Exoplanets Floating Freely, Without A Star
IRA FLATOW, host:
Still talking about galaxies, planets, but some places - some planets not in our own solar system. Not too long ago, you know, if scientists wanted to study the origins of a planetary system and how planets form, there was a pretty limited dataset to work with.
We had just the nine planets in our solar system, then eight planets, plus all the leftovers, like asteroids and comets and, of course, Pluto. But in the last two decades, scientists have confirmed the existence of over 500 planets outside our little corner of the Milky Way, planets with weird, unpronounceable names like HG9446C.
And this week, a group of astronomers added 10 more planets to that list. But these are even more unusual. These are 10 Jupiter-sized planets that seem to be orbiting either very distantly from their host stars, or maybe even floating out there all on their own, orbiting no star at all.
I mean, is it still a planet if it has no star? How did those planets get loose, then, in the first place? Could some planets have been kicked out of our solar system, too, long ago?
That's what we're going to be talking about with my guests. Sara Seager is professor of planetary science and physics at MIT in Cambridge. She joins us from our studios on campus.
Welcome to SCIENCE FRIDAY.
Professor SARA SEAGER (Planetary Science and Physics, MIT): Good afternoon, Ira.
FLATOW: Good afternoon. Joachim Wambsganss is a professor of astronomy at the University of Heidelberg in Germany, and director of the Center for Astronomy there. His research appears in the journal Nature.
Welcome to SCIENCE FRIDAY, Dr. Wambsganss.
Professor JOACHIM WAMBSGANSS (Astronomy, University of Heidelberg): Good afternoon, Ira.
FLATOW: Good afternoon to you. What is - Sara, can you give us an idea of these planets, how unusual are they?
Prof. SEAGER: Well, let's just say that the authors, this week, have discovered a new population, a new class of exoplanets. Until now, we didn't know that there were quite a large number of planets that are either really far from their star, or even free-floating altogether.
FLATOW: Wow. And how - were these hard to discover?
Prof. SEAGER: Well, they weren't - it depends. I mean, think about it this way: 10 or 15 years of background work went into leading up to this discovery. But what the authors did was change their research slightly, and they're monitoring millions and millions of stars, tens of millions of stars at one time.
What they did differently this time was just change the cadence, the frequency for which they take data. So instead of taking data once a day, they took it once an hour.
FLATOW: Wow. And that's a lot of data they had coming in all that time. And they basically saw the planets and discovered that they did not have any stars associated with them.
Prof. SEAGER: Well, there's a lot of analysis behind this. But the overall picture is that they didn't see a star. There actually might be a star there, really, really far from the planet...
Prof. SEAGER: ...but there's no evidence for the host star. And furthermore, the authors use information from a completely different planet-finding technique to come to the conclusion that most of the planets they found are indeed free-floating.
FLATOW: All right. We'll talk about free-floating planets. Our number: 1-800-989-8255. You can tweet us @scifri. We'll be right back after this break. Stay with us.
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FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY, from NPR.
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FLATOW: You're listening to SCIENCE FRIDAY. I'm Ira Flatow. We're talking about some interesting - a couple of handfuls of new planets that seem to be floating out there by themselves. We're talking with Joachim Wambsganss, who is professor of astronomy at University of Heidelberg in Germany, Sara Seager, professor of planetary science and physics at MIT. Our number: 1-800-989-8255.
Dr. Wambsganss, why would you call them planets? Isn't the definition of a planet something that must be orbiting a star?
Prof. WAMBSGANSS: Well, indeed, this is one of the three criteria that currently sets the definition of planets: orbiting around a star. And therefore, I expect wild discussion among astronomers whether these new objects could be called planets or should be called planets, or whatever, great dwarf or objects of planetary mass, formerly called planets. There are already six, eight, 10 suggestions out there. And people are really not sure what to use as the proper name for it.
FLATOW: And so these are not rocky planets, right? These are giant gas balls, even bigger than Jupiter.
