As astronomers are finding even more new extrasolar planets, they’re starting to discover entirely new categories. There are classes of planets out there that we just don’t have any analog here in the Solar System. Let’s talk about them.
What is a Terrestrial Planet? (Universe Today)
The Realm of the Ice Giants: What Exploring These Planets Teaches Us (The Planetary Society)
What are Gas Giants? (Universe Today)
Why Neptune and Uranus are different (EarthSky)
Proto-planetary nebulae (Swinburne University)
Kepler-138 d (NASA)
What’s a transit? (NASA)
Impossibly heavy planet is the first ‘mega-Earth’ (New Scientist)
Kuiper Belt (NASA)
Hypothetical Planet X (NASA)
Kepler and K2 (NASA)
TESS Exoplanet Mission (NASA)
Stellar Populations Of Globular Clusters (Swinburne University)
When Stellar Metallicity Sparks Planet Formation (Astrobiology Magazine)
Born in beauty: proplyds in the Orion Nebula (Hubble Telescope)
Transcriptions provided by GMR Transcription Services
Fraser Cain: Astronomy Cast Episode 585: Super Earths, Mini-Neptunes, Gas Dwarfs. Welcome to Astronomy Casper, for a weekly facts-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me as always is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the Director of CosmoQuest. Hey Pam, how are you doing?
Dr. Pamela Gay: I am doing well. To those of you watching this on video, we are threading the needle between the neighbors’ gardener blowing the leaves and picking up the truly enormous, Hallmark special size leaf pile. And the dogs were afraid of the leaf blower and quiet and enthralled and barking at the picking up the leaves. So, I now have 130 pounds of dog in the studio with me.
Fraser Cain: Right.
Dr. Pamela Gay: And you may occasionally see fur enter the edge of the camera field.
Fraser Cain: Right, if you’re watching.
Dr. Pamela Gay: Yes.
Fraser Cain: And definitely for the people hearing the podcast, you are going to hear the panting of dogs, which is preferable to the barking of dogs, and the leaf blower. It has been a day.
Dr. Pamela Gay: It has.
Fraser Cain: Which has been part of a week. For those of you who are wondering what happened for the last couple of weeks, Pamela was in grant proposal madness and just couldn’t spare any time. So, we just took a couple of weeks off.
Dr. Pamela Gay: No, let’s say what really happened. Nancy Graziano took one look at me last week, because I was willing. And she’s like, “No.” I was mommed, and she made the correct choice. Thank you, Nancy.
Fraser Cain: Yeah, yeah, yeah. We probably could have made a show of whatever was left with your brain maybe. But yeah, I think it makes total sense to me. All right, so astronomers are finding even more new extrasolar planets and they’re starting to discover entirely new categories. There are classes of planets out there that we just don’t have any analog here in the solar system. Let’s talk about them. We’ve been running a whole bunch of these stories. The title is Super Earths, Mini-Neptunes, Gas Dwarfs. I love a gas dwarf. What is that? A gas giant but it’s small? It’s a mini-Neptune, what’s that? And then, of course, the super earth. So, how far of a variety are astronomers starting to find of other worlds than what we have here in the solar system?
Dr. Pamela Gay: We’re seeing two different factors. The first factor is that there’s a fairly good continuum of planets of sizes. There’s some gaps in there. There’s a notable gap right before you hit ice giants. But as things are getting developed, press officers and scientists don’t always know what to call things. So, as near as I can tell, they make things up and move along.
Fraser Cain: Right.
Dr. Pamela Gay: And so, in the preparation for this story, because I’ve honestly been struggling with this myself as I read things, because I’ll see mega earths, I’ll see super earths, I’ll see sub-Neptunes, I’ll see dwarf ice giants and gas giants and super Jupiters.
Fraser Cain: Super Jupiters. Yes, super Jupiters.
Dr. Pamela Gay: It’s just, okay, let’s take a moment and back away and figure out what of this is just adding adjectives and what of this is based on science?
Fraser Cain: Okay, so then let’s talk about the raw materials here that we have to work with. We have, I guess, the size of the planet, the mass of the planet, what it’s made out of, and where it’s located. Does that cover everything that we might need?
Dr. Pamela Gay: Mass and density and radius are trickster-y.
