Astronomers have found that tiny, round, dark clouds called globulettes have the right characteristics to form free-floating planets. The graph shows the spectrum of one of the globulettes taken at the 20-metre telescope at Onsala Space Observatory. Radio waves from molecules of carbon monoxide (13CO) give information on the mass and structure of these clouds. ESO/M. Mäkelä.
Free-floating rogue planets are intriguing objects. These planet-sized bodies adrift in interstellar space were predicted to exist in 1998, and since 2011 several orphan worlds have finally been detected. The leading theory on how these nomadic planets came to exist is that they were they ejected from their parent star system. But new research shows that there are places in interstellar space that might have the right conditions to form planets — with no parent star required.
This artist’s impression shows the free-floating planet CFBDSIR J214947.2-040308.9. Credit: ESO/L. Calçada/P. Delorme/Nick Risinger/R. Saito/VVV Consortium
Rogue planets – also known as free floating planets – are pretty intriguing. They are not orbiting a star but instead are wandering through the galaxy, having been either forcibly ejected from a solar system or having formed very early on in the Universe. While only a handful of these planets have been actually found, astronomers estimate these vagrant worlds could vastly outnumber stars. In fact, it’s been suggested there could be 100,000 times more rogue planets than stars in our Milky Way galaxy alone!
The latest rogue world to be found is exciting in that it is the closest such object to our Solar System so far. At a distance of about 100 light-years, its comparative proximity, along with the absence of a bright star very close to it, has allowed the team to study its atmosphere in great detail. Astronomers say this object gives them a preview of the exoplanets that future instruments will be able to find – and potentially take image of — around stars other than the Sun. But the planet also seems to be loosely tied to a roving group of stars, called the AB Doradus Moving Group.
The new rogue planet, with the ungainly name of CFBDSIR J214947.2-040308.9 (CFBDSIR2149 for short), was found using the Very Large Telescope and the Canada-France-Hawaii Telescope. The astronomers, led by Philippe Delorme from the Institut de planétologie et d’astrophysique de Grenoble, CNRS/Université Joseph Fourier, France, are calling the object a rogue planet candidate for now, as they want to study it further to confirm its free-floating status.
Moving star systems are equally intriguing. The AB Doradus Moving Group is the closest such group to our Solar System, and the stars drift through space together in a pack. They are thought to have formed at the same time. If the new rogue planet actually is associated with this moving group, astronomers say it will be possible to deduce much more about it, including its temperature, mass, and what its atmosphere is made of. There remains a small probability that the association with the moving group is by chance.
The link between the new object and the moving group is the vital clue that allows astronomers to find the age of the newly discovered object. Without knowing its age, it’s not possible to know whether it is really a planet, or a brown dwarf, a “failed” star that lack the bulk to trigger the reactions that make stars shine.
This is the first isolated planetary mass object ever identified in a moving group, and the association with this group makes it the most interesting free-floating planet candidate identified so far.
This closeup of an image captured by the SOFI instrument on ESO’s New Technology Telescope at the La Silla Observatory shows the free-floating planet CFBDSIR J214947.2-040308.9 in infrared light. This object, which appears as a faint blue dot at the centre of the picture, is the closest such object to the Solar System. Credit: ESO/P. Delorme.
“Looking for planets around their stars is akin to studying a firefly sitting one centimetre away from a distant, powerful car headlight,” said Delorme. “This nearby free-floating object offered the opportunity to study the firefly in detail without the dazzling lights of the car messing everything up.”
Free-floating objects like CFBDSIR2149 are thought to form either as normal planets that have been booted out of their home systems, or as lone objects like the smallest stars or brown dwarfs. In either case these objects are intriguing — either as planets without stars, or as the tiniest possible objects in a range spanning from the most massive stars to the smallest brown dwarfs.
“These objects are important, as they can either help us understand more about how planets may be ejected from planetary systems, or how very light objects can arise from the star formation process,” says Philippe Delorme. “If this little object is a planet that has been ejected from its native system, it conjures up the striking image of orphaned worlds, drifting in the emptiness of space.”
If CFBDSIR2149 is not associated with the AB Doradus Moving Group, the astronomers say it is trickier to be sure of its nature and properties, and it may instead be characterized as a small brown dwarf. Both scenarios represent important questions about how planets and stars form and behave.
“Further work should confirm CFBDSIR2149 as a free-floating planet,” said Delorme. “This object could be used as a benchmark for understanding the physics of any similar exoplanets that are discovered by future special high-contrast imaging systems, including the SPHERE instrument that will be installed on the VLT.”