Prof. WAMBSGANSS: Well, so far, we can only determine their mass. And it's roughly the mass of Jupiter. And it's very likely that they are gas giants. But we don't have definite evidence of that yet.
FLATOW: And Sara, you want to tell us what they...
Prof. SEAGER: Sure, yeah. I'll just - I'll add in that out of the 150 or so exoplanets that we do have a mass and a size and has density, none of them are massive rocks. All the massive planets we know are, indeed, mostly hydrogen and helium.
FLATOW: Mm-hmm. And could a planet, since you haven't found a noticeable star yet, could a planet like this support life if it's so far away?
Prof. SEAGER: Well, these gas giants - if they are indeed gas giants, or hydrogen-helium giants - we don't think those types of planets can support life, even their relatives like Jupiter and Saturn in our own solar system.
It seems kind of counterintuitive, but the cold planets like Jupiter and Saturn, they're actually really hot on the inside, and you don't have to go too far down in their atmosphere until it's too hot for the molecules for life.
However, the microlensing surveys that have announced these planets, they're not sensitive to smaller planets yet, lower-mass planets. But we believe there are also a lot of free-floating rocky planets out there, and if we can find them someday, those ones also might support life.
FLATOW: Would that, then, be the norm for planets, so to speak? I mean, out there - and we are the abnormal kind of planet?
Prof. SEAGER: Well, you know, Ira, one thing is you're asking a lot of speculative questions. And in exoplanets, we know we just have to wait for the future, and we will have an answer then. But right now, you know, we could speculate that there are going to be a lot of ejected planets, but we don't know for sure.
FLATOW: What does it mean to be an ejected planet? You mean it was once part of a star, and now something threw it out, or it lost its star? Or what would have happened? Joachim, do you have any idea about that?
Prof. WAMBSGANSS: Yeah. What we can detect from these observations so far is that there might be a star nearby, but at least 10 times the Earth's sun distance. But they could as well be freely floating or ejected, and that means, indeed, that formerly, they were in a bound planetary system.
So the assumption is that they could still form like Jupiter did, in a disk around a star, and then by some gravitational interaction or by some nearby encounter by a different star, they could be kicked out of their orbit and now float freely through the Milky Way.
FLATOW: So we really don't know what might have ejected them or caused them to...
Prof. SEAGER: Well, we do. I mean, it's like - it's gravitational interaction, like a game of pool, billiards. They interacted with each other, and some of those interactions were violent and got kicked out.
Have you ever played pool, where you knock the ball, and it goes right off the table? Well, if you're a beginner that may have happened to you.
FLATOW: Many times. Many times.
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Prof. SEAGER: Okay, well, that's kind of what it's like. It gets hit in just the kind of right or wrong way, and gets ejected.
Now, I want to add one other piece to the puzzle, and that is every planetary system with lots of planets - that means more than four or five, including the solar system - is what we call dynamically full.
If you were to take another planet and stick it anywhere in the solar system, the solar system would become unstable. So some people take that to mean that planetary systems formed with so many extra planets and eventually ejected those to become full.
The analogy is that if every morning, you woke up and found a perfectly full cup of coffee on your table, you might wonder: Did someone have to overfill that cup in order for it to be perfectly full? And that's the analogy that we carry around for exoplanetary systems.
FLATOW: Interesting. Let's go to Donny in Arizona.
DONNY (Caller): Hi. Yeah, I just - I had a couple of questions. And I guess it's well-timed, given what the conversation was. The first one is: How likely would it be for one of these exoplanets to be picked up into a solar system? How likely would it be for them to become part of a solar system that they're just floating around?
And then the other question I had was: What's the likelihood that these planets could collide? And what would that debris look like?
FLATOW: All right, thanks. You know, that's sort of like - sort of the chemistry idea of planetary motion of electrons. You lose one, it goes to another atom.
Prof. SEAGER: Well, let's just say that space is incredibly vast and empty. Anything in space floating around has almost a zero chance to hit another star or planet, unless it's an environment very different from our own solar system's environment, such as the very inside of a globular cluster.
