POTSDAM, Germany, July 20 -- Cosmic vagabonds -- dark, free-floating planets that do not orbit stars -- may be wandering around the galaxy invisible and undetectable except by using a new method called gravitational lensing, according to a German astrophysicist.
"We expect that there should exist a free-floating population of Jupiter-like or even Earth-like planets in interstellar space," Hans Zinnecker, head of the star formation group at the Potsdam Astrophysical Institute, told United Press International.
Astronomer Stephen Maran, of the American Astronomical Society in Washington, agreed. "Astronomers are beginning to realize that there may indeed be free-floating planets out in the Milky Way," he said. Planets are likely to form around individual stars in very wide binary systems -- young, low-mass stars orbiting one another as pairs, Zinnecker explained. "At the same time, a good fraction of planets born in binary systems will, in the long run, be subject to ejection into space due to gravitational perturbations."
In other words, these planets are freed from their suns. Finding them is especially tricky.
Planets outside our solar system are too small and too dark to be seen using telescopes, even those that orbit stars, which obscure nearby objects with their own bright light.
Astronomers detect far away planets nonetheless. By carefully measuring how starlight is shifted in frequency by the gravitational fields of invisible objects, such as orbiting planets, astronomers are able to deduce the presence of those planets. Without a nearby star, however, the process of finding a planet might appear to be hopeless.
Not so, according to Zinnecker, who said, "There is hope to detect free-floating planets through gravitational micro-lensing observations." In simple terms, this involves the force of gravity as a light-bending lens. The first confirmation of Einstein's famous theory of gravity, general relativity, came from the gravitational lens effect of the sun, which bent the light from a nearby star as it passed through the sun's gravitational field. The same technique may be used to find free-floating planets. Light from background stars should be slightly amplified as it passes through the gravity of these massive, hidden objects.
"In fact, gravitational lensing is being explored to find planet-sized objects," Alexander Wolszczan, the Evan Pugh professor of astronomy and astrophysics at Penn State University, told United Press International. "It's very exciting and very promising."
Wolszczan said recent work by Timothy Brown and the High Altitude Observatory in Boulder, Colo., shows great promise in the search for free-floating, extra solar planets.
"By using the background light from dense globular clusters, which are groups of hundreds of thousands of stars, one can hypothetically see gravitational lensing effects caused by planet-sized objects in front of those clusters," Wolszczan said.
Zinnecker said larger detectors will have to be constructed to see such effects.
"New larger detector arrays would help -- to catch many stars at once in one image," Zinnecker said. "Sharper images are needed due to the crowding of background stars towards the center of the galaxy, where one must look in order to sample the whole galactic disk," where the planetary population is highest.
"Within the next few years the project should be doable, with the next generation telescope" in the United States, Zinnecker added. While dark planets may be exciting discoveries, they will not solve the deepest mysteries of the universe, such as the nature of dark matter, an elusive substance physicists are convinced must exist to explain the orbital paths of certain galaxies.
"Dark planets cannot be the ubiquitous dark matter that cosmologists talk about," Zinnecker told UPI. "The total mass in these free-floating planets is too insignificant."
Zinnecker said he is driven to pursue these objects for a far simpler reason than solving difficult cosmological puzzles: "As a scientist and as a human being, I am curious to know what is out there drifting in space."
