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Now that we’ve discovered thousands of exoplanets, we’re learning more and more about what kinds of planetary systems there are out there across the Universe. Are planets like Earth unique or totally rare?
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Show Notes
In Depth | Leonids (NASA)
May 31st Could Be the Most Powerful Meteor Storm in Generations, or Nothing at All (Universe Today)
Comet 73P/Schwassmann-Wachmann 3 – Fragments B, G (Hubblesite)
PODCAST: Ep. 642: Is the Sun… Normal? (Astronomy Cast)
Infographic: Profile of planet 51 Pegasi b (NASA Exoplanets)
Hot Jupiter (NASA Exoplanets)
Kepler and K2 Missions (NASA)
What is the habitable zone or “Goldilocks zone”? (NASA Exoplanets)
Overview | Mars (NASA)
Overview | Venus (NASA)
In Depth | Venus (NASA)
NASA finds evidence two early planets collided to form Moon (NASA)
Methanogen – an overview (Science Direct)
The Great Oxidation Event: How Cyanobacteria Changed Life (American Society for Microbiology)
PODCAST: Ep. 641: Are Planets Alive? (Astronomy Cast)
What is the Radial Velocity Method? (Universe Today)
What is the Transit Method? (Universe Today)
Red Dwarf (Swinburne University)
Wide-Field Infrared Survey Explorer (WISE) (NASA JPL)
The Realm of the Ice Giants (The Planetary Society)
Exoplanet Transformations: Puffy Planets Become Super-Earths (Caltech)
Discoveries Dashboard (NASA Exoplanets)
Largest Batch of Earth-size Habitable Zone Planets Found Orbiting TRAPPIST-1 (NASA Exoplanets)
Neutron star collisions are a “goldmine” of heavy elements, study finds (MIT)
Our Solar System’s “Shocking” Origin (Carnegie Science)
JWST (NASA)
Habitable Exoplanet Observatory (HabEx) (NASA JPL)
Science with the ELT: Exoplanets (ESO)
Transcript
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast Episode 644: Is the Earth Normal? Welcome to Astronomy Cast, your weekly facts-based journey through the does cosmos. We hope you understand, not only what we know, but how we know what we know.
I’m Fraser Cain, the publisher of Universe Today. I’ve been a space and astronomy journalist for over 20 years. With me as always is Dr. Pamela Gay, the senior scientist for the Planetary Science Institute and the director of Cosmo Quest.
Hey, Pamela. How are you doing?
Dr. Gay: I am doing well. It is spring.
Fraser: Yeah.
Dr. Gay: We have tornados, but not today.
Fraser: Yes. So, I need to give the most…man, I don’t even know how to describe this. I need to give the most weasel-word-ridden excitement potential thing to do coming up.
Dr. Gay: Okay, go for it.
Fraser: Okay. And so, that is that there is potentially, maybe, possibly the greatest meteor storm –
Dr. Gay: Yes.
Fraser: – in decades, maybe centuries happening on the evening of May 30th or the morning of the 31st depending on where you live.
Dr. Gay: Tau Herculids.
Fraser: The Tau Herculids meteor.
Dr. Gay: Get your hammock ready. Get your hammock ready.
Fraser: Yes. Yeah. What happened was this comet – and I’m not even gonna say the name, 73P – broke up in 1995. And the great Hubble has pictures of this.
Dr. Gay: Yeah.
Fraser: The comet expanded in brightness 400 times and tore itself apart. And we pass through the Tau Herculids every May 30th/31st every year. But this time it’s possible that we’re gonna be passing through the point where the comet broke up, and it all depends on the math.
But what that means is that we could see – like when you go and you watch the Perseid Meteor Shower during the summer and you have a really good time – you’re like, “Woo-hoo,” – you’re watching one meteor a minute, 60 meteors an hour.
Dr. Gay: Yeah.
Fraser: During a meteor storm like the ’98 Leonids or the ’66 Leonids, people were seeing upwards 1000 an hour. And I saw them, and they were absolutely incredible.
Dr. Gay: Yeah.
