The United States is scheduled to launch on Friday an orbiting telescope designed to help answer one of the oldest and deepest questions of astronomy: Are we – or at least our planet’s microbes – alone in the galaxy?
The Kepler spacecraft will stare at a patch of sky – the same 100,000 stars near the northern constellation Cygnus, all at once – for at least 3-1/2 years. The goal is to detect Earth-like planets orbiting their host stars at distances thought to be sweet spots for life.
Dubbed habitable zones, these are orbits where a planet is bathed in light that is strong enough to permit liquid water to collect and remain a persistent feature of the planet’s surface.
For planet-hunting astronomers, the $591 million Kepler mission promises to cross what they’ve dubbed the “Great Divide” that separates the decidedly uninhabitable planets they have found so far with the Earth-like ones they seek.
During the past 15 years, ground-based telescopes have detected more than 300 so-called exoplanets – planets that are beyond our solar system. But these discoveries have involved a bizarre menagerie of objects ranging in size from 10 times Jupiter’s mass to nearly Earth-scale planets.
They often follow highly elliptical orbital paths, subjecting them to enormous temperature swings. Some are more dense than lead; others would float like foam on water, notes William Borucki, a scientist at National Aeronautics and Space Administration’s Ames Research Center in Sunnyvale, Calif.
Some orbit so close to their host stars that one “year” in those planets is less than one Earth day.
The ultimate goal, however, is “to someday take a picture of a pale blue dot orbiting a nearby star,” says Debra Fischer, an astronomer at San Francisco State University. To decide how to do that in the most efficient and cost-effective way, it’s vital to know how many of these planets are likely to be out there for a given number of stars. The frequency with which Earth-like planets are found will be a key driver for future planet-hunting missions, she says.
And if Kepler finds only a tiny handful of such planets in habitable zones?
“That would be another profound discovery,” says Dr. Borucki, the mission’s lead scientist. “It will mean that Earths must be very rare. We may be the only extant life,” at least in our galaxy.
No aliens? Alien chemistries will do
Even if Kepler detects few Earth-like worlds in Earth-like orbits, the number of other planets it finds is likely to explode. Some solar-system experts would be content to give up the prospect of alien life forms for alien geology and chemistry.
The pressure at the center of the Earth is some 3.5 million times higher than the atmospheric pressure at the surface, but Jupiter’s core is squeezed to some 70 million higher, notes Raymond Jeanloz, a planetary scientist at the University of California at Berkeley.
At these pressures, helium is no longer a gas incapable of carrying electricity; it’s a fluid metal that has no problem conducting electricity.
On some of the larger exoplanets, pressures at the cores are estimated to reach billions of times Earth’s surface pressure.
“By the time you get to billion-atmosphere pressures, it’s not just the chemical bonds between atoms” that change. “Atoms themselves are crushed,” Dr. Jeanloz said during a briefing at the American Association for the Advancement of Science’s annual meeting in Chicago last month. “This is a regime of a new kind of chemistry.”
The one-ton Kepler observatory boasts a 1.4-meter (4-1/2 foot) diameter mirror and a push-the-envelope camera, according to James Fanson, project manager for the Kepler mission at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. A typical digital camera has roughly eight million to 10 million individual picture elements, or pixels, on its light detector. Kepler’s camera boasts 95 million pixels.
The camera won’t take pictures of the stars it monitors, however. Instead, it will measure changes in starlight as an orbiting planet slips in front of its host star. It’s a tough measurement to make – akin to measuring the dimming effect of a flea passing in front of a car’s headlight, Dr. Fanson says.
The amount of dimming yields the planet’s radius. The elapsed time between each dimming episode yields its orbital period. From this data, and from a knowledge of the parent star’s traits, astronomers can calculate the planet’s mass, its density, and whether it falls in the habitable zone.
Not all solar systems will be oriented in ways that allow the telescope to detect planet “transits” across the faces of stars. So Kepler’s “transit” technique for picking out planets has dictated the large number of stars the telescope must monitor.
Tracking 100,000 stars
Picking the targets, 100,000 sun-like stars, was no cake walk. Astronomers spent five years methodically measuring traits of 4.5 million stars in Kepler’s planned field of view.
Out of the resulting 100,000-star catalog, Kepler scientists estimate that perhaps only 10 percent have planets with orbital periods short enough to allow for repeated detections within Kepler’s 3-1/2 year primary mission length. Some 0.5 percent of the 100,000 stars are expected to reveal planets orbiting at Earth-like distances.
That still means there is potential to find hundreds of Earth-like planets orbiting in that sweet spot. The solar systems range in distance from around 50 light-years away to some 3,000 light-years or more.
The architecture of solar systems
But finding planets at habitable distances from their suns does not alone a livable planet make.
Missions to Mars, Jupiter and its moons, and Saturn have yielded locations where simple forms of life might have gained a foothold and may still exist today, notes Renu Malhotra, a planetary scientist at the University of Arizona in Tucson. Jupiter and Saturn in particular fall outside the traditional “habitable” range of orbits.
Solar-system architecture is critical to habitability, she explains, adding that she expects Kepler, as well as follow-up observations from ground-based telescopes, to reveal in more detail how planets are distributed in these new solar systems.
Earth’s moon, for instance, has stabilized Earth’s tilt and limited the periodic wobble in Earth’s spin. This reduces the extremes at which climate could change over periods ranging from seasons to geologic time scales.
Our solar system’s order of planets has led to regions such as Kuiper Belt beyond Neptune or the asteroid belt between Mars and Jupiter, where relics from the solar system’s birth remain in relatively stable, tidy orbits. Without such shepherding, they could hurtle chaotically, boosting the risk of pummeling the rocky inner planets.
“It’s quite an exciting time to be alive,” he says.