FLATOW: And yet they did get hit, if they're not with their star anymore.
Prof. SEAGER: Well, they formed - the idea is that they formed in a planetary system with other planets around a star. But once they're ejected, they have almost no chance of being captured by another star, if they're an environment like our own sun.
Prof. WAMBSGANSS: And this ejection mechanism is not really a physical hit. So the other planet or the other star that kicked it out did not physically touch this planet. It was just sort of a nearby encounter that made it being kicked out of the orbit.
FLATOW: Ah-ha. That explains that. Now, Sara, tell us about these mini-satellites that you're planning to launch the exoplanets - exoplanets at.
Prof. SEAGER: Well, let me first give you a bit of background. There are six different techniques to find exoplanets, and they all have a different type of star or planet that they can find.
So the one we just talked about, microlensing, their stars are really far away, very, very far, halfway between where the sun is and the center of the galaxy.
What we want to move to is to find very small planets around the very nearest and brightest sun-like stars. So here at MIT and at Draper Labs, we have embarked on a different kind of satellite.
This is a very small satellite. It's almost the size of a loaf of bread. And we have a prototype getting ready to launch in about 2013. And what we hope to do is change the way that space telescopes are done.
Instead of taking 30 years from concept to launch, they would just be a few years. You put one up, and then you put more. So instead of one big telescope, you have lots and lots of little, tiny telescopes.
FLATOW: Are they coordinated together, or each one works separately? Or how does that work?
Prof. SEAGER: Each one will work separately, but if one finds something interesting, for example, they could all be tasked to turn to the same object for a short period of time.
FLATOW: Do they - are they - they have their own artificial intelligence, that they look around on their own and find something?
Prof. SEAGER: No. They'll be told where to look for now. But all these ideas are out there, and many other projects want to try to put a kind of artificial swarm intelligence on their satellite fleets. But so far, this fleet concept is a new one.
FLATOW: So it's sort of a swarm, a little swarm, a little fleet of mini, loaf-sized telescopes that are looking for exoplanets?
Prof. SEAGER: Exactly.
FLATOW: How many are we talking about?
Prof. SEAGER: Well, we'll probably put dozens, dozens of them.
FLATOW: And do they hitch a ride on a different satellite being launched? So if you've got a little room on a - yeah, you get a little room...
Prof. SEAGER: They do. Mm-hmm. They do, actually, they're called - yeah. I mean, do you know - have you heard the phrase three peas in a pod? Well, the phrase is three cubes - three cube sats in a peapod. And a peapod is the standardized deployer being bolted to the upper stage of a rocket of more than 10 different launch vehicles.
So NASA and other places support launch of these things, and dozens of them have already been launched - not mine, but other, similar size-and-shaped satellites.
FLATOW: Does that account for their size and shape, so they could fit in these little pods?
Prof. SEAGER: Exactly. Exactly. They're standardized - yes, exactly -standardized mass and volume so that they fit into these standard pods.
FLATOW: So they're hitching a ride, hitching satellites.
Prof. SEAGER: Hitching rides.
FLATOW: Hitching rides. And you think you'll have a...
Prof. SEAGER: But I think the point...
FLATOW: Yeah, go ahead.
Prof. SEAGER: I think I just want to make a point for the audience that there's so many different kinds of planets out there. There are many different ways to find them, and each different technique can find a different kind of planet.
FLATOW: Now, let's talk a bit about this technique. This was using gravitational lensing. That's kind of interesting.
Joachim, go ahead. You can tell us.
Prof. WAMBSGANSS: Yeah, the technique of gravitational lensing works like a magnifying glass. So we use a lot of background stars just as light bulbs at the other side of the Milky Way. And then we wait for other stars or planetary systems to pass in front of them.
They act like a magnifying glass. So for a short duration - a few days or a few weeks - we see the background bulb being brightened and then getting (unintelligible) again in a very predefined way. And this is the signature of this gravitational focusing that the foreground star-plus-planet produces.
