How do exoplanets come to exist? How do they evolve over billions of years? And how do they die? If you’re curious and have 10 minutes, listen to my podcast, The Birth, Life, and Death of Alien Planets, on “365 Days of Astronomy.” (It’s a daily podcast produced by the International Year of Astronomy 2009.) You can also just download the (11 Mb) .mp3 file here and listen to it on your iPod or other media player. This blog post is adapted from the podcast transcript, if you prefer to read rather than listen to the 10-minute broadcast
The race is still on to discover more planets, and scores are promised thanks to missions like the Kepler space observatory. Meanwhile, down here on earth, exoplanets scientists are scratching their heads, mining their data, and tweaking their theoretical models to try and make sense of the diversity of alien worlds we have already found.
Here at NASA’s Goddard Space Flight Center, where I work as a science writer, we’ve got a whole group of scientists obsessed with exoplanets. They took me on a whirlwind tour of the birth, life, and death of planetary systems. It all starts with a collapsing cloud of gas that forms an infant stars surrounded by a spinning disk of gas and dust — the stuff of which planets are made. A protoplanetary disk.
JENNIFER WISEMAN: Young protostars are buried in a large envelope of dense gas, kind of flattened like a fluffy pancake, but it can extend out to thousands of astronomical units, the distance from the Sun to the Earth.”
DANIEL PENDICK: That’s Jennifer Wiseman. She studies star birth and is the new senior project scientist for the Hubble Space Telescope. She’s also the chief of Goddard’s ExoPlanets and Stellar Astrophysics Laboratory, which is home to many of the exoplanet researchers here at Goddard.
WISEMAN: You have this puffy but dense sort of pancake of gas, swirling around, and in the interior part of this, material is being gravitationally sucked into a tighter accretion disk that’s right around this young forming star.
PENDICK: OK, so far so good. We’ve got an accretion disk, which is where planets come from. What happens next? I asked Hannah Jang-Condell, a post-doctoral researcher at Goddard and the University of Maryland. She’s also a member of the Goddard Circumstellar Disks Group, about a dozen scientists here active in exoplanet research.
JANG-CONDELL: So basically you’ve got a star. It’s not burning hydrogen yet. You’ve got this disk of gas and dust surrounding it. And planets are starting to form in this disk
PENDICK: Hold on — did she say dust? As in those fluffy dust bunnies that inhabit the underside of my couch? Not exactly. When astronomers say dust, they mean tiny bits of solid stuff, like minerals and ices, floating around in space. The dust grains are on the scale of a micron—a millionth of a meter—in diameter.
JANG-CONDELL: It’s assumed that as you build these things up from the micron size to the centimeter size, that things stay fluffy. So sort of loosely bound aggregates. So they are a lot like dust bunnies at that stage.
PENDICK: So much for interstellar dust bunnies. Now, back to the planet building stage of our story.
JANG-CONDELL: So there’s two main scenarios for the way planets form. There is the core accretion scenario. So you start out with dust particles and they collide and coagulate and become larger and larger bodies. When it gets about 10 to 20 times Earth’s mass it’s able to accrete gas, and then the gas will stay on it. From that point it can accrete gas and become a gas giant planet like Jupiter.The alternative scenario is called gravitational instability. In that case, you have a massive disk, and it’s cool enough and dense enough for it to start self-gravitating. So in other words, the disk will fragment, it will start to form a clump, the clump will become self gravitating, and eventually it will collapse to form a giant planet.
PENDICK: This all takes place in the space of a few million years — a cosmic blink of an eye. Gas giants have to form before all the gas in the system has either accreted onto the star or is blown away by the star’s radiation.Once the gas goes away, the infant planetary system evolves into something called a debris disk. As Goddard exoplanet researcher Aki Roberge explains, the planet-building process continues in debris disks, creating larger and larger bodies called planetesimals. In today’s solar system, planetesimals are known as asteroids and comets.
AKI ROBERGE: They start colliding and sticking. Roughly speaking, it’s just hit-stick-hit-stick, get bigger and bigger and bigger.
PENDICK: Sometimes the collisions are not so sticky. The planetesimals smash together and generate lots of smaller debris particles. In fact, huge dusty disks were discovered around other stars for the first time in the 1980s. Astronomers dubbed them ‘debris disks.
