Eating planets gives stars indigestion

November 09, 1999

PLANETS are tiny, insignificant things compared with their parent stars-little more than moths fluttering around the stellar camp fire. You might think that they could have no discernible effect on their stars and, until recently, astronomers would have agreed with you.

But it is becoming clear that when old stars grow huge enough to swallow their planets, they can become quite ill. "It seems that planets can have a profound effect on stellar evolution," says Mario Livio of the Space Telescope Science Institute in Baltimore, Maryland. Planet gobbling could explain the properties of many giant stars and the beautiful shapes of glowing clouds known as planetary nebulae.

Any star that becomes a giant can swallow nearby planets, and almost all stars eventually evolve into giants. When a star is in its so-called main sequence phase-the phase our Sun is currently going through-it "burns" hydrogen nuclei in its core, fusing them and releasing energy. The energy that pours out balances the great weight of the star and prevents it from collapsing. However, when all the hydrogen fuel in the core is exhausted and there is no longer any heat being generated, the core is crushed by the star's weight, heats up tremendously and ignites unburnt hydrogen in a shell around it. The surge of new heat inflates the outer parts of the star, swelling it into a monstrous red giant often more than a hundred times the diameter of the Sun. The red giant phase lasts about a tenth as long as the main sequence.

No one dreamt that planets could influence this process, because the only planets likely to be swallowed are close-orbiting ones, and in our Solar System the close planets are tiny. Mercury, Venus and the Earth would be little more than a light snack for our Sun. But in the past few years, astronomers have discovered about 20 planets orbiting other stars. Most of these planets are giants at least as massive as Jupiter. "It now appears that at least 5 per cent of nearby stars have planetary companions," says Livio. The big surprise was that in several of these systems, the giant planets orbit searingly close to their stars-most notably in the case of 51 Pegasi B, which is a hundred times closer to its star than Jupiter is to the Sun. "This was a totally unexpected discovery," says Livio.

And it seems that such planets, if swallowed, could drastically change their stars. In the June issue of Monthly Notices of the Royal Astronomical Society, Livio and his colleague Lionel Siess reported computer simulations of this catastrophe. Surprisingly, they found that a large planet continues to orbit inside the star for thousands of years, only slowly being vaporised by the heat. "You have to remember that these stars are super-tenuous gas balls with their matter smeared over an absolutely huge volume," says Livio. "Their outer regions are as rarefied as what we would consider a good vacuum on Earth."

Despite the ethereal nature of the stellar envelope, it still applies a frictional drag that slows anything passing through it, so the swallowed planet gradually spirals down towards the core. How close it gets depends on its mass. According to the simulations, a planet with the mass of Jupiter would evaporate long before it reached anywhere near the core. But objects more massive than 20 Jupiters would survive almost all the way to the just outside the core, where the temperature is about 2 million degrees. There, they would be dissipated by the combined effects of the heat and the tidal forces exerted by the core, which should stretch a planet and rip it apart.

But the star doesn't get off scot-free. Livio and Siess found that gravitational energy dumped by the sinking planet should heat up the star, causing it to puff off its cool outer layers as expanding shells of warm gas and dust glowing with infrared light. And the rapidly orbiting planet should raise the spin rate of the star-red giants normally rotate only sluggishly-and contaminate the star with its heavy elements.

Clearly, these effects depend on the mass of the swallowed planet, with more massive planets having a greater effect. And planets are not the only stellar companions that can be swallowed by a star when it becomes a giant. In the past three years, astronomers using the infrared sky surveys DENIS and 2MASS have discovered a few dozen solitary brown dwarfs-small failed stars which can be between 10 and 80 times the mass of Jupiter. As these are bigger than planets, they can have a more profound effect on a star. They may even merge with the stellar core.

Whether they merge or not depends on the stage in the star's life when they are swallowed. Livio and Siess's simulations show that if the star's envelope is thin, containing less than a hundredth of a solar mass of gas, the gravitational energy unleashed by a sinking brown dwarf may be enough to eject the envelope, leaving the dwarf in orbit around the stellar core. If the envelope is thicker, the brown dwarf will either be dissipated or collide and merge with the core.

