Quarks to Quasars:The Science of Science Fiction

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New Earths

Written by Ben Bova

Will we ever be able to find a New Earth? Probably pretty soon.

The search for other worlds, especially other worlds that might harbor life, or worlds we ourselves could live on, has been going on for centuries. The payoff is tantalizingly close.

It’s not easy to find planets orbiting other stars.

For one thing, the stars are awfully far away. The nearest star to our solar system is the Alpha Centauri system, at 4.3 lightyears distance. Remember, a lightyear is the distance light travels in a year. Moving at 186,000 miles per second, it takes light eight minutes to go from the sun to the earth, about five and a half hours to reach distant Pluto, and 4.3 years to span the distance to the next nearest star.

And while stars are shining from light that they generate internally, planets are not self-luminous; they shine only in the light they reflect from their parent star. Also, planets are much, much smaller than stars.

For generations, astronomers spoke of the “firefly and searchlight” problem. Stars are like big, brilliant searchlights. Planets are like tiny dim fireflies. Tough to see, especially when they’re huddling close to the searchlight.

But astronomers are a canny lot. And stubborn.

If you can’t see ‘em, maybe you can detect them some other way.

Unseen Companions

Peter van de Kamp, at Swarthmore College’s Sproul Observatory, championed the astrometric technique for finding exoplanets (i.e., extrasolar planets, planets of other stars). Astrometric means, literally, “star measuring.” The technique depends on the fact that a planet’s gravitational field exerts a force on the star it’s orbiting around. The planet’s tug on its star is minuscule, rather like the impact of a flea on an elephant’s rump, but it is real and—in a precious few cases—it might be measurable.

Starting in the 1940s, Van de Kamp and a few other astronomers announced they had discovered “unseen companions” at several of the stars closest to us, including Barnard’s star (six lightyears away) and the double star 61 Cygni (11 lightyears away).

They could not see the “unseen companions,” of course. They spent years measuring extremely small fluctuations in the movement of those stars across their line of sight (called the star’s proper motion) and assuming that those little wobbles were caused by planetary-sized bodies orbiting around the star.

In 1975, though, Van de Kamp’s claims were thrown into the trash heap. An independent study of the images of Barnard’s Star, which Van de Kamp had studied for more than 20 years, showed that the wobbles Van de Kamp observed were due to defects in his telescopic apparatus. Although Van de Kamp insisted that his work was valid right down to his death in 1995, the scientific world discounted his findings.

Pulsar Planets

In 1991, Andrew Lyne and his colleagues at Britain’s Jodrell Bank radio telescope reported they had detected a planet orbiting the pulsar PSR1829-10. Although this “discovery” turned out to be an error, shortly afterward Alexander Wolsczan of the Aricebo radio telescope in Puerto Rico and Dale Frail of the National Radio Astronomy Observatory in New Mexico announced they had found evidence of two planets at the pulsar PSR1257+12.

Pulsars are the collapsed, incredibly condensed cadavers of massive stars that have blown up in titanic supernova explosions. Could planets survive the billion-degree-hot plasma of a supernova explosion? Or had these planets coalesced out of the tangled cloud of gas and dust surrounding the pulsar after the original star had blasted itself into oblivion?

Pulsars emit regular pulses of radio energy, and the planets were detected because of a slight shift in the frequency of the pulsar’s emissions.

Pulsar planets seemed to be a cruel “good news-bad news” joke. Yes, planets orbiting another star had finally been found. But they were either burned-out cinders or newborns bathed in lethally intense radiation.

Planets of Sun-like Stars

Finally, in 1995 Michael Mayor and Didier Queloz of the Geneva Observatory in Switzerland discovered a planet orbiting the Sun-like star 51 Pegasi, which lies 42 lightyears from us.

They used a new technique in their hunt. As we have seen, the old astrometric method depends on detecting perturbations in a star’s proper motion, that is, its motion across our field of view.

