TO THE LAYPERSON, THE HEAVENS FOLLOW FAIRLY PREDICTABLE PATTERNS. Moons orbit planets. Comets, asteroids and planets, such as the Earth, orbit stars. So when the news broke in late February that astronomers have estimated an almost incomprehensible number of planets drifting through interstellar space – unbound to any star – the story was everywhere.
Previous studies have found evidence for nomad planets. Last year, researchers detected about a dozen of them through gravitational microlensing – a technique that looks for stars whose light is momentarily refocused by the gravity of passing planets. But the new study, aptly titled “Nomads of the Galaxy” and published in the Monthly Notices of the Royal Astronomical Society, proposed an upper limit to the number of nomad planets that might exist in the Milky Way Galaxy: 100,000 for every star. And because the Milky Way is estimated to have 200 to 400 billion stars, that could put the number of nomad planets in the quadrillions. (Note: The researchers calculated their estimate by considering the known gravitational pull of the Milky Way, the amount of matter available to make such objects, and how that matter might be distributed into objects that range from the size of Pluto to larger than Jupiter.)
The Kavli Foundation spoke recently with two of the authors of the new nomad planets study, as well a leading researcher in the search for planets and life beyond Earth. The participants:
- Roger D. Blandford, Director of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University and the SLAC National Accelerator Laboratory, Professor of Physics at Stanford University and at the SLAC National Accelerator Laboratory, and a co-author of the study.
- Dimitar D. Sasselov, Professor of Astronomy at Harvard University and the Harvard-Smithsonian Center for Astrophysics, and the Director of the Harvard Origins of Life Initiative, which bridges the physical and life sciences to study issues ranging from planet formation to the origin and early evolution of life.
- Louis E. Strigari, Research Associate at KIPAC and the SLAC National Accelerator Laboratory, who led the team that reported the result.
The following roundtable discussion has been edited by the participants.
THE KAVLI FOUNDATION: Let's pretend 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?
LOUIS STRIGARI: We still don't have a great inventory of how many nomad planets there are in our galaxy, at the very small scales. 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.
TKF: Can we say anything about how large they might be?
ROGER BLANDFORD: I’ll pick up the story there. Of course, there have been some well-publicized discussions of what the definition of a planet is, and in our paper we talk about nomad planets from Jupiter on downward to very small masses. We considered nomad planets down to the size of Pluto because we have a particular technique in mind for trying to detect them. So we asked, “What is the maximum number we can imagine being there?” We then presented this as an observational challenge to either detect them or place better constraints on how many of them are out there.
A Pluto-sized nomad planet is a convenient size for the microlensing technique. But if you have some other technique like your hypothetical interstellar spacecraft whizzing around at warp speed, then you could encounter smaller bodies more frequently than that. At a maximum, the closest one to Earth might be, say, 10 percent of a light year away, with a light year equal to 5.87 trillion miles. Smaller nomad planets the mass of Pluto could be encountered much more frequently than that. The short answer, of course, is that we don't know. But the exciting thing is the opportunity and the wherewithal is there to start finding out.
DIMITAR SASSELOV: 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. 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.
TKF: Dr. Strigari, you're planning some computer simulations that may help us better understand this very process of how planets could be ejected from solar systems, is that right?
LOUIS STRIGARI: That's correct. 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. Questions we can ask for a computer simulation include, “How often during the formation of these star clusters, and then during their subsequent dispersal, do planets get ejected?” “How often do these planets get exchanged between stellar systems?” 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.
TKF: Nomad planets, by definition, are cast completely into interstellar space and therefore aren’t bound to any star. How could this happen?
DIMITAR SASSELOV: You could say this has happened in our own solar system with small objects, typically comets, which pass close to Jupiter. 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.
TKF: And one of the other ways is just forming independently in large-scale clouds of gas and dust from which stars also develop, is that right?
DIMITAR SASSELOV: Yes, certainly, that would be an extension of the mechanism by which stars form.
LOUIS STRIGARI: Theoretical calculations say that probably the lowest-mass nomad planet that can form by that process is something around the mass of Jupiter. So we don’t expect that planets smaller than that are going to form independent of a developing solar system.
DIMITAR SASSELOV: This is the big mystery that surrounds this new paper. How do these smaller nomad planets form?
TKF: What is the possibility that a planet in our own solar system was once a nomad planet?
DIMITAR SASSELOV: 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.
ROGER BLANDFORD: I think there is evidence that Sedna, which many scientists refer to as a “dwarf planet,” is not indigenous. 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.
LOUIS STRIGARI: Extrapolating that further, you could even ask the question, “What's the probability or likelihood that we've been visited by an interstellar type of comet?” 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.
ROGER BLANDFORD: 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.
TKF: So, future studies should be able to better characterize the population of nomad planets, their size and so on.