This was, in fact, predicted by Einstein 75 years ago. But in his favor, he was very skeptical. He says: Of course, there's no hope that we can ever observe this. And the reason is that this very close alignment of a background bulb, a background star and this foreground planetary system is very unlikely.
So Einstein already estimated that we have to monitor at least a million stars to find one of these events, and that's why he was so skeptical. But by now, our techniques make it possible to monitor millions of stars, literally.
This team, in fact, monitored 50 million stars, as Sara had said, for two years, once an hour. So they have 2,000 observations for each of these 50 million stars, and then they could sort of pick the needle in the haystack and find these 474 microlensing events, 10 of them being shorter than two days, and these are the ones produced by these planetary (unintelligible) objects that are either very far away from the host star or even freely floating.
FLATOW: Fascinating. We have a...
Prof. SEAGER: Yeah. I just want to emphasize how crazy these idea is and how amazing that it's successful. Imagine, a dark, unseen planet or planet plus star, thousands of light years from Earth. You can't even see it, but it just happens to line up perfectly with the background star and magnify it enough so that people on Earth can detect it.
FLATOW: That - it has to be perfect.
Prof. SEAGER: Not perfect, though, but almost.
FLATOW: Almost perfect. Then that's why you have to look through so many different pictures...
Prof. WAMBSGANSS: Right.
FLATOW: ...to find one. Let's - a tweet came in from Sia Salate(ph). He says, what's the difference between an exoplanet and brown dwarf? If they're - it's - you know, they are giant gas right?
Prof. WAMBSGANSS: The definition for a brown dwarf is that it has enough mass so that in the central, hottest, and densest part - the more (unintelligible) part of hydrogen, which is called deuterium, can fuse into helium and produce energy. This is - this fusion is the process that makes a stars shine, but this is normal hydrogen. And the limit for this is about eight percent of the mass of the sun.
If a star has a mass that is below eight percent of the solar mass but more than one percent, then it can still produce energy, very small amount of energy, by this deuterium fusion. And this is called a brown dwarf. And only objects with masses lower than that are then probably called a planet if they're in orbit of the sun.
FLATOW: Doctor Seager, you really sound passionate about the subject.
Prof. SEAGER: Yes. What I can I say?
FLATOW: Well, I know. I mean - why - have you always been interested? Is there something about this...
Prof. SEAGER: Well, you know, the thing is that - imagine that when I was child and growing up, there were no exoplanets. So I couldn't be excited about it back then. But certainly, I remember - one of my clearest memories is when I was a child and on my first camping trip and waking up in the middle night and going out to the dark sky and I was just literally blown away. I had no idea there were so many stars out there. And I carried that around with me for, I guess, many, many years until I realized there could be a lot more out there than stars.
FLATOW: Yeah. So are finding stuff beyond your widest imagination or is it pretty tame compared to what you were thinking about?
Prof. SEAGER: I think it's all beyond all of our wildest imaginations. Who would have expected all of these - well, we did expect all these free floating planets because planetary systems, naturally, we think (unintelligible) and knock it out there. There are so many crazy planets out there.
Let's switch for a second to Kepler. The Kepler science team, NASA's Kepler space telescope, which is looking for the frequency of other earths by monitoring the 150,000 sun-like stars for three and a half years. And that Kepler science team is meeting right now as we speak. I'm skipping the meeting. But they actually - Kepler has so far found some completely crazy systems. They have found a multiple planet system with six planets and five of those planets had orbits closer to their star than Mercury is to our sun.
FLATOW: Wow. We're talking...
Prof. SEAGER: All packed of - mm-hmm.
FLATOW: Yeah. That is whacky, as you would say.
Prof. SEAGER: It's a very tightly - one of those tightly packed full systems that I was referring to. And moreover, they're not all Mercury-like planets. These planets have a mass and a size such that most of them do have hydrogen, helium and (unintelligible).
Prof. SEAGER: They're not Jupiters. They're more like mini Neptunes.
FLATOW: Yeah. Mini - and then if you read this in science fiction books, you would have said never possible.
