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Hunting for Phantom Worlds

Many astronomers are looking for livable, Earth-like planets. Alexander Wolszczan studies the wounded survivors of stellar suicides.

By Corey S Powell
Oct 2, 2014 5:00 AMNov 12, 2019 4:27 AM
As a red giant star expands, it can engulf and vaporize any planet that orbits too close. | James Gitlin/STScI AVL


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I'm tucking in to dinner with astronomer Alexander Wolszczan in a bustling bistro near his office at Penn State, and he’s in a cheerfully gloomy frame of mind. He has reason enough to be gloomy. He discovered the first planets beyond our solar system but gets little recognition. His follow-up research has been slow and painstaking, hitting repeated dead ends. Neither of these things seems to bother him in the least, however. No, what puts Wolszczan in his intriguingly contradictory mood are the planets themselves.

Earth leads a charmed existence, circling a stable sun that provides just the right amount of warmth for life. The planets that Wolszczan found 22 years ago aren’t so lucky. They orbit a pulsar — a tiny, rapidly spinning stellar cinder that blasts them with ferocious surges of radiation. “There could be a permanent aurora lighting up the sky there from the wind of particles from the pulsar. Some of those particles could get down to the surface and blast it smooth,” he says. And the misery of the planets’ current existence pales in comparison with their traumatic birth, in the 100-billion-degree debris from a supernova that shredded most of the original star.

More recently, Wolszczan has turned his attention to another class of doomed planets. He has begun finding and studying worlds around red giants, elderly stars that have nearly exhausted their nuclear fuel. In a last spasm of activity, they swell up, brighten tremendously and shed enormous clouds of gas. Any planets circling a red giant would get baked and buffeted in the process. That fate awaits Earth in about 5 billion years, when our sun will join the ranks of the red giants. For the distant planets Wolszczan is scrutinizing, the future is now.

These areas of research set Wolszczan distinctly apart from his peers. Since his pulsar planet discovery in 1992, lots of other scientists have joined the search for worlds around other stars, but almost nobody else does it the way he does. He half-smiles: “I’m not exactly a mainstream person.” Today’s planet hunters tend to fixate on planets similar to Earth, eagerly touting newfound worlds that even vaguely resemble our own in size, temperature or composition. My conversation with Wolszczan covers none of that.

In an even, Polish-inflected cadence, he talks not about living planets, but about dying ones, dead ones and the ones that — in the case of the pulsar planets — have entered an uneasy afterlife. I come to think of the objects Wolszczan studies as ghost worlds. They are far removed from terrestrial standards of comfort and stability. They have, in a real sense, moved from one plane of existence to another. They are celestial oddities that most of his colleagues look right past, as if they weren’t even there. All of those factors are exactly what makes them so fascinating to Wolszczan: They are the extreme cases that test the limits of how planets can form, and where they can survive.

It soon becomes clear that Wolszczan is concerned with living worlds after all — he just comes at the topic from the opposite direction. “The effects on planetary systems as stars age relates to the long-term survival of life,” he tells me. “It doesn’t concern only us. If life is as abundant as some people think, then stellar evolution has to be taken into account everywhere.” He sees a cautionary tale about the dangers to human survival, and yet the planets he studies have made him unexpectedly bullish on life’s cosmic prospects. Hence the gloom. Hence the cheer.

Taming the Wild Pulsar

The journey to the ghost worlds began with a chunk of stolen time at the 1,000-foot Arecibo radio telescope in Puerto Rico. Normally it was fully booked, but when Wolszczan visited in January 1990, the giant dish was sitting idle because it was under repair and could not be steered. He recognized a rare opportunity and arranged to perform a blind survey, letting Earth’s rotation bring different parts of the sky into view and then checking out anything unusual that showed up.

“You would see one particular spot in the sky for just 30 seconds, but with a telescope that big, that was good enough to discover lots of interesting things,” Wolszczan says. He calls it “cowboy science” — going for a ride just to see what’s out there.