Fraser: So, some predictions for this storm are I’ve seen 10,000 an hour, 100,000 an hour. I’ve seen 40 meteors a second, which is 140,000 an hour. And with that comes a distribution of, not just regular meteors, but also fireballs and everything, bolides. It’s just gonna be bonkers. But it’s not certain.
Dr. Gay: Yeah.
Fraser: And so, it’s going to be peaking at 10:00 p.m. Pacific Time on May the 30th, 1:00 a.m. Eastern Time, or 0500 Universal Time. And so, that means it’s best positioned for North America, South America, a little bit of Europe. But I know people are like, “Oh no, I’m in Australia. Oh no, I’m in South Africa. Oh no.” Meteor storms are famously unpredictable.
Dr. Gay: Yeah.
Fraser: We get these kinds of warnings all the time, and they never show up. So, this almost certainly won’t happen. But if it does, it’ll be amazing. And it’s almost certain that it’ll be early or late. So, don’t panic. Just on that night, go out. And if it’s happening, we’ll see a rise in meteor activity as you get closer and closer to the peak. And then the peak happens, and then we’ll see declining meteors.
So, everyone on Earth should be able to get a shot at seeing this. Whether you’re in the Northern Hemisphere, the Southern Hemisphere, in is for everybody. It could be the meteor storm of a generation, but it could also be nothing. So, don’t yell at me.
Dr. Gay: This is an excuse to go camping if you like to go camping.
Fraser: Yeah.
Dr. Gay: This is an excuse to upgrade your hammock if you’re thinking maybe this summer I need a better hammock.
Fraser: Yeah. Yeah.
Dr. Gay: We won’t be recording next Monday.
Fraser: Yeah.
Dr. Gay: It is Memorial Day here in the US. But also, I plan to use this as an excuse to put more hammocks in the backyard, and have people over, and have a fire.
Fraser: More hammocks, yeah. Yeah. And even if you’re in a city –
Dr. Gay: Yeah.
Fraser: – like if you’re in the middle of a city, you’ll still be able to see it.
Dr. Gay: Oh, bolides.
Fraser: Yeah, it’ll be like, again, 40 a second. So, the sky’s is gonna be just – it’s gonna be raining stars. So, you’d see more in the dark if you’re far away from the city lights, but you’ll still see a lot if you’re in a big city. So, this is for everybody. Everybody.
Dr. Gay: Yeah.
Fraser: And again, there’s no guarantee this gonna happen. But the way I always say it is that you miss 100 percent of the meteor storms that you don’t stand outside to watch.
Dr. Gay: It’s true.
Fraser: So, don’t blame me if you decide to not watch it and then it turns out to be the storm of the century.
Dr. Gay: Get your bug spray. Get an old AM/FM radio that you can toon in to hear them striking the atmosphere, and then tell us what you saw.
Fraser: Yeah. Awesome.
All right, we have done an episode about whether or not our solar system is normal. Now we wanna talk about our planet. We’ve now discovered thousands of exoplanets. We’re learning more and more about the kinds of planetary systems that are out there across the universe. But are planets like Earth unique or totally rare?
So, when we think about this idea of is planet Earth normal, right, like we now know of many, many exoplanets out there. Are we getting a sense? Because I think in the past we would think in every star system there will be a Venus-like planet, and an Earth-like planet, and a Mars-like planet. There will be a terrestrial planet in the habitable zone of the star, and there could be life there.
Are we getting a sense of how similar Earth is to other kinds of terrestrial planets out there?
Dr. Gay: So, we’re in this super weird time right now where we have started finding rocky worlds. We have started finding things that are roughly the size of Earth, but we can’t find them around sun-like stars yet. So, we’re only finding rocky worlds next to tiny, tiny stars that have tiny, tiny habitable zones right up next to their surface. So, we have found potentially habitable worlds that orbit every 11 days, while probably being tidally locked. And –
Fraser: Right.
Dr. Gay: Yeah.
Fraser: Experiencing flares from the red dwarf star.
Dr. Gay: But it’s still encouraging because we’ve gone from this point where the first planet found around a normal star was found – it was a super-Jupiter snuggled up right next to its star, 51 Peg. And it was far hotter than we knew planets could exist. And so, we started out with this canonical idea prior to that of solar systems are gonna be rocky, rocky, rocky, gassy, gassy, icy, random stuff.