Prof. SEAGER: Well, it's possible if you did a calculation, because it is dynamically stable. But you would have thought - you probably wouldn't imagine a system like that.
And one more think about Kepler I want to mention, is that it found a planet just called Kepler-10b, and that planet is so close to the star. It's 20 times closer to its star than Mercury is to the sun. And that means, you know what, its surface is hot enough not just to melt lead but to melt rock. So on the surface of that planet, we're thinking, there must be lava lakes, not caused by volcanoes but just caused by heating from the star.
FLATOW: Wow. What the SPF factor must be there. We're talking about exoplanets this hour on SCIENCE FRIDAY from NPR, with Sara Searson(ph) and Joachim Wambsganss.
It's 800-989-8255. Let's go to John Addo(ph) in Saugatuck, Michigan. Hi, John.
JOHN (Caller): Hi, Ira. How are you?
FLATOW: Hi there.
JOHN: Hello, Ira.
FLATOW: Hi. Hi, John. I should say Seager instead of Sara Seager. I said Sara Searson. I'm sorry. Hi, John. How are you?
JOHN: Very good.
FLATOW: I think you got a question for us.
JOHN: Well, I was - I wanted to ask you and the professors if indeed they'd heard of Immanuel Velikovsky's hypothesis that Venus is indeed a new member of our solar system and was a - an exoplanet until not too long ago, maybe 4,000 years ago.
FLATOW: Well, that's an old theory. It goes back decades. Let me - Sara, what do you think of that?
Prof. SEAGER: I haven't thought of that but - I mean, I have not heard of it but I can give it some thought while we turn to our other guest.
FLATOW: Joachim, you ever heard Velikovsky's idea?
Prof. WAMBSGANSS: Similarly, I haven't heard of this idea and I think it's unlikely. Our planetary system seems to be in a pretty stable orbit. And if only 4,000 years ago, another planet would have sort of joined us and keep in - I don't - I would not expect the motion of the known planets, Mars, us, Venus, to be as stable and as smooth. But, again, similar to Sara, I haven't really given any thought to the idea.
FLATOW: Would you be able to tell if one of our planets had come from somewhere else?
Prof. WAMBSGANSS: Well, if the - we have tested the material of some of the planets with NASA missions. And as far as I understand, the sun is attached that all planets are of the same age. And I think this is a pretty robust result, which indicates that they are formed in the same -at the same time and the same place.
I would expect a different planet, which formerly was orbiting another star consisting of different material, different ratios of whatever iron and other metals and also being of a different age.
FLATOW: Mm-hmm. Doctor Seager, where would you like to go from here? What would you like - what would be exciting?
Prof. SEAGER: Well, what we really want to do is try to find planets around the very nearest stars, especially the very nearest sun-like stars. What we wanted - the way I look - like to look at exoplanets is think ahead to thousands of years from now when people look back to our early 21st generation of people - scientists, engineers and just everybody else.
And what do you think they're going to remember from this time? I mean, I hope they'll remember a lot of things. But I'm sure they will remember that we were the first people to go out there and find exoplanets and to hopefully find planets like ours out there. We're hoping that these people, thousands of years from now, have a way to travel to those other planetary systems and they'll look back at us, our generation - just like we look back at the great explorers hundreds of years ago - that we actually were the first to start this whole movement.
FLATOW: Well, good luck to you and good luck to your exoplanets idea. And we'll check in from now on. OK? Stay with us and tell us what's going on.
Prof. SEAGER: Thanks, Ira. Thanks.
FLATOW: You're welcome. Thank you, Joachim, for joining us, and good luck to you.
Prof. WAMBSGANSS: Well, thank you.
FLATOW: Joachim Wambsganss is a professor of astronomy at the University of Heidelberg in Germany, director of the Center for Astronomy there. And Sara Seager, professor of planetary science and physics at MIT.
We're going to take a short break and come - when we come back, get out your niobium-titanium party hats. We're celebrating the 100th birthday of superconductors. I'll get out the little party treats. Stay with us. We'll be right back.
I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.
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