This artist's conception illustrates a Jupiter-like planet alone in the dark of space, floating freely without a parent star. Astronomers recently uncovered evidence for 10 such lone worlds, thought to have been "booted," or ejected, from developing solar systems.

The planet survey, called the Microlensing Observations in Astrophysics (MOA), scanned the central bulge of our Milky Way galaxy from 2006 to 2007. It used a 5.9-foot (1.8-meter) telescope at Mount John University Observatory in New Zealand, and a technique called gravitational microlensing. In this method, a planet-sized body is identified indirectly as it just happens to pass in front of a more distant star, causing the star to brighten. The effect is like a cosmic funhouse mirror, or magnifying lens--light from the background star is warped and amplified, becoming brighter.

Based on these results, astronomers estimate that free-floating worlds are more common than stars in our Milky Way galaxy, and perhaps other galaxies too.

Credit: NASA/JPL-Caltech

This artist's animation illustrates the technique used for finding free-floating, Jupiter-mass planets in space. Astronomers found evidence for 10 of these worlds, thought to have been ejected early on from their developing solar systems.

The movie begins by showing the busy, central region of our Milky Way galaxy, where the planets were found with a ground-based telescope. It then zooms in on a star that brightens. This brightening is due to the passage of an unseen, free-floating planet (and has been exaggerated here). As a planet just happens to cross in front of a more distant star, its gravity causes the starlight to warp, and this warping resulted in an overall brightening of the star as seen by the telescope. In this effect, called gravitational microlensing, the planet's gravity plays the role of a magnifying lens.

The next part of the animation shows a zoomed in view of what the microlensing of a star would look like if it could be seen at much higher resolution. The blue dot represents the planet, but has been enlarged to make it easy to see. The main star is the brightest dot in the center, shown amidst other smaller, red and yellow stars. As the planet passes by, its gravity causes light from the stars to split into multiple, mirrored and reversed images. When the planet is directly in front of the main star, that star's multiple images are stretched into arcs. The overall result is a temporary brightening of the star.

Astronomers refer to the circular shape that can be seen as the planet passes by the stars as an Einstein ring. When a planet is directly in front of star, it will cause the starlight to bend into a full Einstein ring. When the planet is near stars, it will cause the star images to either appear deflected away from the ring, or inverted and reversed within the ring.

The duration of the microlensing event will reveal the rough mass of the passing body. Jupiter-mass objects will cause a star to brighten more quickly, over just a day or two. A passing star would cause a more distant star to brighten over a period of weeks.

The overall density of stars, as well as the brightness of their inverted images within the Einstein ring, have been exaggerated in this animation to help show the effects of the gravitational lensing. It is very rare for one passing planet to distort the light from multiple stars at once.

The movie ends with an artist's conception of a free-floating, Jupiter-mass world.

The gravitational microlensing shown is based on simulation data from M. Freeman (University of Auckland, New Zealand).

Credit: NASA/JPL-Caltech