Wolszczan’s roughly 10-day trot through the sky paid off quickly with the detection of a special kind of pulsar, called a millisecond pulsar, that rotates hundreds of times each second. At the time, it was only the fifth such object ever found. The pulsar now bears the designation PSR B1257+12 (PSR for pulsar, the rest indicating its sky coordinates), and Wolszczan affectionately refers to it as “1257.” But he was not feeling so affectionate back then as he attempted to explain the peculiarly irregular timing of radio pulses from this city-size star: “It was really a lot of pain because the pulsar didn’t want to fit any standard models. I just had to struggle with it.”

In short order, Wolszczan realized that the Arecibo data made sense if a small object were pulling the pulsar back and forth, causing its signal to arrive a little early at some times, a little late at others. By June 1990 he was certain that an orbiting planet was the only sensible explanation, but it took painstaking analysis to prove that PSR B1257+12 actually has three planets, called A, B and C.

He knew his results would be scrutinized closely: These would be the first confirmed planets beyond our solar system, found in a place where most people thought planets could not exist. Did he have any doubts before he went public? “No. That may sound a little bit strange, but pulsar timing is an extremely precise method,” he says.

In January 1992, Wolszczan announced his results at a meeting of the American Astronomical Society. Planets B and C are each about four times the mass of our world. Planet A is a mere 1/50th of an Earth mass, just slightly heftier than the moon. It is still, by far, the smallest known planet around another star. What followed was a wave of wonder and confusion. A wide variety of influences, from Copernicus to Carl Sagan, had convinced astronomers that the galaxy must be full of other solar systems much like our own. Now at last, here was an airtight discovery of planets orbiting another star, and everything about them was all wrong. “Everybody expected planets around normal stars. They were wondering, ‘What is going on?’ ” Wolszczan recalls.

The inferred life story of PSR B1257+12 could hardly be more different from the story of our solar system. Our sun is middleweight and modest; the star that evolved into the pulsar started out massive and dazzling. Earth and its neighboring planets arose as part of the sun’s birth; the pulsar planets emerged from their star’s death.

Wolszczan sketches out a history that goes like this: In its youth, the star that became PSR B1257+12 had at least eight times the mass of the sun. Goaded by the force of its prodigious gravity, it burned bright and hot, consuming the bulk of its nuclear reserves in just a few million years. At the end of its life, the star exploded as a supernova, flinging most of its material outward violently. All that remained was an ultradense, fast-spinning fragment of the original star’s core — the pulsar. Any planets that might have existed in orbit before the supernova were eradicated in the conflagration. But the star’s transformation was not yet complete.

A disk of gas formed around the pulsar, originating either from a nearby companion star, or from the so-called supernova fallback material — part of the star’s expelled debris that did not develop enough speed to escape into space. That disk then condensed and gave rise to a new family of planets, composed of heavy elements created by the supernova.

Whole New Worlds

With his discovery, Wolszczan earned himself a place in the textbooks as the first person to find a new planet since the discovery of Pluto in 1930 (or, if you grumpily dismiss Pluto as a mere dwarf, since the discovery of Neptune in 1846). But PSR B1257+12 A, B and C quickly faded from the public consciousness. They were simply too weird, too unexpected. “The discovery didn’t fit in the NASA plan for extrasolar planets. It did not fit in a very dramatic way, and that showed in the reactions of people,” Wolszczan says. Starting in 1995, other astronomers found planets around nice, proper, sunlike stars. Most of the scientific attention soon turned that way.

But quietly, in the background, ghost worlds kept getting more intriguing. In 1993, Stephen Thorsett, then at Caltech, identified a planet-mass object circling another pulsar, called PSR B1620-26.