With spectrographic analysis, however, it is possible to make much more sensitive measurements of a star’s radial velocity: its motion toward or away from the observer.

When a planet crosses in front of its star’s face, it causes a slight dip in the star’s brightness. Slight, but measurable—in some cases. It depends mainly on the size of the planet and the sensitivity of the astronomer’s equipment.

George W. Henry of Tennessee State University in Nashville made the first visual detection of an extrasolar planet in 1999. Note that “visual detection” does not mean that the planet was actually seen.

Henry was able to measure a dip in brightness of the star HD209458, deducing that a Jupiter-sized planet was passing in front of it. HD209458 lies about 150 lightyears from Earth. In 2001 astronomers used the Hubble Space Telescope to detect sodium in the planet’s atmosphere. Hydrogen was detected two years later.

From the star’s 1.7 percent decrease in brightness during the planet’s transit across its face, Henry estimated that the planet’s radius is 1.6 times bigger than Jupiter’s. By determining the inclination of its orbit, a mass of 63 percent of Jupiter was deduced, showing that the planet must be largely gaseous, like the Jovian planets in our own solar system.

The temperature of the planet’s atmosphere was measured at 1100 degrees Celsius, so hot that some 10,000 tons of hydrogen are being boiled away from it every second.

In 2006 France launched COROT, a spacecraft intended to find Earth-sized planets by measuring fluctuations in stars’ brightnesses. In March 2009 NASA launched the Kepler spacecraft, with the same goal.

Kepler is the brainchild of astronomer William J. Borucki, of NASA’s Ames Research Center in California. In a laboratory mockup of the Kepler system the equipment detected the equivalent of Mars-sized planets, half the size of Earth.

Basically, the Kepler system consists of a 1.4-meter Schmidt reflector telescope linked to photoelectric detectors capable of measuring differences in light input down to 20 parts in a million. When an Earth-sized planet passes in front of a star of the Sun’s brightness, the drop in light output is eight parts in a thousand, well within Kepler’s capability. Distance doesn’t matter; the drop in brightness is the same, near or far.

Early in 2009 COROT found a planet about twice Earth’s size—the smallest detected so far. But the exoplanet is so close to its star that its “year” takes only 20 Earth days. It is undoubtedly too hot to hold liquid water.

Kepler should be able to detect hundreds of Earth-sized planets orbiting at Earth-type distances from their stars: planets that could hold liquid water and be abodes for life.

Of course, the exoplanets’ orbits must be aligned so that they cross their parent stars’ disks. But Kepler’s optics are able to look at thousands of stars within a few hundred lightyears of Earth, so the chances of finding Earth-sized planets look good–if they’re really there.

Once Earth-sized planets are found, the race will be on to try to detect oxygen and water vapor in their atmospheres. And, in time, to actually get photographic imagery of them. For that, we will have to wait for new, bigger, more sophisticated spacecraft such as NASA’s James Webb Telescope, or the Terrestrial Planet Finder (TPF).

The Webb Telescope is intended to replace the aging Hubble. TPF is a complex of telescopes dedicated to finding new Earths. Both of them are in the planning stages. Both could be cut out of NASA’s budget by Washington.

But if COROT or Kepler find Earth-sized worlds out among the stars, that could supply the public and political demand to move onward and seek details about these potential New Earths.

Astrometry Again

Meanwhile, a team from NASA’s Jet Propulsion Laboratory reported in July 2009 that they had found an exoplanet using the old astrometric technique. The planet is about six times the mass of Jupiter. It orbits the star VB 10, which lies about 20 lightyears from Earth.

Van de Kamp redeemed?

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Request for input: What scientific subjects are you curious about? Let me know and I’ll try to write a column about them.

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Ben Bova is the author of more than 120 books of futuristic fiction and nonfiction about science. He is a past president of Science Fiction and Fantasy Writers of America, President Emeritus of the National Space Society, and a Fellow of the American Association for the Advancement of Science.