’ROBERGE: Over the years, there’s been lots of pieces of evidence collected that these debris disks, they really are young planetary systems. So they are like young, dense versions of our own Kuiper and asteroid belts, and our own solar system probably went through a phase very much like it, a debris phase, when it was young.So any giant planets that would form in the system have already formed because there is no gas left to form any giant planets. And some planetary embryos, maybe Mars sized bodies, are there already. So what’s happening is the late stage of terrestrial planet formation. So you are building up from Mars to real Earths.
PENDICK: Terrestrial planets can have violent births, as embryonic planets up to the size of Mars slam into others and build up larger planets. Also at this time, water rich comets may stream in and collide with the young terrestrial planets. This provides the raw material for oceans and atmospheres.Theory tells us these events must be happening in the dusty disks astronomers study. But we don’t see any of this directly.
ROBERGE: All you can really see, ironically enough, is the very smallest portion. So what you see is the dust, tiny, tiny little dust [grains.] This is the dust that’s produced when two asteroids crash together and break up, or the dust that’s in a comet’s coma that’s being expelled as they evaporate. So actually we see the indirect signs. We can see the tiniest material but we know it has to be coming from bigger things.
PENDICK: At some point, things do settle down a bit. But even in a mature planetary system, the action is far from over. Planets continue to migrate in their orbits, or even be ejected from the system in hair-raising close encounters. And if a planet orbits close enough to its star — even closer than Mercury orbits the sun — it could spiral inward and be consumed. In short, entire planets disappear from planetary systems. Goddard exoplanet researcher Brian Jackson explains.
BRIAN JACKSON: Once you get that close, tides raised on the host star and tides raised on the planet can affect the orbit of the planet. Because the rotation of the star is so much slower than the rate at which the planet is going around, the bulge tends to point a little bit behind the planet. And you can think about the gravitational interaction with that bulge always pointing behind the planet a little bit kind of yanks back on the planet and that can reduce the orbital distance between the host star and the planet.Eventually its orbit will shrink enough that it will be destroyed. That can happen within a few billion years. So a lot of these close-in planets that we see aren’t going to last more than a few billion years.
PENDICK: And even planets farther out from the star can experience dramatic changes because of tidal forces.
JACKSON: If the planet’s orbit is non-circular, then what happens is the size of the tidal bulge when the planet is closest to its host star is bigger than when the planet is farther away. The shape of the planet will change as it goes around in its orbit. That change, that periodic flexing of the planet, dissipates energy inside of the planet. It can drive volcanism, which can cause outgassing and provide an atmosphere for the planet. And we see this sort of volcanism powered by tides in our own solar system, for example, Jupiter’s moon Io undergoes the same sort of tidal heating…and that drives the volcanoes that erupt on the surface of Io.
PENDICK: In fact, tidal flexing could hypothetically turn the surface of a rocky planet into a lava sea fuel massive supervolcanoes. Or it could cause just enough heating to maintain a warm and stable climate, as earthly plate tectonics does on our world.We used to think that solar systems eventually settle down and become middle-aged and sedentary, with stable and predictable behavior. But this does not appear to be the case.J
ACKSON: Among planetary systems, the rule seems to be that interactions can be very violent and dynamic and the orbits can evolve pretty dramatically over time
.PENDICK: Planetary systems can even come back from the dead after the most violent event nature has to offer — the supernova explosion of a star. Goddard post-doctoral scientist John Debes has studied these born-again planetary systems. In fact, the first planets ever discovered around other stars were found orbiting a pulsar — the superdense rotating remnant of a star that went supernova.
JOHN DEBES: What people think happened is that after the initial supernova explosion some of the material fell back into a disk, and that allowed these smaller planets to form. And the only reason we found them is because pulsars are amazingly precise clocks, and you could measure the timing of the pulses of the pulsar, and see that that would change due to the orbital wobble of these planets.What’s great about that system, even though it’s the only one that’s been found, is it really shows the sort of basic process for forming a planet must be pretty easy to do, because if you can do it in the fallback disk of a supernova, you can do it just about anywhere if you have the right amount of material and the right conditions
.PENDICK: Hot Jupiters spiraling to their fiery doom… planets par boiled in molten lava… worlds born from the ashes of dying stars. It sure isn’t your grandfather’s solar system science anymore, with well-behaved old planets in their stately settled orbits. As telescopes give us even sharper views of alien worlds, it’s hard to predict what strange world await discovery.
