Milky way may swarm with nomad planets

Figure : This image is an artistic rendition of a nomad object wandering the interstellar medium. The object is intentionally blurry to represent uncertainty about whether it has an atmosphere. A nomadic object may be an icy body akin to an object found in the outer solar system, a more rocky material akin to asteroids, or even a gas giant similar in composition to the most massive solar system planets and exoplanets.

By Stanford University
Published: February 24, 2012

Our galaxy may be awash in homeless planets, wandering through space instead of orbiting a star. In fact, there may be 100,000 times more nomad planets in the Milky Way than stars, according to a new study by researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) in Stanford, California.

If observations confirm the estimate, this new class of celestial objects will affect current theories of planet formation and could change our understanding of the origin and abundance of life.

“If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist,” said Louis Strigari from KIPAC. Although nomad planets don’t bask in the warmth of a star, they may generate heat through internal radioactive decay and tectonic activity.

Searches over the past two decades have identified more than 500 planets outside our solar system, almost all of which orbit stars. Last year, researchers detected about a dozen nomad planets, using a technique called gravitational microlensing, which looks for stars whose light is momentarily refocused by the gravity of passing planets.

The research produced evidence that roughly two nomads exist for every typical, main sequence star in our galaxy. The new study estimates that nomads may be up to 50,000 times more common than that.

To arrive at what Strigari called “an astronomical number,” the KIPAC team took into account the known gravitational pull of the Milky Way Galaxy, the amount of matter available to make such objects, and how that matter might divvy itself up into objects ranging from the size of Pluto to larger than Jupiter. Not an easy task, considering no one is quite sure how these bodies form. According to Strigari, some were probably ejected from solar systems, but research indicates that not all of them could have formed in that fashion.

“To paraphrase Dorothy from The Wizard of Oz, if correct, this extrapolation implies that we are not in Kansas anymore, and in fact we never were in Kansas,” said Alan Boss from the Carnegie Institution for Science in Washington, D.C. “The universe is riddled with unseen planetary-mass objects that we are just now able to detect.”

A good count, especially of the smaller objects, will have to wait for the next generation of big survey telescopes, especially the space-based Wide-Field Infrared Survey Telescope and the ground-based Large Synoptic Survey Telescope, both set to begin operation in the early 2020s.

A confirmation of the estimate could lend credence to another possibility mentioned in the paper — that as nomad planets roam their starry pastures, collisions could scatter their microbial flocks to seed life elsewhere.

“Few areas of science have excited as much popular and professional interest in recent times as the prevalence of life in the universe,” said Roger Blandford from KIPAC. “What is wonderful is that we can now start to address this question quantitatively by seeking more of these erstwhile planets and asteroids wandering through interstellar space, and then speculate about hitchhiking bugs.”

Latest Findings on Moon’s Impact History

Figure: Post-lunar cataclysm diagram of our solar system.
Credit: LPI/Marchi/Bottke/Kring/Morbidelli

Published : February 28, 2012
By : NASA's Ames Research Center in Moffett Field, California

During Earth’s earliest days, our planet and other bodies in the inner solar system, including the Moon, experienced repeated impacts from debris that formed the building blocks of the planets. Over time, as material was swept up and incorporated into the inner planets, the rate of impacts decreased. Then, roughly 4 billion years ago, a second wave of impacts appears to have taken place, with lunar projectiles hitting at much higher speeds. This increase could reflect the origin of the debris where main belt asteroids were dislodged and sent into the inner solar system by shifts in the orbits of the giant planets.

A team of researchers from the NASA Lunar Science Institute (NLSI) at NASA’s Ames Research Center in Moffett Field, California, has discovered that debris that caused a “lunar cataclysm” on the Moon 4 billion years ago struck it at much higher speeds than those that made the most ancient craters. The scientists found evidence supporting this scenario by examining the history of crater formation on the Moon.

Scientists analyzed digital maps of the lunar surface to learn about its history. Their analysis shows that craters formed near the 533-mile-diameter (860 kilometers) Nectaris impact basin were created by projectiles hitting twice as fast as those found on more ancient terrains. This was represented by a subtle shift in crater sizes, with the craters themselves 30 to 40 percent larger on average than those found in comparable populations with older craters. The scientists believe this can be best explained by an increase in the velocities of the projectiles that produced the younger craters.

The increase in velocities may indicate a change in the solar system when the craters were created. The analysis supports the lunar cataclysm hypothesis that the brief pulse of impacting objects 4 billion years ago was due to gravitational disturbances caused by the reorganization of the giant planets as their orbits changed. Nectaris, a crater close to the Apollo 16 landing site, appears to have recorded the spike in asteroid impacts during the lunar cataclysm.

Determining the magnitude and duration of any impact cataclysm and testing that hypothesis is a top science priority for future exploration of the Moon, according to a previously published report by the National Research Council.

When Apollo astronauts gathered rock samples from the Moon, many samples had ages dating back 3.9 to 4 billion years ago, suggesting an enhanced pulse of bombardment. If a bombardment of asteroids hit the Moon as theorized, there could be indicators left on the lunar surface that would help validate the theory. Detailed mapping by the United States Geological Survey has previously identified small regions of the lunar surface that might contain clues about the bombardment. The team re-studied those ancient surfaces and measured the sizes of the impact craters using new data obtained from the Lunar Orbiter Laser Altimeter, an instrument on NASA’s Lunar Reconnaissance Orbiter (LRO) currently orbiting around the Moon.

“This is an exciting time for lunar research with LRO and other spacecraft providing so much new data,” said Simone Marchi from NLSI. “Collaborating with scientists of different disciplines allowed us to link these observational data to dynamical models to put new constraints on solar system history.”

The inferred increase in velocity seems to have occurred after the Moon’s 1,550-mile-diameter (2,500km) South Pole-Aitken Basin was produced, but before the formation of the largest lava-filled impact basins on the lunar nearside, visible from backyards around the world.

“It is fascinating that the surface of our own Moon records evidence of orbital changes in Jupiter and Saturn that took place so long ago,” said Yvonne Pendleton from NLSI.