ROGER BLANDFORD: That's definitely what we were pointing out in our paper. We are encouraging the use of existing facilities, or telescopes that are going to be coming online. Perhaps there could also be some inexpensive new technique that would make the estimate for the number of nomad planets more precise. Nomad planets are certainly enticing targets for observational astronomers. Remember, it wasn't so long ago that we had no good evidence for any extrasolar planets. We just had a lot of false alarms. And now, we are talking about up to 2,000 or 3,000 of them. The subject has developed at an extraordinary rate.
TKF: One more question about the exchange of planets between solar systems. Is this something that we need to worry about, here in our own solar system? Could a nomad planet be swept up into our solar system and perturb the orbits of planets here, even collide with Earth?
LOUIS STRIGARI: If we’re talking about an object impacting the Earth and causing a significant amount of damage, we should be more worried about an asteroid-type object that we know about, that already resides in our solar system.
ROGER BLANDFORD: The threat is internal, not external.
TKF: Right now, we can indirectly detect nomad planets with this microlensing technique, but we don’t know where it is other than somewhere between the Earth and the background star whose light it’s changing. Will we ever be able to actually pinpoint the location of these nomad planets? And what kind of technology would be required to do that?
ROGER BLANDFORD: Using this microlensing technique, all we can probably say is there's a probability distribution of where the nomad planet would be found. Another technique – it's a bit of a long shot – is to look for infrared radiation, the heat, from a very nearby nomad planet. Using this data, you could determine its motion and other characteristics, which would give you additional clues to the distance. As I mentioned before, smaller nomad planets are expected to be closer than larger ones, in part because we think there are many more smaller nomad planets than larger ones. So if you are going to find a nomad planet by detecting its infrared radiation, it will likely be small and very close to our solar system. And in that case, I think you’d be able to get a much better estimate of the distance.
TKF: I’d like to switch gears a bit and talk about the speculation that some nomad planets might harbor life – a topic discussed briefly in the paper. It seems easy to imagine that these planets, unattached to a star and drifting aimlessly in deep space, are very cold and lifeless. But under the right conditions, they could be something very different. Dr. Sasselov, you direct a program at Harvard that studies what conditions are essential for life to develop. Why are nomad planets not necessarily cold and lifeless?
DIMITAR SASSELOV: It's good to separate the subject of life into two parts. First, we know that a water environment is essential for life, as we know it, to arise after a transition from chemistry to biochemistry. Secondly, we need to consider what kind of environmental conditions are necessary for that biochemistry, once it emerges, to survive over astronomically significant periods of time.
Planets the size of the Earth or larger provide very good potential habitats, because they have their own internal heat. But we should pause here and recognize that we still don’t know how life emerges. We don’t know, for example, whether ultraviolet radiation or other conditions that a nearby star provides could be critical to the emergence of life.
"If you imagine the Earth as it is today becoming a nomad planet... life on Earth is not going to cease. 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." - Dimitar Sasselov
TKF: Of course, our thinking on this subject is limited because we have only one example of what life is.
DIMITAR SASSELOV: That’s right. Human science does not have a good definition, nor do we understand the true nature of life as a phenomenon. On the other hand, we understand organic chemistry fairly well, and also the essentials of biochemistry. We can therefore compare conditions in the galaxy to what it takes to have the biochemistry we know about. You find these necessary conditions on planets, in particular planets that range in mass from Mars to Uranus and Neptune. This is exactly what we are talking about today when we talk about nomad planets. These kinds of planets have their own internal heat, which enables complex chemistry. And it doesn’t matter whether they are associated with a star or not.
TKF: This internal heat – is it going to be created by some kind of radioactive decay?
DIMITAR SASSELOV: Yes, our own Earth is a good example of this. The majority of the heat that goes through the crust to the surface of the Earth is due to latent heat still preserved from its formation – and in no small order radioactive heat from radioactive elements, including uranium, thorium and potassium.
ROGER BLANDFORD: If one of these nomads was in interstellar space and had some moons associated with it, like Mars has moons associated with it, that could provide tidal heating – the squeezing and stretching of the surface of the planet. That would cause a certain amount of frictional heating and that could be an additional source of heat, which should last quite a while.
TKF: And which could help drive some of the tectonic action at the surface.
ROGER BLANDFORD: That's right. The other key to this – and there've been several rather speculative papers, but very interesting ones – is 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. We already know that bacteria and other organisms can survive under the most inclement conditions at the bottom of the deep ocean, in vents around volcanoes, and even in the human gut – and that must be as miserable as anywhere and they do just fine. It's at least a reasonable question to ask: “How long could such extremophiles survive in conditions such as those around a suitably-configured wandering nomad planet?”
