Worlds Without Suns: Nomad Planets Could Number In The Quadrillions

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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.

Rogue Planets Could Drive By And Scoop Up Life

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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!

Read: Rogue Planets Can Find Homes Around Other Stars

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.

Rogue Planets Can Find Homes Around Other Stars

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.”

Read the team’s paper.

Source: CfA