Are there any signs of these processes in real stars? Fortunately, some of the effects of swallowing a planet or brown dwarf, such as a high spin rate and lithium abundance, should persist for hundreds of thousands of years. Livio and Siess say that many red giants have all of these tell-tale traits-they are spinning very rapidly and emitting unexpectedly large amounts of infrared radiation. And their light shows the spectral fingerprint of lithium, an element that does not normally survive for long in a star, which suggests that it was added by a planet fairly recently.

The two astronomers estimate that between 4 and 8 per cent of red giants show evidence of planet swallowing. "This agrees well with more direct estimates of how common planets are," says Livio. Caty Pilachowski of the National Optical Astronomy Observatory at Kitt Peak in Arizona agrees. "The planet-swallowing hypothesis is the best explanation I've seen for the origin of these lithium-rich giants."

Planet swallowing may also explain another puzzle. By the end of its giant phase, a star's radiation has completely blown away its outer layers, exposing the core-whose eventual fate is to end up as a super-dense white dwarf. Intense ultraviolet radiation from the core ionises the surrounding cloud of ejected gas, causing it to fluoresce and glow. This is a planetary nebula, so-called because of the glowing cloud's resemblance to a planetary disc. There seems no reason for the gas to be ejected more forcefully in one direction than another, so you might think these nebulae would be spherical. But many are bipolar, with material ejected along a preferred axis.

Twenty years ago, Livio suggested that this bipolar shape may arise when the nebula has another star as a close companion. He proposed that the companion becomes enveloped in the main star's outer layers and spirals inwards, dumping enough energy to eject these layers, albeit at a leisurely speed of about 20 kilometres per second. This material stays in the plane of the companion's orbit to form a doughnut-shaped cloud around the main star. Later, when the star's blistering core is exposed, its radiation drives a fierce stellar wind of matter away from the star at about 1000 kilometres a second. The wind blows in all directions, but because it is impeded by the slow-moving gas in the doughnut, it emerges perpendicularly as a bipolar outflow. "The doughnut acts like a corset, and the wind blows two bubbles perpendicular to the corset," says Livio.

The problem with Livio's idea was that only a few planetary nebulae appeared to have both a white dwarf and a spiralled-in companion in their hearts. But, working with his student Noam Soker, who is now at the University of Haifa in Israel, Livio showed that it's no problem if the companion was a massive planet or brown dwarf. "It could have completely dissipated or merged with the white dwarf," he says.

All this applies to stars near the end of their lives. But it seems that planets might be gobbled up by young stars too. There are high quantities of heavy metals in the atmospheres of several stars with massive planets-a sign that they once ate one or more planets ("Death stars", New Scientist, 23 October, p 10). These unfortunate bodies may have been dragged inwards by tides in the disc of gas surrounding each young star.

Despite all the successes of the planet-gobbling idea, however, there are still a few puzzles left to solve. Pilachowski points out that the story became more complex with the recent observation of a lithium-rich giant in the globular star cluster M3. A team led by Robert Kraft of the University of California at Santa Cruz reported finding the star earlier this year. The astronomers argue that it contains far more lithium than a planet could have carried-in fact almost ten times as much lithium as any planet could have delivered to it. "They argue that the lithium must have actually been made in the star by nuclear reactions," says Pilachowski. "I think the jury is still out. As usual, we need more data." But Pilachowski says this apparent discrepancy takes nothing away from Livio and Siess's work. "Their work highlights an important weakness in our theory of stellar evolution," she says. "Namely that planets can have a profound, and until now unappreciated, effect on the evolution of stars."

As to whether the Earth will one day be consumed by the Sun, there is still some doubt (see "Earth to ashes"). "But whatever happens," says Livio, "it won't be for another 5 billion years. So we can all sleep safely in our beds tonight."
Author: Marcus Chown

New Scientist issue 13th November 99


New Scientist

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