DIMITAR SASSELOV: That's a very good point, and I'd like to continue in that direction. If you imagine the Earth as it is today becoming a nomad planet – it gets expelled and then it's flung into interstellar space. Life on Earth is not going to cease. That we know. It's not even speculation at this point. People who study extremophiles, in particular in the deep biosphere in the crust of the Earth, 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. And because the internal heat of the Earth is going to continue at this level for at least another 5 billion years, this entire deep biosphere is going to be completely uninhibited by the Earth becoming a nomad planet.
TKF: That could make nomad planets carriers of life throughout the galaxy. We’re veering toward the subject of panspermia, the hypothesis that life exists throughout the universe and is spread by meteoroids, asteroids and other objects, aren’t we?
ROGER BLANDFORD: People have been talking about how life may have been seeded throughout the cosmos since Anaxagoras in the 5th century B.C. (the Greek Philosopher who first wrote about the concept of panspermia). 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. 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. 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. We want to write a sort of “Hitchhiker’s Guide to the Galaxy.”
"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. 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." - Roger Blandford
DIMITAR SASSELOV: Having larger bodies like these nomad planets allow microbes to not simply survive the journey but in fact prosper and be robust.
TKF: Turning back to more immediate work on nomad planets, what are the next steps for better understanding them? More comprehensive surveys of the sky are needed, but NASA is facing huge budget cuts.
LOUIS STRIGARI: I think we have to use our imagination a little bit in terms of how we can do this. A space-based telescope survey like WFIRST is important, but several other types of space-based observatories that will happen in the next decade or so will have the capability to make these observations – without interfering with their primary missions. Of course, it would be outstanding if larger more dedicated observatories such as WFIRST would really nail this topic.
ROGER BLANDFORD: Pioneers like Dimitar have been extraordinarily ingenious at using relatively inexpensive observatories to make great discoveries. They've used existing facilities and existing projects in new ways, and I think we’re going to see that this is a subject that attracts lots of young people. They don't know what's impossible, and therefore they make discoveries. I am optimistic on those grounds, but I'm also optimistic about some of the more directed space missions and ground-based facilities coming online over the next decade.
"I'm really curious about the exchange of planets between solar systems, as we’ve talked about. 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." - Louis Strigari
TKF: Most people think the biggest mysteries in astronomy today are dark matter and dark energy, which make up 96 percent of the universe but are a complete mystery. The possibility of a galaxy teeming with nomad planets, planets we never before thought existed, means there’s still a lot to learn about the remaining four percent of the universe - that is, the ordinary stuff that makes up stars, planets, moons and all the other so-called “baryonic” matter in the cosmos. What does each of you think are the biggest unanswered questions related to this four percent?
DIMITAR SASSELOV: The four percent holds the key to another of the big mysteries of human knowledge, which is life. What is the nature of life?
ROGER BLANDFORD: Just looking at the inanimate properties of this four percent, of baryonic matter, there are many great mysteries. For example, we can’t locate some of it but we know it is there. The dwarf galaxies seem to have rather carelessly lost a good fraction of it. This is one of the big observational challenges in modern cosmology.
LOUIS STRIGARI: We still don't know about the mix of dark matter and luminous matter in our galaxy. One of the questions that I tend to get asked about is, “Could these nomad planets make up all the dark matter?” In other words, could they be the source of both the mysterious dark matter and ordinary, baryonic matter that is “dark” simply because we haven’t detected it yet? Nomad planets would not be able to account for much of the unseen baryonic matter. And that’s the problem: we still don't have a very great inventory of ordinary matter – even on the scale of our own sun. That sort of highlights what we don't know.
ROGER BLANDFORD: Maybe I could put Dimitar on the spot. Ten years down the road, what do you think we’ll know about extrasolar planets both attached and detached from stars?
DIMITAR SASSELOV: I hope we can better understand how planets form in protoplanetary disks. That is still a big mystery, although the data is accumulating at fast pace right now. Also, I think we are going to identify, in the vicinity of our solar system, extrasolar planets that are potentially habitable. We’ll learn as much as we can about their atmospheres, and we’ll search for geochemistry that may lead to a signature of life.
In the meantime we’ll figure out a way, in the infrared, to find nomad planets that are even closer – within a couple of light years. And that will be the next big breakthrough. I hope that this happens in the next ten years. But I'm not sure if we will be able to pull it off.
LOUIS STRIGARI: I'm really curious about the exchange of planets between solar systems, as we’ve talked about. 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.
TKF: And then, as you said earlier Dr. Blandford, nomad planets could also be intergalactic travelers.
ROGER BLANDFORD: Yes, at very high velocities you can escape the galaxy. 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.
TKF: Our own Milky Way is headed for a collision with the Andromeda galaxy, which I suspect will lead to a fair number of nomad planets.
ROGER BLANDFORD: Yes. But don't frighten people. It's not for many billions of years.