Fraser Cain: Sure.
Dr. Pamela Gay: So, start with those three, then add composition and phase state.
Fraser Cain: Phase state, yeah, of course. Not just what they’re made out of, but are they a liquid or are they a solid or whatever? Yeah, okay.
Dr. Pamela Gay: Right.
Fraser Cain: All right, so let’s just pick one of those and we’ll start.
Dr. Pamela Gay: Okay. So, we’re going to start with the three big classifications that people then put stuff in between. And so, the way I think of this is on the color wheel we have our primary colors of red, yellow, blue, red, green, blue, depending on whose color wheel you’re using, and it’s really red, yellow, blue, people, let’s face it, and you can mix these things. So, when it comes to planets, our primaries are we have terrestrial worlds, which is stuff that is solid for the majority of it, and may have an atmosphere, but that is not where the dominant amount of mass has gone.
Fraser Cain: Right. Okay.
Dr. Pamela Gay: Then we have the ice giants, so we’re increasing in size as we go. And ice giants are something that have a rocky core that is substantial, but they’re also surrounded by enough gas that their radius is dominated by this gassy atmosphere and that gassy atmosphere is dominated by things that are heavier than hydrogen and helium.
Fraser Cain: Got it. Okay. Metal, as astronomers would call it.
Dr. Pamela Gay: And what I’ve learned is planetary scientists call it ice, because it’s volatiles.
Fraser Cain: Right.
Dr. Pamela Gay: So, this is stuff like methane and ammonia, and stuff that you can make into ice.
Fraser Cain: Right. So, if you pour alcohol in front of them, they’ll call that ice. If you pour methane in front of them, they’ll call that ice. Yeah. It is very reductionist of the science, isn’t it?
Dr. Pamela Gay: The planetary scientists do add more classifications of material than the astronomers, so I can’t complain too much. Then gas giants, which initially, and many of you learned there were terrestrial worlds and gas giants, so the thing that makes the gas giants and the ice giants different is not the mass, it’s the composition which turns out to be related to the mass. So, the gas giants have atmospheres that dominate them by radius and they are predominantly hydrogen and helium.
Fraser Cain: Okay. So, if we were to think about the kinds of worlds, we have the ones made of rock and metal, mostly with a thin on envelope of gases.
Dr. Pamela Gay: Yes.
Fraser Cain: The ones made of hydrogen and helium with potentially some kind of rocky core down at the very middle of them, dominated. And then, the ones that are sort of in between that have a rocky core, but then have heavier volatiles than hydrogen and helium. And those are the three major classifications that you can expect to find out there. You can pretty much stick every world that we find into one of those groups.
Dr. Pamela Gay: And they don’t necessarily form in the places we find them in our own solar system. The thing about these worlds is they can get moved around by gravitational interactions. And there are some theories that have Uranus and Neptune forming between Jupiter and Saturn, so it’s more a matter of what kinds of conditions existed in the protoplanetary nebula.
There’s some really cool research that is finding that eddies and turbulent pockets inside protoplanetary discs may have the oomph needed to put together a planet, and so it could be that these icy objects, which are really gas but have heavier atoms, are formed in these turbulent pockets, whereas the rocky worlds tend to be things that have had the potential for atmospheres blown away and have formed through the building up of planetesimals, so put rocks together until you get a big rock and that’s how you get to a terrestrial world. The gas giants, it’s looking like, and again, we don’t know anything for certain with planetary formation, it’s looking like these, you start out with something that’s like a 10-Earth mass core and let it gravitationally pull in hydrogen and helium and that’s how you get a gas giant.
Fraser Cain: Right. But as we went into the introduction here, a gas giant can be only a little bigger than the Earth, so you can have teeny tiny versions of them.
Dr. Pamela Gay: Except those are called –
Fraser Cain: Gas dwarfs.
Dr. Pamela Gay: – And you’re still – Yeah, and gas dwarfs are still, in most cases, thought to be 10 times the size of Earth, which as an astronomer, I’m like, “That’s not a lot bigger than the Earth,” but for a planetary scientist is massive and they start calling those “mega-Earths.”
Fraser Cain: Well, we did a story fairly recently and the smallest known gas dwarf is 60% bigger than the Earth. Now, not in terms of mass, just size. That’s really small.