Fraser: Right.
Dr. Gay: And then we were like, oh no. It’s hot Jupiter, hot Jupiter, and that was all we found. And then Kepler came along, and it was like, “No, no, wait. You were impatient,” and started finding other kinds of solar systems, leading us to understand we have no idea how solar systems are gonna form, and they’re gonna do whatever they want and form however they want.
And the more we’re able to see, the more we hope to be able to find that Earth 2.0 out there around that sun 2.0 star. But right now, we’re kinda unique as far as we know.
Fraser: So, what can we learn about the kinds of terrestrial planets that have been found? And also, what do we know about the Earth and which factors we think might be contributing to us being maybe more unique? That’s not really a word you should use. More special.
Dr. Gay: Special. Every child is special.
Fraser: Yeah.
Dr. Gay: That can mean so many things.
Fraser: You can’t be more unique or less unique.
Dr. Gay: It’s true.
Fraser: Either you’re unique or you’re not. That’s all. Yeah.
Dr. Gay: It’s true. So, what we’re looking at is we still have this Goldilocks problem in our solar system of Venus is too hot, Mars is too cold, and we’re just right. But our understanding of how we got here has evolved.
Mars doesn’t have enough of a magnetic field, so it lost its atmosphere. But it once had oceans. Venus, it doesn’t really have a magnetic field, all sorts of chaos went on other there. It has a super thick atmosphere. And we now think that something catastrophic occurred anywhere from a few million to a few hundred million years ago that took it from being an ocean world to being what we see today, and that’s a bit terrifying to think about.
Fraser: Were those Venus and Mars problems as opposed to habitable zone problems?
Dr. Gay: Those were Venus and Mars problems as opposed to habitable world problems.
Fraser: Yeah. Right. So, if Venus had a different atmosphere composition or a different mass, it could be as habitable as Earth, and even same with Mars –
Dr. Gay: Yeah.
Fraser: – if it had maybe a thicker atmosphere but a lot more mass. There could have been three completely habitable worlds in the Solar System if you just fine tuned their chemicals a little bit.
Dr. Gay: Yeah. Mars, you’d have to like change its mass to allow it to –
Fraser: Yeah. No, the same mass as Earth. If Mars was the same mass as Earth –
Dr. Gay: Yeah.
Fraser: – Venus was the same mass as Earth –
Dr. Gay: Oh, right.
Fraser: – Venus had maybe a thinner atmosphere than Earth, Mars had a thicker atmosphere than Earth, you could probably balance out all three in terms of energy budget.
Dr. Gay: And I think the rotation rate is something we also have to pay attention to with Venus. Because Venus somehow got flipped upside down rotationally.
Fraser: Right.
Dr. Gay: And it’s rotating so slowly that its day exceeds its year. And that changes the mixing that’s able to go on in its core. So, had the sets of collisions that occurred in our solar system been different, had that Mars-sized world that hit the proto-Earth maybe hit the proto-Mars and made something much bigger, had wherever hit Venus not hit at the angle it hit at and changed its rotation it way it did, it could have been a completely different scenario.
And we’re still trying to figure out even the role Earth’s moon has. There are some ideas that without Earth’s moon and the tidal forces it puts on our world, we might not have been able to support life the way that we do today. And so, there’s all of these what ifs. And until we’re able to start consistently finding and studying smaller worlds around sun-like stars where that habitable zone is further out and you’re not getting tidally locked, it’s just computer models all the way down.
Fraser: Right. It is interesting how we’ve got these other examples just in our solar system how things can go wrong and make planets that are uninhabitable today.
Dr. Gay: Yes.
Fraser: Where with Venus, no planet-wide magnetosphere, a horribly thick atmosphere of carbon dioxide, sulfuric acid, intense temperatures and pressures.
Dr. Gay: It personifies death from the skies.
Fraser: Yeah. Yeah, it’s hell.
Dr. Gay: Yes.
Fraser: And then on Mars, low mass, dead interior, not a thick enough atmosphere, couldn’t hold onto its atmosphere, etc., etc. And then with Earth, we’ve got a large moon keeping us stable, but also probably no giant impact that rolled us over on our side since the moon.