Planet PSR B1620-26 b is a totally different kind of world from the ones Wolszczan found. It is two and a half times the mass of Jupiter, more than 100 times as weighty as Wolszczan’s planets. Its orbit is radically different, following a huge 100-year path that carries it around both the pulsar and a separate companion star. Finally, with about 12.7 billion years under its belt, it is the oldest planet known, prompting the nickname “Methuselah” (which, let’s face it, has more music than “PSR B1620-26 b”). Methuselah is almost certainly a captured world, snatched from that companion star.

Then in 2011, a team of radio astronomers led by Matthew Bailes of Australia’s Swinburne University of Technology found a third planetary system around a pulsar, one unlike either of the previous two. This time the planet has a mass similar to Jupiter’s but a density at least 10 times as great — denser than lead. That tremendous heft “provides a clue to its origin,” Bailes noted at the time. No normal planet could pack so much mass into such a small space. Most likely, planet PSR J1719-1438 b is all that remains of a close-orbiting star that was mostly devoured by the pulsar’s birth explosion, leaving only its compressed core behind. If Bailes is correct, the planet’s outer layers consist of carbon, probably in crystalline form. You know crystal carbon by a more common name: diamond. That’s right, the pulsar is wearing a diamond planet.

No additional pulsar planets have been found, leaving something of a mystery: three examples, three completely separate kinds of planets. Wolszczan suspects plenty more systems like his original 1257 exist out in the galaxy, but their planets are too small to show up in today’s radio searches. He has sketched out plans to return to Arecibo and find them, but that will take a lot of telescope time. For now, he has turned his attention to another, far larger class of ghost worlds.

Roasted Remains

Only the most massive stars evolve into pulsars, and such stellar bigwigs are rare. Roughly 97 percent of the stars in the Milky Way are smaller ones that take another evolutionary path, leading eventually to the red giant stage and continuing beyond. A mainstream star like the sun grows gradually brighter and hotter as it ages (it’s happening right now) until the final spasm that inflates it into a red giant. In short order, astronomically speaking, the red giant blows off its outer layers and leaves behind a white dwarf — essentially the naked heart of the star — which slowly cools to eternal blackness.

As a red giant star expands, it can engulf and vaporize any planet that orbits too close. | James Gitlin/STScI AVL

If you want to understand what happens to most dying, dead and reborn planets, red giants are the place to start. Wolszczan has shifted much of his attention in this direction, and he enlisted a variety of collaborators to help. Since 2004, they’ve looked at a set of about a thousand stars, mostly red giants. “If we start detecting planets around stars like that, we can tell what happens to planetary systems when their stars begin to evolve and lose mass and swell up and do all those unpleasant things,” he says.

Wolszczan and others have already found more than 40 planetary systems around red giants, allowing them to sketch out how the same process will play out here at home. As the sun grows more luminous, the solar system’s habitable zone will shift outward. Earth will overheat in about a billion years, but Mars will become balmy. Then the moons of Jupiter and Saturn will melt, with Europa and Titan turning into temporary ocean worlds. At the sun’s ruddy peak, it will radiate so much energy that even Pluto will reach comfy temperatures, according to Alan Stern, leader of the New Horizons mission that is heading there next year.

The red giant phase is make-or-break time for planetary survival. As the star sheds its outer layers, it becomes less massive, loosening its gravitational grip. In response, the planets migrate outward into new orbits, potentially turning the whole system chaotically unstable. “You may get really dramatic evolution in the system, including orbit crossings, planet collisions and all kinds of interesting things,” Wolszczan says. All the while, the star also keeps expanding, threatening to consume its children.

Wolszczan has seen these outcomes, planets with highly oval orbits, or others persisting only as phantom gas clouds in their star’s outer layers. The data do not yet definitively show which way Earth will go. Probably it will survive as a ball of rock, but thoroughly sterilized. Mercury and Venus will almost surely be vaporized — about as ghostly as you can get.

