Too Darn Hot

Imagine if Earth nearly touched the sun. It would take only one day for our planet to whip around the entire star, and Earth’s surface would be hot enough to melt iron. That may sound like a hellish situation for a terrestrial planet, but if theorist Doug Lin is right, the heavens might be riddled with such orbs.

Of the more than 130 extrasolar planets identified so far, the smallest has been the size of Neptune, about 20 times the size of Earth. Lin, of the University of California, Santa Cruz, predicts a host of smaller planets, similar in mass to Earth but far higher in temperature.

Dubbed hot Earths, these planets would be the lightweight siblings of hot Jupiters—the massive, close-in planets that researchers have detected traveling around more than 25 stars since 1995. Researchers this week reported directly observing the light from two hot Jupiters (see “Alien Light: Extrasolar planets are detected in new way,” in this week’s issue: Alien Light: Extrasolar planets are detected in new way). Hot Earths would orbit less than half as far from their parent stars as do these bigger, gaseous bodies.

Lin’s theory holds that a hot Earth may reside within the orbit of every hot Jupiter. Other than shedding light on the formation and abundance of terrestrial planets throughout our galaxy, the presence of hot Earths could answer a burning question among planet hunters: How do giant, Jupiterlike planets form?

Jupiters are the kingpins of any planetary system. They’re not only the biggest planet, but also when and how they form control such features as the location of the smaller planets and a star’s asteroid belts. Jupiters direct the delivery of chemical elements that might spawn life on habitable planets.

Researchers have long wrestled with two competing theories about the formation of giant planets. The search for hot Earths may settle the debate. These high-temperature terrestrial analogs can form in one of the models, but not in the other, Lin says.

New telescopes and detectors about to begin operation on the ground and a specially equipped spacecraft set for launch in 2007 should give astronomers the means to identify hot Earths, notes planet hunter Debra Fischer of San Francisco State University.

While the ultimate dream remains to find a planet just like home, hot Earths are easier to detect and could be far more abundant.

A hot recipe

Lin’s theory of hot Earths relies on an assumption about how hot Jupiters form. The raw material for making all planets comes from flattened disks of gas, dust, and ice that swaddle newborn stars. These protoplanetary disks last about 10 million years.

Astronomers generally agree that the inner part of a protoplanetary disk, close to the parent star, doesn’t contain enough material to make a planet as heavy as Jupiter. Therefore, a Jupiterlike planet would have to be born farther out and would march inward, toward its parent star. It would do so by gradually losing energy through interactions with the swirling disk from which it arose.

In Lin’s version of the process, which he described in February at a meeting on planet formation in Aspen, Colo., the migration begins soon after the fledgling Jupiter forms. The massive orb’s gravity clears a ring-shaped gap in the rotating protoplanetary disk, similar to a deep groove in a phonograph record. This gap marks a boundary between the inner and outer parts of the disk.

The inner part of the disk revolves around the star faster than the planet does, just as our solar system’s innermost planet, Mercury, whips around the sun faster than its nearest planetary neighbor, Venus does. The difference in speed causes the inner disk to transfer some of its spin, or angular momentum, to the newly formed, Jupiterlike planet. The planet transfers some of this rotational energy to the outer, slower-moving part of the disk. During this sequential transfer, both the inner part of the disk and the planet lose energy.

The energy loss causes the inner sector of the disk to spiral inward. The massive planet, which is gravitationally tied to this sector, moves along with it. The outer part of the disk shifts inward too.

In one of the proposed models for making Jupiters, known as the gravitational-instability model, a Jupiter-mass planet forms wholesale, in a sudden fracturing of the protoplanetary disk. The large planet develops so swiftly that material in the inner part of the disk doesn’t have a chance to coalesce into other planets. So, when the Jupiter journeys inward, it sweeps through very little solid material in front of it and thus travels solo.

Not so in the competing model, a theory known as core accretion. In this model, a Jupiterlike planet forms gradually over a million or more years. First, ice and dust grains in the disk collide and coalesce into a solid core about 10 times as massive as Earth. Next, the core attracts a cloak of gas large enough to make a Jovian mass, equivalent to about 300 Earths.

In this framework, by the time a Jupiterlike planet begins its inward spiral, a mother lode of debris and small, embryonic planets has already built up close to the parent star. These solid objects, swept up in front of the migrating Jupiter, join the inward march.

If the orbital period of the Jovian planet and the period of the lighter weight bodies ahead of it happen to relate to one another in simple whole-number multiples, then the gravitational interaction is enhanced. In those cases, the gravity of the giant planet nudges the smaller objects forward. It also piles up the debris, packing many small objects into planets as massive as Earth or Neptune.

“As long as a hot Jupiter is able to form and migrate in this manner, it would catch all these little guys and shovel them in,” says Lin.

The planetary march comes to a halt when the inner part of the protoplanetary disk thins. Some of the material has fallen onto the star; some of it has gathered into planets or smaller clumps. With the inner disk depleted, the Jovian planet can no longer transfer energy from the inner to the outer part of the protoplanetary disk.

