this young free-floating planetary-mass object is surrounded by a disk of at least 10 Earth masses, and thus eventually, can form a mini planetary system.[7] Spectroscopic observations of OTS 44 with...