Dr. Gay: Right. And more than that, we have this constant mixing going on from the tidal forces where we see the ocean tides; we see the surface of the planet actually rising and falling as the moon goes by. And this creates a small amount of heating. We have the radioactive interior that has the heating. All of this comes together to put us in a situation where our planet is still mushy on the inside and driving a nice, healthy magnetic field. And Venus just doesn’t have that.
And this also makes us start to wonder just what role did life have on preserving life on our world as well. We know that in the past there were methanogens that created a methane-rich, more greenhouse gassy atmosphere. Then the oxygen creating life came along, changed the atmosphere wildly. And all the wildlife has its own effects on the biosphere, creating as we talked about with our episode three episodes back a world that itself may be called alive in so far as having a biosphere impacts how the planet evolves.
Fraser: That’s interesting, that there are effects on the planet in terms of, say, its albedo –
Dr. Gay: Yes.
Fraser: – with the reflectivity or the amount of absorption that happens with the forests and things like that that can be factors to life. Now, are started out this conversation taking about how all these hot Jupiters were found, and that really was an outflow, an outcome of the most powerful telescopes and techniques that we had at the time, the radial velocity technique.
Dr. Gay: Exactly.
Fraser: Well, isn’t it weird that all the planets we find are giant orbiting their star really closely and happen to be lined up perfectly so that we see them passing to the right and left of their star from our perspective?
Dr. Gay: Yeah. Or top and bottom.
Fraser: Right, or top and bottom. Yeah, yeah. Or isn’t it interesting that the – but side to side – that it’s passing almost directly in front of its star.
Dr. Gay: Yeah.
Fraser: And then with the transit method, the same thing. What do you know, with the transit method, we find even more planets that happen to be passing exactly in front of their star. So, have we found enough? I mean, we found these super-Earths which are pretty interesting.
Dr. Gay: Yes.
Fraser: And not anything like we have in the Solar System.
Dr. Gay: Yeah.
Fraser: And then more terrestrial Earth-sized worlds and even smaller found around these red dwarf stars. We don’t even really have a sample set yet of what – to find analog earths right now.
Dr. Gay: Exactly. It’s sort of like once upon a time when you asked what is the distribution of stars. Astronomers kinda went, “Uhhh.” And the issue was we didn’t have enough infrared capability to understand the population of red dwarf stars out there. And so, there were hints that they existed in huge numbers.
But to truly understand what is the ratio of tiny stars to big stars you need to look at I big enough value of space that you have those rare giant stars. And you need to be able to see that entire value to a faint enough level that you can make out those little red dwarfs all the way out to the edges. And until we had the ability with telescopes like WISE to survey all the little red dwarfs out there, we couldn’t even tell you what the ratio of stars was.
Well, now instead of having the what’s the ratio of red dwarfs to big ol’ white giants, it’s more a matter of what is the relationship between the Mercuries and the super-Jupiters? And I find it kind of fascinating that there’s this sweet point in the histogram right now of planet sizes where ice giants seem to dominate, and we don’t yet know how much of that is observational bias versus, no, the universe just likes to create ice giants.
Fraser: Like super-earths, or mini Neptunes, or just regular old ice giants.
Dr. Gay: Right. Yeah. And I love the research that is starting to say that is starting to say that you may get super-earths by removing the atmosphere from ice giants. I just love that idea, and I’m gonna bring it up regularly apparently.
Fraser: So, then the presence of these super-earths and even mini Neptunes and stuff –
Dr. Gay: Yeah.
Fraser: – does that give us any kind of sense of what could be waiting for us? Like I wonder if there’s a crossover. If you find under a sunlight star you find a bunch of super-earths and various other worlds orbiting it, and then you look at a red dwarf stars and you see a certain distribution, is there some way to kind of overlap the two to get a sense of what might be waiting for us once we can start to image these planets, the other earths?
Dr. Gay: So, part of my hesitancy is every time we try to do that, the universe is like, no, you’re wrong. We initially thought that red dwarfs could never have large numbers of planets because how could solar nebula that they formed out of, that cloud of gas that only produced this itty-bitty tiny star. How could it possibly produce a large number of planets?