Dr. Pamela Gay: Okay. Yeah.
Fraser Cain: Right? Obviously, we have experience with terrestrial planets, rocky planets being very small, and we have this definition of these mini-Neptunes, so I mean, there’s no rules.
Dr. Pamela Gay: They attempt to have rules. They attempt to have rules and one of the problems that they run into is trying to figure out, “Well, what’s the density of the stuff making this thing up? Is it rock and water? Is it predominantly water?” With transient method detections, we can get the radius of these objects and radius is a fabulous thing to have when you can have it. It’s because of the radius that we are starting to think that there are particularly fluffy planets, so these are planets that appear to have super-heated atmospheres that have poofed out, so we have puffy worlds. That’s one format. You can also have, however, gas giants that, because they’re snuggled in against their star, are getting their atmosphere stripped away, and as that occurs, are they evolving to become mega-Earths? What do you call the intermediate stage?
Fraser Cain: Is that one idea for how you might get a super-Earth, is you get the atmosphere of the planet just blown away, and then you’re left with whatever remains, like just the craziest comet ever?
Dr. Pamela Gay: Yeah. You essentially take a Jupiter-ish world, or up to larger, and there’s been some stuff that people argue about found up at the 30-Jupiter mass range where you’re starting to wonder, “Is this actually a brown dwarf?” We’re not going into that today, but you take a Jupiter world, you migrate it in, and we’ve figured out lots of different ways to migrate planets and we don’t know which one is correct. You migrate it in until it hits the point where the solar radiation, the solar wind, everything that it’s experiencing is blowing away its atmosphere, and they do appear like crazy comets, we believe. But over time, you only can blow away atmosphere for so long when there’s before there’s absolutely no atmosphere to be blown away and what you get left behind is essentially a 10-Earth mass object, which is what we think the core of a Jupiter-like world is, so is that now a super-Earth?
Fraser Cain: Right, and super-Earths are definitely a thing.
Dr. Pamela Gay: Yeah.
Fraser Cain: They have definitely been found and definitely pegged down to the point that they know that they’re rocky, so what are the limits of – ? When you think about a super-Earth, what are we talking about here?
Dr. Pamela Gay: In general, when they say “super-Earth,” they mean something that, depending on which paper you’re reading, is two-and-a-half, three times the size of the earth up past those 10-Earth mass objects that are not surrounded by gas, the greater than 10 will also get sub-classified into mega-Earths, so for certain, if it’s greater than 10-Earth masses, it’s a mega-Earth. If it’s –
Fraser Cain: A mega-Earth? I have never even heard that term before, “mega-Earth.” I’m looking this up. You’re just yanking my chain here now.
Dr. Pamela Gay: – I am not. I went down –
Fraser Cain: Oh, my god, you’re right.
Dr. Pamela Gay: – I went down a very sad rabbit hole of, “Can you all talk to each other and come up with names?” That was really how I felt.
Fraser Cain: I’m totally going to follow you down this rabbit hole. This is the best.
Dr. Pamela Gay: It was a fabulous rabbit hole. But at the end of the rabbit hole, what I discovered is it was not built by a single rabbit. It was built by many different rabbits. Some of them were cottontails, some of them were jackrabbits, some of them were jackalopes.
Fraser Cain: Right, so it’s a war in – Yeah. Right, mega-Earths.
Dr. Pamela Gay: We have super-Earths, we have mega-Earths, we have mini-Neptunes, we have sub-Neptunes, we have dwarf gas giants, we have super-Jupiters. When you see all these words, what I want you to think about is if they use the word “Neptune,” it means something that they believe has volatiles in its atmosphere.
Fraser Cain: Right.
Dr. Pamela Gay: If they say “Jupiter,” they mean something that is hydrogen/helium-dominated. If they say “Earth,” they mean something that is rocky and may have a veneer of atmosphere, but really, it’s a rock.
Fraser Cain: And any other description is nonsense.
Dr. Pamela Gay: It’s an adjective.
Fraser Cain: It’s an – Yeah, yeah, so a mega-Mars, a mini-Mercury, a super-Pluto.
Dr. Pamela Gay: You can use any adjectives you want. That doesn’t mean it has meaning.