This video shows an artist’s impression of the free-floating planet CFBDSIR J214947.2-040308.9. In the first part of the sequence the planet appears as a dark disc in visible light, silhouetted against the star clouds of the Milky Way. This is the closest such object to the Solar System and the most exciting candidate free-floating planet found so far. It does not orbit a star and hence does not shine by reflected light; the faint glow it emits can only be detected in infrared light. In the final sequence we see an infrared view of the object with the central parts of the Milky Way as seen by the VISTA infrared survey telescope as background. The object appears blueish in this near-infrared view because much of the light at longer infrared wavelengths is absorbed by methane and other molecules in the planet’s atmosphere. In visible light the object is so cool that it would only shine dimly with a deep red colour when seen close-up.
Artist's concept of a free-floating Jupiter-like planet. (NASA / JPL-Caltech)
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The concept of nomad planets has been featured before here on Universe Today, and for good reason. Not only is the idea of mysterious lone planets drifting sunless through interstellar space an intriguing one, but also the sheer potential quantity of such worlds is simply staggering. If some very well-respected scientists’ calculations are correct there are more nomad planets in our Milky Way galaxy than there are stars — a lot more. With estimates up to 100,000 nomad planets for every star in the galaxy, there could be literally quadrillions of wandering worlds out there, ranging in size from Pluto-sized to even larger than Jupiter.
That’s a lot of nomads. But where did they all come from?
Recently, The Kavli Foundation had a discussion with several scientists involved in nomad planet research. Roger D. Blandford, Director of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University, Dimitar D. Sasselov, Professor of Astronomy at Harvard University and Louis E. Strigari, Research Associate at KIPAC and the SLAC National Accelerator Laboratory talked about their findings and what sort of worlds these nomad planets might be, as well as how they may have formed.
One potential source for nomad planets is forceful ejection from solar systems.
“Most stars form in clusters, and around many stars there are protoplanetary disks of gas and dust in which planets form and then potentially get ejected in various ways,” said Strigari. “If these early-forming solar systems have a large number of planets down to the mass of Pluto, you can imagine that exchanges could be frequent.”
And the possibility of planetary formation outside of stellar disks is not entirely ruled out by the researchers — although they do impose a lower limit to the size of such worlds.
“Theoretical calculations say that probably the lowest-mass nomad planet that can form by that process is something around the mass of Jupiter,” said Strigari. “So we don’t expect that planets smaller than that are going to form independent of a developing solar system.”
“This is the big mystery that surrounds this new paper. How do these smaller nomad planets form?” Sasselov added.
Of course, without a sun of their own to supply heat and energy one might assume such worlds would be cold and inhospitable to life. But, as the researchers point out, that may not always be the case. A nomad planet’s internal heat could supply the necessary energy to fuel the emergence of life… or at least keep it going.
“If you imagine the Earth as it is today becoming a nomad planet… life on Earth is not going to cease,” said Sasselov. “That we know. It’s not even speculation at this point. …scientists already have identified a large number of microbes and even two types of nematodes that survive entirely on the heat that comes from inside the Earth.”
Researcher Roger Blandford also suggested that “small nomad planets could retain very dense, high-pressure ‘blankets’ around them. These could conceivably include molecular hydrogen atmospheres or possibly surface ice that would trap a lot of heat. They might be able to keep water liquid, which would be conducive to creating or sustaining life.”
And so with all these potentially life-sustaining planets knocking about the galaxy, is it possible that they could have helped transport organisms from one solar system to another? It’s a concept called panspermia, and it’s been around since at least the 5th century BCE when the Greek philosopher Anaxagoras first wrote about it. (We’ve written about it too, as recently as three weeks ago, and it’s still a much-debated topic.)
“In the 20th century, many eminent scientists have entertained the speculation that life propagated either in a directed, random or malicious way throughout the galaxy,” said Blandford. “One thing that I think modern astronomy might add to that is clear evidence that many galaxies collide and spray material out into intergalactic space. So life can propagate between galaxies too, in principle.
There could be quadrillions of nomad planets in our galaxy alone -- and they could even be ejected into intergalactic space. (Image: ESO/S.Brunier)
“And so it’s a very old speculation, but it’s a perfectly reasonable idea and one that is becoming more accessible to scientific investigation.”
Nomad planets may not even be limited to the confines of the Milky Way. Given enough of a push, they could be sent out of the galaxy entirely.
“Just a stellar or black hole encounter within the galaxy can, in principle, give a planet the escape velocity it needs to be ejected from the galaxy. If you look at galaxies at large, collisions between them leads a lot of material being cast out into intergalactic space,” Blandford said.