And then there’s one of the TRAPPIST systems being like, “Hey, I’ve got planets for days. You want a planet? I’ve got many.” And so, the more we look, the more we realize planets are just gonna be everywhere. But the one thing we have seen so far is little red dwarfs have for the most part littler planets. And so, there’s that tendency.
And so far we have only been able to find massive planets around massive stars. And so, there’s this human tendency to say, well massive stars must only have massive planets, and it’s not gonna work that way. And figuring out what the transition’s gonna look like from little stars have cute little, tiny planets – many of which can be in their habitable zones – to massive stars can have any possible kind of planet, and what is the trend, we don’t know how to get in between those two point right now.
Fraser: Right. Right.
Dr. Gay: There’s many things possible.
Fraser: Now, we’ve been taking mostly just about sort of physical characteristics of these planets.
Dr. Gay: Yeah.
Fraser: But I wanna sort of think in the time domain as well. We’re here 13.7 billion years after the Big Bang. The Earth formed 4.5 billion years ago. Does our time in the universe have any factor on whether or not we’re normal? Were there more planets earlier do we think?
Dr. Gay: No, just the opposite is likely true. As our universe has evolved with time, it has had the ability through stellar evolution, through supernovas going off, through neutron stars merging to create more and more heavy elements. We are, as Carl Sagan pointed out, star stuff. To think of it more grisly, we are the left over bits of stars’ dead bodies that have been spewed across the universe in amazing explosions.
Fraser: Right. Right. We are made of rotting star corpses.
Dr. Gay: Yeah, exactly.
Fraser: Right. Take that, Carl Sagan.
Dr. Gay: It’s just one of these things when you have to think about what a star has to go through to get its material to be in a human being. The universe is a violent place, but that violence is what led to us being able to exist because so much stuff has been mixed together.
Fraser: Doesn’t it give like a bit of a tension then because we know we’re way beyond the age of star formation.
Dr. Gay: Yes.
Fraser: We are billions of years late to the star formation party, but we’re fairly well-positioned in the metals party.
Dr. Gay: Exactly. And looking around the universe, we once thought that we were of average metallicity, and it turns out we are far from average metallicity. We are one of the more metal-rich stars out there.
Fraser: Whoa.
Dr. Gay: And this gives us a solar system full of rocky bodies and worlds that are made up of far more than just hydrogen and helium. Now, admittedly, when I say “far more” I mean that we think that I believe it’s 1.03 percent of the sun is made of things other than hydrogen and helium, and that is still very metal-rich. So, it takes a lot to be able to get to planets. And by being late to the party, we are luckily enough metal-rich.
Now, we are starting to find that there’s giants elliptical galaxies out there that have had bursts of star formation in the past and have become extremely metal-rich. So, in other galaxies, we would probably be late to the party. But in our galaxy, which is the only one we can see planets in right now –
Fraser: Right.
Dr. Gay: – we are right on time.
Fraser: And then what about position? So, we are I guess not too close to the core of the Milky Way, not too far out into the outskirts of the Milky Way. Is our position normal?
Dr. Gay: It helps. The volume that exists to put stars in increases as you look at larger and larger radiuses. So, when you look at the inner part of the solar system, it is a smaller volume, but it also happens to be way denser. So, there’s a lot of crazy math involved in saying, are we normal or are we not normal?
What I can say is, our distance from the center of the galaxy means we are not somewhere where the stars are so dense that we had to worry about necessarily having planets stolen on a regular basis early on before we had a chance to form life. And we probably don’t need to worry about having planets stolen into the future. The crossing distances are just great enough where we are that it’s pretty good. But at the same time, we’re not so far out in radius or so far high up from the disc that we’re any more a metal-poor region of the galaxy.
So, we found this nice sweet spot where there’s enough of the stuff that we need to form rocky planets, but there’s not such a density of objects that it’s like trying to get through a crowded subway station at rush hour.
Fraser: And even factors like being above or below the galactic plane could have a factor as well in the formation of the solar system?
Dr. Gay: Yeah.
Fraser: And also, the things that came before us. So, for example, astronomers are fairly confident that a binary pair of neutron stars collided in our general vicinity shortly before the formation of the solar system.