Fraser Cain: I guess. I mean, eventually, I’m sure somebody’s going to say there’s a super-Pluto because they’re talking about something that has the characteristics of a Kuiper belt object, but –
Dr. Pamela Gay: So, now you’re talking about Triton.
Fraser Cain: Sure, but maybe it’s the size of the Earth.
Dr. Pamela Gay: Okay. There’s where you start adding compositional artifacts.
Fraser Cain: For something that’s made of ice and that is made of water, ice, and rock, as opposed to –
Dr. Pamela Gay: Yeah, and nitrogen.
Fraser Cain: – Yeah, as opposed to, say, something that’s made of, and then you could have a water world. What’s a water world? Is that a Earth planet?
Dr. Pamela Gay: A water world is a weird density Earth. It is something that’s density appears to be lower than what you would get if something was pure rock. It has a radius that is not consistent with it having a massive gas atmosphere like a Neptune or a Jupiter.So, in order to get that combination of lower density than rock, radius that behaves like a terrestrial world, you have to either cover the thing in water or fill it with water. Pick one, we don’t really know. That’s where it starts getting cool, is we can start to figure out small hints about the composition of planets when they’re in systems that are such that, through a combination of measuring Doppler shifting of the star, the motion of the star caused by the planet’s gravity and the radius of the planet as it transits in front, this ability to measure both mass and radius allows us to get at the density.
Dr. Pamela Gay: These are inaccurate measurements though, which is where things go slightly sideways because we don’t know how non-round the orbit is. We don’t know the tilt of it. We can put limits on both of these factors. We can put limits on the tilt by the fact that it’s transiting. We can put limits on the ellipticity by knowing how long it takes to transit and how long it takes for it to go around the star. But these are limits, and so there’s room for mistakes, and they’re mistakes at the level of how much rock to water ratio is there.
Fraser Cain: Right, and then we’ve got some even more interesting things. I mean, did you hear about this world, K2-141b, the one with the magma ocean that’s 100 kilometers deep?
Dr. Pamela Gay: Yeah.
Fraser Cain: So, you’ve got this world that is orbiting its star every couple of hours, seven hours long. It is so close and so hot that the front of the planet – It’s tidally locked, and the front of the planet is molten lava that is then, or is, gasified on the front side, blown around to the backside at winds of 8,000 kilometers per hour, and then falls as rain. But it’s like it’s raining lava, and then the lava is flowing back around, could be hundreds of kilometers deep, and it is flowing back to the front.
Dr. Pamela Gay: As far as we know from computer models. I just want to add that caveat.
Fraser Cain: yeah, the world exists. It’s definitely this close and –
Dr. Pamela Gay: It’s definitely this lava covered. We just don’t know the kinetics of how it’s all moving, and that comes from software.
Fraser Cain: Yeah, and they assume that it’s tidally locked. So, now, you’ve got a world that is starting to grow a tail of rock as opposed to a tail of ice. So, you could just see the location really does seem to matter, and the tidal locking seems to matter as well about what you get.
Dr. Pamela Gay: My personal favorite exoplanet is KELT-9b. This is a world going around a super hot star, which is the kind of star we didn’t think would have worlds, but researcher Scott Gaudi and his team at, I believe, the Ohio State University, took a look at this star, and lo and behold, there is this massively heated planet. Super Jupiter, hot Jupiter, and the surface of the planet on the side facing the really hot star is roughly the temperature of our sun surface.
Fraser Cain: Right.
Dr. Pamela Gay: To be fair, this does not mean it has nuclear reactions going on or anything like that. There are not nuclear reactions going on at the surface of our sun. It’s just to say this world’s atmosphere is getting heated by the really bright, hot star to star like temperatures.
Fraser Cain: Yeah. The surface of the sun doesn’t hold a candle to the core of the sun where the fusion is happening.
Dr. Pamela Gay: Yeah.
Fraser Cain: The surface of the sun is the point that things finally cool down enough that it can escape into space. The center of the earth is as hot as the surface of the sun.
Dr. Pamela Gay: I think that’s true.
Fraser Cain: It is.
Dr. Pamela Gay: I have never thought of that before.