The discussion is a fascinating one and can be found in its entirety on The Kavli Foundation’s site here, and watch a recorded interview between Louis Strigari and journalist Bruce Lieberman here.
The Kavli Foundation, based in Oxnard, California, is dedicated to the goals of advancing science for the benefit of humanity and promoting increased public understanding and support for scientists and their work.
Artist's rendering of an Earth-sized rogue planet approaching a star. Credit: Christine Pulliam (CfA)
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Free-floating “rogue” planets may occasionally dip into the inner Solar System, picking up dust containing organic compounds — a.k.a. the ingredients for life — and carry it back out into the galaxy, according to new research by Professor Chandra Wickramasinghe, Director of the University of Buckingham Centre for Astrobiology in the UK.
Rogue planets are thus called because they are not in orbit around a star. Either forcibly ejected from a solar system or having formed very early on in the Universe — even within a few million years after the Big Bang, the team proposes — these vagrant worlds may vastly outnumber stars. In fact, it’s been suggested there are as much as 100,000 times more rogue planets than stars in our Milky Way galaxy alone!
Professor Wickramasinghe — a proponent of the panspermia hypothesis whereby the ingredients for life can be transported throughout the galaxy on dust, comets, and perhaps even planets — and his team have suggested in a paper published in the journal Astrophysics and Space Science that Earth-sized rogue planets could pass through the inner Solar System, possibly as often as once every 25 million years on average. Like a cosmic drive-thru these planets could gather zodiacal dust from the plane of the Solar System during their pass, thus picking up organic compounds along the way.
The planets would then take the material gathered from one solar system and possibly bring it into another, serving as a type of interstellar cross-pollinator.
Wickramasinghe’s team propose that, by this process, there could be more life-bearing, Earth-sized planets existing between the stars than orbiting around them — a lot more. Based on their estimates there may be as much as a few hundred thousand billion such worlds in our galaxy… that’s several thousand for every star.
It will be interesting to see how this idea is received, but it definitely is an intriguing concept. As we hunt for the “Holy Grail” of life-friendly exoplanets around other stars, they may be drifting through the darkness in number, hiding in the spaces between.
In this artist's conception, a rogue planet drifts through space. Credit: Christine Pulliam (CfA)
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As crazy as it sounds, free-floating rogue planets have been predicted to exist for quite some time and just last year, in May 2011, several orphan worlds were finally detected. Then, earlier this year, astronomers estimated that there could be 100,000 times more rogue planets in the Milky Way than stars. Now, the latest research suggests that sometimes, these rogue, nomadic worlds can find a new home by being captured into orbit around other stars. Scientists say this finding could explain the existence of some planets that orbit surprisingly far from their stars, and even the existence of a double-planet system.
“Stars trade planets just like baseball teams trade players,” said Hagai Perets of the Harvard-Smithsonian Center for Astrophysics.
Astronomers now understand that rogue planets are a natural consequence of both star and planetary formation. Newborn star systems often contain multiple planets, and if two planets interact, one can be ejected in a form of planetary billiards, kicked out of the star system to become an interstellar traveler.
But later, if a rogue planet encounters a different star moving in the same direction at the same speed, be captured into orbit around that star, say Perets and Thijs Kouwenhoven of Peking University, China, the authors of a new paper in The Astrophysical Journal.
A captured planet tends to end up hundreds or thousands of times farther from its star than Earth is from the Sun. It’s also likely to have a, orbit that’s tilted relative to any native planets, and may even revolve around its star backward.
Perets and Kouwenhoven simulated young star clusters containing free-floating planets. They found that if the number of rogue planets equaled the number of stars, then 3 to 6 percent of the stars would grab a planet over time. The more massive a star, the more likely it is to snag a planet drifting by.
While there haven’t actually been planets found yet that are definitely a ‘captured’ world, the best bet would perhaps be a planet in a distant orbit around a low-mass star. The star’s disk wouldn’t contain enough material to form a planet that distant, Perets and Kouwenhoven said.
The best evidence of a captured planet comes from the European Southern Observatory, which announced in 2006 the discovery of two planets (weighing 14 and 7 times Jupiter) orbiting each other without a star.
“The rogue double-planet system is the closest thing we have to a ‘smoking gun’ right now,” said Perets. “To get more proof, we’ll have to build up statistics by studying a lot of planetary systems.”
As for our own solar system, there’s no evidence at this time that our Sun could have captured an alien world, which would lie far beyond Pluto.
“There’s no evidence that the Sun captured a planet,” said Perets. “We can rule out large planets. But there’s a non-zero chance that a small world might lurk on the fringes of our solar system.”