At this juncture, the Jupiter parks itself in a 3-day orbit—one-eighth as far as Mercury’s path is from the sun. There, the giant planet heats up to 1,100 K. But the temperature of the hot Earth that lies in front of it, circling the star every 1.5 days or so, can soar to 1,500 K, Lin calculates.

With the star’s gravity always keeping the same side of the planet facing toward it, the surface of an Earth-mass planet would become hot enough to melt metal. Oceans would either evaporate or would persist as superheated liquids underneath the lid of a high-pressure atmosphere. This wouldn’t be a likely place for life, at least not the kind on our planet.

Looking up

For now, theorists are intrigued but remain skeptical of Lin’s vision of a universe rife with hot Earths. Alan Boss of the Carnegie Institution of Washington (D.C.), and one of the originators of the gravitational-instability model, agrees that his model doesn’t permit the formation of hot Earths, but he’s not convinced that the core-accretion model permits their formation either.

According to Boss, even if a Jovian planet took a few million years to form, as in the core-accretion model, that’s still not enough time for Earth-size planets to have coalesced in the inner part of a protoplanetary disk. At best, a Jupiter migrating inward might plow debris together into a few moon-size chunks, an assemblage that doesn’t add up to an Earth.

Lin’s “basic idea is reasonable,” says John Chambers, also of the Carnegie Institution of Washington. Suppose that a giant planet begins migrating inward a few million years after the formation of the protoplanetary disk. At that time, a fledgling planet in the inner disk would have attained only about one-tenth its final mass. But the gravity of the giant planet might bunch together several of these embryonic planets, enough to make an inwardly migrating Earth, Chambers says.

Another theorist, Nader Haghighipour of the University of Hawaii in Honolulu, says, “I’m keeping an open mind, but I don’t think [the hot-Earth model] is a firm prediction, just an idea at present.”

It’s an idea that is already being put to the test. Last year, observers announced the discovery of the smallest extrasolar planets yet detected—three bodies about as massive as Neptune. Those three planets, says Boss, could be at the high end of a group of smaller, roughly Earth-mass planets.

“We may have already made this leap into a new class of extrasolar planet,” he notes.

Astronomers found evidence for these Neptune-mass planets the same way that they detected nearly all the other extrasolar planets identified to date. As each orbiting planet tugs on its parent star, it causes the star to wobble ever so slightly. To identify the Neptune-mass bodies, researchers had to measure a star’s movement of just over 1 meter per second—about the speed of a jogger.

With detector improvements scheduled over the next 6 months at the Keck Observatory on Hawaii’s Mauna Kea, Fischer says that she and her colleagues, veteran planet hunters Geoff Marcy of the University of California, Berkeley and Paul Butler of Carnegie, expect to observe even smaller stellar wobbles. What’s more, she adds, a new telescope devoted to planet hunting is set to begin its search in October on Mount Hamilton near San Jose, Calif. With this telescope, called the Automated Planet Finder, scientists will search for wobbles induced by hot Earths among stars already known to harbor hot Jupiters.

A spectrograph already in use at the European Southern Observatory in La Silla, Chile, has begun to find wobbles slower than 1 m/s. Called the High-Accuracy Radial Velocity Planet Searcher (HARPS), this instrument is one of several that detected the least-massive extrasolar planet now known, an orb about 14 times Earth’s mass.

HARPS sidesteps a stumbling block that has limited previous investigations, says planet hunter Michel Mayor of the Geneva Observatory in Sauverny, Switzerland. A star’s low-frequency global oscillations can mimic the signal of a small wobble. “We have adopted a new observing strategy with HARPS to eliminate this noise,” says Mayor. “Already, the results are promising, but we have to still increase our effort to try to detect real Earth-mass planets on tight orbits, as proposed by Doug Lin.”

Astronomers are considering a second way, beyond a stellar wobble, to find Earthlike planets. This approach requires a special alignment: The circling planet must pass, or transit, directly in front of its star as seen from a ground-based or an Earth-orbiting telescope. Each time the planet transits, it would block a ten-thousandth or so of the star’s light, says Matthew J. Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. A hot, closely orbiting planet is most likely to have this alignment. The Kepler spacecraft, scheduled for launch in 2007, is designed to detect such mini-eclipses.

Even if the transit of an Earth-mass planet isn’t detected directly, tiny deviations in the periodic transit of larger planets might also indicate the presence of a hot Earth, says Holman. He and Norman W. Murray of the Canadian Institute for Theoretical Astrophysics in Toronto propose this detection scheme in the Feb. 25 Science. Eric Agol of the University of Washington in Seattle and his colleagues describe the same method in an upcoming Monthly Notices of the Royal Astronomical Society.

The prospects are exciting for planetary scientists. Whereas the wobble method permits them to infer only the minimum mass of a planet, transits reveal its exact mass and radius. Once astronomers have this information, they can calculate a newfound planet’s density and find out whether it is gaseous like Jupiter or rocky like Earth.

Astronomers are right at the verge of detecting hot Earths, according to Lin. “This will be a big claim, and researchers will be extraordinarily careful before they make an announcement,” he says. But he suspects that the first such discovery will be only months away.