Fraser Cain: The center of the earth is 6,200 Kelvin, and the surface of the sun is 5,800 Kelvin.
Dr. Pamela Gay: Yeah, and I had never – Thank you. I learned something.
Fraser Cain: You’re welcome. I picked that up in a book or a story somewhere. Yeah, there’s these little tidbits that you hold on to where you’re just like, yeah, I’m using that.
Dr. Pamela Gay: Yeah. You just have to see the numbers side by side, and kudos.
Fraser Cain: This is a total rabbit hole. Feel free to cut this if you want, Richard, but I’m sure people dig it. When the universe cooled down to the point that the cosmic microwave background radiation could be released, the universe was the color of a red star.
Dr. Pamela Gay: Yeah.
Fraser Cain: So, it was red. The light that was – the point that it could actually be released, it was red. It was about 3,000 Kelvin, and so everything was red. Then suddenly, it cooled to this perfect moment that now light could actually be released, so..
Dr. Pamela Gay: Some of the colors today, when you average across the components given by all the different sources of light out there, is the color of a latte.
Fraser Cain: Yes. Yeah. All right, Richard, you can decide whether you want to hold onto it. I’m sure you’re going to keep it, but let’s get back to the topic at hand. So, then, based on the – It feels to me now, that with these raw materials that we described at the beginning of this episode, that we’ve got the mass, got the radius, we’ve got the composition, we’ve got these gas planets, these terrestrial planets, these ice planets that they can be almost any mass and they could be in almost any location. So, what are the limits? What should we expect to never find?
Dr. Pamela Gay: The limits come with how long something stays the way it is. So, you can’t expect a senior citizen of a star to have a hot Jupiter snuggled up next to it that still has a complete atmosphere. Because at a certain point, you’re going to have solar systems settle into a more finished format. Our own solar system probably had additional planets when it formed. We know it had at least a few additional planets when it formed that got consumed up by the planets that remain. Once things start to settle into that final form, then evolution sees its way through to the end.
Dr. Pamela Gay: You’re not going to find an ocean world snuggled up against its sun. This is where you start to think, okay, so how are we able to get what we see today? What are all the different routes? This is where it starts getting interesting to imagine that Mercury, which we know is still undergoing low levels of mass loss from interacting with the sun’s high energy particles, cosmic rays, massive amounts of light, it could have had water wherever it initially formed, and has been made smaller. It has crack lines in it from this, all because it’s so close to the sun and lost every volatile it could possibly lose, except for those in the permanently shadowed craters, and that water was brought later, new water.
Fraser Cain: Right, so it’s a dried out apple?
Dr. Pamela Gay: Yeah, exactly.
Fraser Cain: Yeah.
Dr. Pamela Gay: And –
Fraser Cain: Cracked and shrunk with all of its volatiles evaporated out.
Dr. Pamela Gay: And Uranus and Neptune couldn’t have formed where they are.
Fraser Cain: Right.
Dr. Pamela Gay: And if there is a planet nine, it probably got stolen from some other system. And so, there’s all these neat limits on where can things stay over time. So, the limits are, in a very young solar system that hasn’t had time to go through all the wild kinematics, you’re not going to find the world’s in the outskirts, the 30 to 50 astronomical unit distances. In old solar systems, you’re not going to find the hot Jupiters that still have very thick atmospheres. And it’s in how things evolve that have limits. And other than that, the universe is going to do what it will. It just may not be allowed to keep the outcomes.
Fraser Cain: It’s funny. We’ve been so excited about finding planets. And at this point, we know of however many thousands of planets that we know of thanks to Kepler and tests and all this kind of stuff. But it still, it’s nowhere near enough to start answering some of these deeper questions that we need to know about how it all works. We need to know about hundreds of thousands, millions, tens of millions of planets then until we can finally go, “Okay, if you’ve got a star of this age with this kind of temperature output and this kind of existing planets, then this is what you tend to see. And if you’ve got those, this is what you tend to – And here’s how they tend to migrate,” and to come up with the simulations that finally describe how a place like even the solar system and just to get to the point that we can say, where did the solar system come from? You’ll need to have surveyed a huge chunk of the Milky Way to be able to do that.