Dr. Gay: Yes.
Fraser: Would we be here without that? Is that a necessitating factor for metal-dense star systems?
Dr. Gay: We’re still trying to figure that out. So, we think we understand that neutron-neutron star collision are required to produce large amounts of things like gold. Now, at the same time, other kinds of supernovae give off specific distributions of various every heavier elements. And there’s other things we know are necessary for life, like phosphorus for instance.
And so, the question becomes, what are the different ways that you can get at you have enough heavy stuff – the irons, the nickels – to be able to form iron-nickel core planets. What are the combinations of supernova that you need in order to get all of the atoms that go into organic molecules that we see for amino acids and other useful nucleic proteins?
Gold is awesome, but was it necessary to get to having life versus life as we know it? And so, these are questions that because we have a single set we just don’t know.
Fraser: Yeah, yeah.
Dr. Gay: But for the jewelry economy, it was probably necessary.
Fraser: I mean, where we stand right now, we’re just a month away from James Webb coming online and being able to start taking images.
Dr. Gay: Yeah.
Fraser: It’ll probably go after the TRAPPIST system to try and learn more about that.
Dr. Gay: I hope.
Fraser: There’s a few other planetary systems this it’ll be able discover. But it can’t see earth-sized worlds around sun-like stars. We need something bigger, better.
Dr. Gay: Yeah.
Fraser: We need something like the LUbex telescope or the Extremely Large Telescope, or something even bigger, to come after that. And so, I think as we kind of rolled through close here, we’re still too earlier. We’re probably – pick a date. When do you think we could do the show again and know with more certainty how normal the Earth is compared to other planets out there?
Dr. Gay: See, you’re requiring me to prognosticate what the global budget for telescope building’s gonna be. And that is a lot harder to figure out than how long is the going to be until we have the technology we need. Because having –
Fraser: But you get to guess. So…
Dr. Gay: Ah, man. My guess would be –
Fraser: Ten years?
Dr. Gay: I’m gonna say closer to 20.
Fraser: Twenty years?
Dr. Gay: Yeah, because we’re gonna to build something that we can fly, and JWST has left me bitter.
Fraser: Right. Well, we will be closing in on episode 2000 then of Astronomy Cast when we can do the episode about how common the Earth is compared to other systems.
Dr. Gay: Exactly.
Fraser: Wonderful. Well, thank you, Pamela.
Dr. Gay: Thank you, Fraser. And thank you so much to all the myriad people that allow us to do all the things that we do.
This week I would like to thank Will Hamilton, Zero Chill, Felix Goots, Simon Parten, Smenski, Planetar, Consia Pinflinico, Andrew Stevenson, Bart Fliardy, Steven Coffee, Glen McDavid, Carthic Fectromin, Sean Marts, Sean Freeman, Blixa The Cat, Rachel Fry, John Drake, Joe Wilkonson, John Asef, Benjamin Davies, Roland Vormerdom, Dean, Bryan Celby, Nyla, Connor, Peter, Lu Zealand, Arctic Fox, Tim Garish, Coreen Dump Truck, Claudia Mastrianni, Jordan Turner, Lee Harborn, Chris Wheelwright, Jason Cardukus, Olivia Brunazank, Ron Thorson, Papa1062, Robert Hundu, Kim Baron, Vitaly, Paul Espizita, Arthur Latshov, Frank Stewart, Ganesh Swormanthine, Bob Zatski, Disastrina, Scott Cone, The Air Major, Kemi Rosain, Rubin McCarthy, Timelord Iroh, Jeff McDonald, Iggy Hammock, and Dave Masfield.
If you would like to hear me desperately try to pronounce your name correctly and probably fail, go join our Patreon at patreon.com/astronomycast. We really wouldn’t be here without you. And more to the point, we wouldn’t be able to provide all the people behind the scenes the income that we can thanks to you.
Fraser: Thanks, everyone. And we’ll see you next week.
Dr. Gay: Bye-bye.
Female Speaker: Astronomy Cast is a joint product of Universe Today and the Planet Science Institute. Astronomy Cast is released under a Creative Commons attribution license. So, love it, share it, and remix it. But, please, credit to our hosts Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, astronomycast.com.
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