Wolszczan and colleagues recently caught both possible outcomes at work around a red giant star called BD+48 740. In a 2012 paper, the researchers report that the surface layers of BD+48 740 contain high levels of lithium, an element common in planets but almost never seen in stars. They interpret the lithium as the chemical remains of a cremated planet. At the same time, one major planet, slightly more massive than Jupiter, still orbits the star, but on a disturbed, elongated path.

Even white dwarfs, the tiny leftovers of red giants, could theoretically host habitable planets. | David A. Aguilar (CfA)

Life Post-Apocalypse

The story is getting pretty grim. What about Wolszczan’s upbeat musings on a universe full of inhabited worlds? I’ve been chewing on that question since I came across a provocative paper about searching for life around white dwarf stars. Two things immediately jumped out at me. First, the paper suggests that there could be habitable planets around white dwarfs — during the dead-end stage that comes after the inferno of the red giant. Second, the lead author is Avi Loeb, a creative and extremely well-respected theorist at Harvard University. This is not crank speculation.

“There are clearly debris disks around white dwarfs, and this material can, in principle, condense to make planets,” Loeb explains. “So the question is, then, if there are planets, some of them rocky, around the white dwarf, could they have life?” A billion years after a white dwarf forms, he notes, it has a temperature similar to that of the sun. It is much smaller and fainter at that point, but if planets formed in tight orbits — about 100 times closer than Earth’s orbit around the sun — they would get enough light and heat that life could take hold. He is talking about a second genesis: new worlds born from the ashes of the old, new life emerging from the wreckage of a sterilized planetary system.

White dwarfs keep cooling, so even if a planet started out balmy, it would gradually sink into deep freeze. Still, habitable conditions could persist long enough (hundreds of millions of years, at least) for life to get restarted. The process could happen even faster if life never really went away. Organisms might hop from world to world through an evolving planetary system, either moving deliberately or spreading accidentally as a result of asteroid impacts. They would migrate outward during the star’s path to red giantism, then sharply inward once the white dwarf emerges.

Loeb’s ideas about white dwarfs are surprising, but I am downright floored to learn that Wolszczan has considered the possibility of life on his pulsar planets. “If you look at the astronomy textbooks, they say there is no way life as we know it can be supported on planets like that,” he says. He’s not even a little deterred; he learned long ago to think past the textbooks.

He imagines the planets might have powerful protective magnetic fields, and creatures on the surface would lumber around in thick, radiation-resistant armor. It turns out that Wolszczan has a vibrant, almost animist view of the universe. “My thinking is that life is just another planetary property, like the planet having an atmosphere, continental drift, volcanoes, a greenhouse effect, the right location with respect to a star,” he says. “Life is part of the business, one of the many properties that we may or may not have depending on the initial conditions. It’s that simple.” Ghost worlds are the places where the properties are unfavorable, but even there, he imagines life might still find a way to take hold.

His pessimism is directed not at life on the grand scale — he is concerned much more specifically about the little tribe of humans huddled here on Earth, about surviving the next five centuries rather than the next 5 billion years. “We are governed by evolutionary principles. Everything we do is a struggle to elevate ourselves above everybody else; you want to squash everybody else,” he says. “The big question is: Is there a way to break through that? To become something that can exist in the universe without the permanent danger of getting extinguished?”

In that context, Wolszczan’s studies of pulsars and red giants serve both as inspiration and warning. He has staked out a line of work where he is required, day in and day out, to adopt what he calls a “cosmic perspective” on the world. Not everyone has that luxury, but if more people can share at least some of that perspective — call it the trillion-mile-high view — then we have a shot of transcending the petty competitions that pit us endlessly against one another. And if not, the alternative is not pretty: “We are going to be replaced by something else unless we do something to break out of this evolutionary train.”

We’ve finished dessert, and while Wolszczan takes a contemplative pause, I scan the dining room, trying to see it through his eyes. Everyone looks so happy, so animated, so … oblivious. Maybe he’s right. Maybe there is no need to search far off into space and time to find a ghost world. The next ghost world could be our own.

[This article originally appeared in print as "Phantom Worlds."]

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