Ask an astronomer how many stars populate the Milky Way, and the usual answer will be 200 to 400 billion. It's not that all those suns have actually been counted; instead, it's a statistical guesstimate based on the census in our immediate interstellar surroundings.
But a new study, published in today's issue of Nature, suggests that a complete census of "big bodies" drifting loose in our galaxy might actually total nearly one trillion — because Jupiter-mass "planets" in interstellar space might well outnumber the stars themselves.
The evidence for this sudden glut of planet-mass objects comes from a dedicated search by two teams of observers: the Microlensing Observations in Astrophysics (MOA) Collaboration and the Optical Gravitational Lensing Experiment (OGLE) Collaboration.
In 2006-07, the MOA and OGLE teams used telescopes in New Zealand and Chile, respectively, to monitor the brightnesses of 50 million stars located in the huge stellar bulge surrounding the Milky Way's center. Led by Takahiro Sumi (Osaka University), observers used these facilities to record the brightness of each star at least once per hour. After boiling down all that data, the teams found that 474 stars had briefly surged in brightness in a way that indicated gravitational lensing of their light by unseen foreground objects passing nearly front of them. During these incidental syzygies, the gravity of the foreground object bends and concentrates the light from the background star — an event known as microlensing.
Microlensing searches aren't new: they've long been used to search for massive dim or dark objects in the galaxy. But the MOA and OGLE teams found that 10 of these little surges lasted less than two days — too short to be caused by stars but just right for Jupiter-mass objects. Based on these statistics, the teams estimate that big planets must be far more common than believed and in fact must outnumber all the Milky Way's normal stars by about two to one.
Surprisingly, during these 10 brief events there were no corresponding lensing surges to betray the presence of nearby stars. So the observers conclude that these "Jupiters" must either be at least 10 astronomical units from their host stars (at least Saturn's distance from the Sun), or they are orphans drifting freely across interstellar space. They're more likely to be free-floaters, because previous direct-imaging searches found that giant planets rarely exist in very wide orbits.
"The implications of this discovery are profound," notes lensing specialist Joachim Wambsganss (Heidelberg University) in an accompanying Nature perspective.
Theorists are chuckling, "We told you so!" They've argued for years that the galaxy should teem with unbound planets. Some have proposed that objects with masses almost as low as Jupiter's form the way normal stars do, directly from collapsing clouds of gas and dust. Think of these as undersized brown dwarfs. Others point out that the chaos that seems to prevail in many just-formed solar systems must cause many close encounters among planets that yield "winners" (those that remain in orbit) and "losers" (those that get flung out of the system entirely).
During the 1990s, while studying gravitational lensing of the distant quasar Q0957+561, Rudolf Schild (Harvard-Smithsonian Center for Astrophysics) inadvertently picked up brief brightness blips. Wesley Colley (University of Virginia) and Schild eventually concluded that these little flashes resulted from lensing by free-floating substellar objects (ranging down to an Earth's mass) in the intervening galaxy, roughly a billion miles away.
Later, Schild and others suggested that Mikly Way and other galaxies contains enough "micro brown dwarfs," nearly a billion trillion in each, to provide the long-sought missing mass (dark matter). But others counter that there's no way these unseen bodies can exist in such vast numbers.
In any case, the MOA-OGLE statistics imply that most of the loose planet-mass objects aren't low-mass stellar wannabes. Instead, the researchers believe they're finding bodies that have been ejected from unstable planetary families — and, by extension, that planetary systems should be the norm, not the exception, for the Milky Way's hundreds of billions of stars.
This also implies that early chaos in planetary systems is common. Exoplanet researchers had already concluded that this is the case from the large number of explanets that have been left in highly eccentric orbits, which they could not have formed with.