Back in 2012, a team of leading astrophysicists reported on Arxiv that 100,000 rogue, free-floating planets might exists for for every star in the Milky Way, which could put the number of nomad planets in the quadrillions. They suggested that it could be that nomad planets play a dynamic role in spreading life throughout universe: if life can exist without the warmth of a nearby sun, it raises the possibility that, along with sustaining life, nomad planets could be transporting life as well.
In an interview with the distinguished team members at the Kavli Foundation at Stanford University, astrophysicist Louis Strigari, currently with Texas A& M University, was asked that in we’re in a spacecraft looking for unexplored planets in the Milky Way Galaxy, how often are we going to be running into nomad planets as we travel from star to star? “We still don’t have a great inventory of how many nomad planets there are in our galaxy, at the very small scales,” he relied. “There could be 105 nomad planets greater than say, the mass of Pluto, per star. But I think you have to realize that interstellar space is vast. Much of it is empty, as we perceive on a human scale. Still, it’s possible that maybe within a light year of us going out toward the nearest star, there could very easily be a few of these planets floating around for us to find.”
“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 Roger D. Blandford, co-author of the study and director of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University and the SLAC National Accelerator Laboratory. “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.”
“I’m really curious about the exchange of planets between solar systems,” said Strigari, lead author of the study and at the time a research associate at KIPAC and the SLAC, ” How often does it happen, and how far can a nomad planet travel? How many trips around our galaxy does it make? I think these are brand new, basic questions. And I think that’s an exciting place to be.”
Imagining Earth as a Nomad Planet
“If you imagine the Earth as it is today becoming a nomad planet… life on Earth is not going to cease,” said Dimitar D. Sasselov, part of the Kavli interview team and Professor of Astronomy at Harvard University and the Harvard-Smithsonian Center for Astrophysics, and the Director of the Harvard Origins of Life Initiative. “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.”
“Based on what we know about how planetary systems form, we would expect a lot of the smaller nomad planets to be ejected into interstellar space, due to close encounters with planets the size of Jupiter and Saturn and sometimes even binary stars in the original protoplanetary disk,” continued Sasselov. “So, it is almost a given that the galaxy should have a lot of free-floating small nomad planets the size of Pluto and smaller.”
Nomad planets, by definition, are cast into interstellar space unbound to any star. “You could say this has happened in our own solar system with small objects, typically comets, which pass close to Jupiter,” added Sasselov. “They don’t really hit the planet but instead come very close. They get what’s called a gravitational assist, which our own spacecraft use in order to speed up toward the edge of the solar system and leave it altogether – as is the case with NASA’s Voyager 1 and Voyager 2 spacecraft. This can happen also to a comet. It would have happened much more frequently early on when there were a lot of these small objects roaming around, and Jupiter probably did its fair share of expelling them from the solar system. When we consider larger nomad planets floating around today, it’s important to recognize that the ones that formed in a protoplanetary disk and then were ejected probably account for only part of the total number we expect – maybe one or two per star in the galaxy. That means we have to think of other ways in which they formed.”
“There are smaller bodies in our solar system and one of them that is still difficult to explain – way out beyond the orbit of Pluto – is Sedna,” Sasselov pointed out to the team members in the interview.
“I think there is evidence that Sedna, which many scientists refer to as a “dwarf planet,” is not indigenous,” observed Roger Blanford, “It’s probably pretty implausible that an object that large would have landed in the solar system but it’s not out of the question.”
“Extrapolating that further,” interjected Strigari, “you could even ask the question, what’s the probability or likelihood that we’ve been visited by an interstellar type of comet?” foreshadowing the recent visit to our solar system by the alien interstellar object, Oumuamua. “You could start addressing that question by looking at the inventory of the comets that are known to exist. Many of them are under hyperbolic orbits and are very weakly bound, if bound at all, to our solar system. I think it’s interesting to note that in the near future we will have a better inventory of these transient-type comets with large-scale surveys. That will allow us to speculate if we are being visited by one of these smaller-mass nomad planets.”
“The odds go up, as you get to smaller and smaller planets, of transferring from one planetary system to another. There are many more of them, of course,” concluded Blanford.
Perhaps the next “front-page” interstellar visitor to our Solar System of Oumana type object will be a life-bearing rogue planet.
Journal reference: Monthly Notices of the Royal Astronomical Society
The Daily Galaxy-Great Discoveries Channel, Sam Cabot, via The Kavli Foundation
Image credit top of page: Milky Way Warped and Twisted