The first day of the American Astronomical Society’s annual meeting here in Long Beach was obsessed with life: The search for habitable planets around other stars and the implication that life could be common throughout the universe. Day two touched on darker themes of death and reincarnation. No metaphysics going on here, though. The talks were still about planets—but this time planets that have been disturbed, cast away, destroyed, or born out of destruction.
The remarkable studies from NASA’s Kepler observatory, which I discussed in my last post, prove definitively that most sunlike stars have planetary systems. But those systems tell only a small part of the story. Many more planets are born but end up as orphans, because planets form too easily. Crowds of newborn planets, jockeying for space around their infant stars, lead to violent gravitational interactions that eject some of them into lonely, deep-space trajectories. Kepler cannot see these runaways, but other new kinds of observations can. Recent estimates suggest such free-floating planets could outnumber the stars in our galaxy: hundreds of billions of worlds sailing forlornly through the dark.
Even the planets that survive that early melee may not stick around for the long haul. At the Long Beach meeting, Nathan Kaib of Queens University in Ontario explained another way that planets can get kicked out of home. If a star has another star in orbit around it—even a small, very distant star—the companion will occasionally approach closely enough to stir things up, sewing chaos. If our sun had a red dwarf companion, he showed, Uranus and Neptune probably would have taken a trip to palookaville.
We don’t have to worry about such a fate. Our solar system has been largely unchanged for 4.5 billion years, proving that our sun is truly stable and alone. Different worries lie ahead for us. Eventually our sun will swell up into a red giant star, roasting and possibly swallowing the Earth in the process. For us, this devastation lies about five billion years in the future. For other, older planetary systems it is happening right now. Earlier this year, Alex Wolszczan of Penn State University caught a red giant star in the process of digesting one of its planets. And on Tuesday, two other teams described details of the next stage of planetary misery.
After a star’s brief, puffed-up existence as a red giant, it sheds off its outer layers, leaving behind its exposed core—a compact but searing-hot stellar cinder known as a white dwarf. Ben Zuckerman of UCLA and John Debes of the Space Telescope Science Institute, working on separate but related projects, found persuasive evidence that some planets do survive all the way through all these changes. Looking at groups of white dwarfs, the two astronomers and their collaborators noticed that about 20 percent of the stars seem to be coated with a layer of heavy elements that looks more like rock than star-stuff. They conclude that surviving planets—now orbiting those white dwarfs in crazy, looping orbits—are sending a rain of asteroids down onto the surface of the star. That could be exactly what our solar system will be reduced to roughly eight billion years from now.
But as I hinted, the day ended on a theme not of death but of resurrection. Wolszczan stepped forward at the meeting to talk about planets that are born from places of extreme destruction. Massive stars end their lives in ultraviolent supernova explosions that leave behind nothing but an ultradense chunk called a neutron star, no bigger than Manhattan but as heavy as the sun. A supernova would seem guaranteed to destroy or discard any nearby planets. Yet Wolszczan has detected planets orbiting three different neutron stars. In one case the planet was probably captured. In another, it seems to be a companion star that was whittled away to just a remnant ball of carbon: “It could be a huge diamond,” the size of a planet, he says.
The third case, around a star called PSR1257+12 (seriously, these people need help with names) is the most intriguing, because it has four orderly planets that appear to have formed in place. They literally rose from the ashes of destruction, Wolszczan says, as some of the supernova debris fell back toward the neutron star and started forming planets all over again, just as happens when stars are born. When he originally discovered these planets, nobody was sure what to make of them. Now, as planets are showing up in more and more places, the message is loud and clear. Nature loves to make planets, and just about anywhere they can form they do form.
The diversity of worlds out there is staggering, far richer than anyone imagined even a few years ago, and astronomers are just starting to explore it. That theme kept buzzing around the Long Beach meeting, and that theme is certainly buzzing through Wolszczan’s head. “We’re just waiting for someone to find planets around a black hole,” he said with an eager grin. “Why not?”