Dr. Pamela Gay: And this is where we really wished that we had the globular clusters that had planets in them because we can get a deep and rich understanding of stellar evolution by looking at these individual laboratories that each have their own metallicity, each have their own age, and allow us to start to see what are the relationships between how stars look as a function of age, and then how does your metallicity change those evolutionary tracks.
Dr. Pamela Gay: With stars that have planets, we sometimes are able to see the youngest stars still in the open cluster where they formed, where we can start to compare the formation of [inaudible] [00:26:45] of planetary disks among different protostars. But once those open clusters have gone a couple of times around the galaxy, they are no more. And we don’t have that laboratory left where we can say, “All these stars are definitely the same generation. All these stars have the same composition. Here’s now how we look at planet formation as a function of mass,” which we’d love to do.
Fraser Cain: Yeah.
Dr. Pamela Gay: Okay. Let’s now look at hold the mass constant, change the metallicity which is, these are all things we could do if only globular clusters had planets and they don’t.
Fraser Cain: Right. Yeah.
Dr. Pamela Gay: So, these are things that we really, really wish
Fraser Cain: But you can get a hint at the metallicity of a star, can’t you?
Dr. Pamela Gay: You can definitely get the metallicity of the stars, but you don’t have this laboratory of 10,000 identical stars all in one field of light.
Fraser Cain: Right, right.
Dr. Pamela Gay: It’s the convenience that you’re missing.
Fraser Cain: And there was another study, fairly recent, where they’re trying to see if there’s any combination between the type of star, the age of the star, or the type of star and the metallicity of the star, I believe, and whether or not it had planets, and they couldn’t find any correlation whatsoever. So, if you see a star, that tells you nothing about whether or not it has planets, unfortunately.
Fraser Cain: Yeah. So, it just shows you, we’re so excited about all the planets that we know about, and yet the reality is we don’t know anything. We don’t know a hint of a fraction, that we can see so many different kinds of planets, things that we never expected to see, and now we have no idea beyond just the most basic understanding of how they got to be the sizes that they are in the places that they got to be and the composition that they are. And so, there is a long, exciting future ahead for exoplanetary astronomy.
Dr. Pamela Gay: It’s such a young field. Planets were first discovered 28 years ago, and normal planets were only found 25 years ago. It’s such a young field.
Fraser Cain: Yeah.
Dr. Pamela Gay: So, I can’t wait to see where it develops as we increase our spacecraft and our spectrographs here on the planet.
Fraser Cain: Yeah. I did a video about six months ago talking about someone had done an estimate. If you follow the exponential curve of just planet discoveries and you just follow it, you just continue to map it up, like Moore’s law but planetary discoveries, we should hit 50 million planets discovered by 2050.
Dr. Pamela Gay: That’s awesome.
Fraser Cain: Then we can do some nice statistical analysis and get a sense of what’s actually going on.
Fraser Cain: All right, Pamela, that was an awesome. Do you have some names for us this week?
Dr. Pamela Gay: I do, as always, we are supported through the generous contributions of, well, you. And this allows us to pay the wonderful people working behind the scenes to simulcast this to Richard, thank you for all the audio mistakes you put up to get the videos posted, to do transcripts, keep our website up to date. There is a small crew of all part-time people that make this happen and your donations from a much larger crew allow us to do that. So, I would like to thank this week, Donald Munda, Scott Bieber, Kenneth Ryan, Bart Flannerty, Andrew Stevenson, Steven Coffee, Glen McDonald, Benjamin Davies, Anthony Burgess, Gabriel Gelfan, Martin Dawson, Russell Petto, Ryan James, Dean [inaudible] [00:00:30:32], Kimberly Reich, Shannon Humber, Claudia Master Lonnie, the air major Eric Fanager, Jessica Felts, Daniel Loosely, Rachel Fry, Dwayne Isaac. And I’m going to stop there and say thank you all of you for what you do to make everything we do possible.
Fraser Cain: Thank you, everybody. And we’ll see you next week.
Dr. Pamela Gay: Bye-bye. Astronomy Cast is a joint product of Universe Today and The Planetary Science Institute. Astronomy Cast is released under a Creative Commons Attribution license. So, love it, share it and remix it, but please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, astronomycast.com. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep this show going, please consider joining our community at Patreon.com/astronomycast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.