Galaxy Full of Unbound Planets

A good supply of floating planets is not a surprise, nor even the numbers indicated which may well be quite conservative.  A star mass acts as a huge sponge for dust and material and also inevitably hooks up with at least one gas giant that then acts as the driver for inner solar system clean up and development.    Outside the influence of such a star mass, we will have gas giants forming that never ignite because they are simply too small.  The numbers of such objects should be well in excess of actual stars.

It also implies that actual planets passing through the solar system on an hyperbolic orbit is completely creditable and even probable.  This is a vindication of Velikovsky’s ideas relating to a planetary visitation in the recent past, although he was arguing for a capture.   A one time pass through would be swift and disruptive and we do need to model the impact properly because we need to understand just how survivable such an event would be.

I make the following conjecture.  The gas giants or at least the outer ones are all captured.  Jupiter may well be captured also, but also acts as our guardian of the inner solar system.  Please recall that Jupiter is at the edge of dynamic instability and will produce earth sized planets when sufficient material is sponged up.  It is possible that Venus is a recent calving of Jupiter as supported by the present circumstantial evidence.  This has been discussed in earlier posts.

The sheer volume of planets is now plausibly confirmed and we can assume that an order of magnitude of smaller planets also exist that we cannot spot.  However on the path to the nearest star, we are still unlikely to run into a planet.  There may only be a couple of gas giants inside a light year or two of Earth and perhaps ten to twenty smaller planets inside that same envelop.  We will have to go looking for them

Unbound planets could abound in the universe

May 19, 2011 

http://physicsworld.com/cws/article/news/46022

One of many lonely gas giants

Ten planets that appear to be drifting in interstellar space have been spotted by an international team of astronomers. The planets are so far from any host stars that they may not orbit a star at all, and could be drifting unbound through space. The team believes that such rogue planets could outnumber normal stars almost 2:1 and their existence could confirm computer simulations of solar-system formation.

More than 550 planets have so far been found beyond our solar system. The vast majority of these extrasolar planets – or exoplanets – have revealed themselves by their gravitational influence on their host star, or by the dip in brightness that they cause as they pass in front of their star. However, a clutch of 12 worlds had previously been found by gravitational micro-lensing.

This technique relies on the object of interest passing directly between the observer and a more distant background object. The mass of the foreground object acts like a lens and magnifies the light from the object beyond. If the foreground object is a star, then any orbiting planet leaves its own tell-tale fingerprint in the shape of the magnification. However, due to the need for an exact alignment, fewer than one in a million stars in the central part of the Milky Way are micro-lensed at any given time. This is why the number of exoplanets detected this way is low.

Sifting through 50 million stars

In an attempt to get around this problem the Microlensing Observations in Astrophysics (MOA) collaboration observes many stars at once. The new rogue planets were found in MOA observations of 50 million stars within the Milky Way between 2006 and 2007.

"Over all the stars observed we are very confident that we witnessed 474 definite lensing events," lead-author of the study Takahiro Sumi, of Osaka University, Japan, told physicsworld.com. Of these 474 events, 10 lasted for less than two days. Seven of these 10 events were later confirmed by data from the Optical Gravitational Lensing Experiment (OGLE) collaboration.

The more fleeting the duration of the event, the less massive the lensing object; a duration of less than two days implies the mass of the foreground object to be much less than that of a star. In fact, Sumi believes the culprits to be planets roughly the mass of Jupiter. What is more, no stars were observed within 10 astronomical units of the lensing objects – one astronomical unit is the distance between the Sun and the Earth and Saturn orbits at about 9 astronomical units. "There is a possibility that these planets do have a host star. However, direct imaging of exoplanets by other teams suggests that such distant planets are very rare," Sumi explains. "This led us to conclude that the lensing objects are freely floating planets, unbound from any star," he adds.

Because they are short-lived events, and the result of chance alignments, Sumi didn't expect to uncover such a high yield of planet-lensing events with MOA. From statistical analysis of his data he was able to extrapolate a figure for how common these free-floating planets might be. "We found that unbound planets, with roughly the mass of Jupiter, should be 1.8 times more common than the stars we observed," Sumi explains.

Scattered into space

The existence of rogue planets isn't completely unexpected: they have been predicted from computer models of solar-system formation. "We think they are formed in the same way as other planets but get scattered from the system by gravitational interactions between them," says Sumi. Joachim Wambsganss, of the University of Heidelberg, Germany, who was not involved in the work, says that this research quantifies this process for the first time. "We just didn't know how often this happened," he said. "This research gives us an idea," he adds.

Wambsganss went on to describe the research as using a "clear and solid method", however he thinks some people may not believe the claims of the rogue planets' abundance. "They used a very extensive statistical analysis, using several different factors, but others may argue with the numbers they used," he explains. One way of strengthening the research's claims will be to use the next stage of data from the MOA experiments. "There are three more years of data for 2008–2010 that they can work through in the same way. They should find more of these events and this will provide an even stronger statistical basis for their claims," he says.

The planets are described in Nature 473 349.

400 Billion wandering planets in the Milky Way

MAY 18, 2011

http://nextbigfuture.com/2011/05/400-billion-wandering-planets-in-milky.html

Free-floating planets may be more common in our Galaxy than stars.

BBC News - Japanese astronomers claim to have found free-floating "planets" which do not seem to orbit a star. They say they have found 10 Jupiter-sized objects which they could not connect to anysolar system. They also believe such objects could be as common as stars are throughout the Milky Way. Using a technique called gravitational microlensing, they detected 10 Jupiter-mass planets wandering far from light-giving stars. Then they estimated the total number of such rogue planets, based on detection efficiency, microlensing-event probability and the relative rate of lensing caused by stars or planets. They concluded that there could be as many as 400 billion of these wandering planets, far outnumbering main-sequence stars such as our Sun

Nature - Unbound or distant planetary mass population detected by gravitational microlensing

Nature - So many lonely planets with no star to guide them

Scattered about the Milky Way are floating, Jupiter-mass objects, which are likely to be planets wandering around the Galaxy's core instead of orbiting hoststars. But these planets aren't rare occurrences in the interstellar sea: the drifters might be nearly twice as numerous as the most common stars. 

Planetary scientist David Stevenson at the California Institute of Technology in Pasadena has considered how the temperatures on ejected planets might compare with those on star-bound bodies2. If Jupiter were kicked out of the Solar System, its surface temperature would drop by only about 15 kelvin, he says – although it would still be unsuitable for supporting life. However, "when you eject a planet that is quite massive, it could have carried along an orbiting body", Stevenson adds. "And that might be a more attractive possibility for life."

Unbound Earth-mass planets might still be capable of carrying liquid water, Stevenson says, even in the frozen reaches of interstellar space – as long as they have a heat-trapping hydrogen atmosphere. "That can bring the surface temperature up to 300 kelvin [about 27 °C]," he says. "And then you can have oceans."

Since 1995, more than 500 exoplanets have been detected using different techniques of which 12 were detected with gravitational microlensing. Most of these are gravitationally bound to their host stars. There is some evidence of free-floating planetary-mass objects in young star-forming regions but these objects are limited to massive objects of 3 to 15 Jupiter masses with large uncertainties in photometric mass estimates and their abundance. Here, we report the discovery of a population of unbound or distant Jupiter-mass objects, which are almost twice as common as main-sequence stars, based on two years of gravitational microlensing survey observations towards the Galactic Bulge.

These planetary-mass objects have no host stars that can be detected within about ten astronomical units by gravitational microlensing. However, a comparison with constraints from direct imaging suggests that most of these planetary-mass objects are not bound to any host star. An abrupt change in the mass function at about one Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.