“The Kepler Six”: Hints Page-One Story of the Century May Be Coming Soon


The exo-planet Kepler-11 system, located approximately 2,000 light years from Earth, sounds like something out of a science fiction blockbutser. It is the most tightly packed planetary system yet discovered. All six of its confirmed planets have orbits smaller than Venus, and five of the six have orbits smaller than Mercury's. The only other star with more than one confirmed transiting planet is Kepler-9, which has three.

"Kepler-11 is a remarkable system whose architecture and dynamics provide clues about its formation," said Jack Lissauer, a planetary scientist and Kepler science team member at Ames. "These six planets are mixtures of rock and gases, possibly including water. The rocky material accounts for most of the planets' mass, while the gas takes up most of their volume. By measuring the sizes and masses of the five inner planets, we determined they are among the lowest mass confirmed planets beyond our solar system."

All of the planets orbiting Kepler-11 are larger than Earth, with the largest ones being comparable in size to Uranus and Neptune. The innermost planet, Kepler-11b, is ten times closer to its star than Earth is to the sun. Moving outward, the other planets are Kepler-11c, Kepler-11d, Kepler-11e, Kepler-11f, and the outermost planet, Kepler-11g, which is half as far from its star as Earth is from the sun.

The planets Kepler-11d, Kepler-11e and Kepler-11f have a significant amount of light gas, which indicates that they formed within a few million years of the system's formation.

"The historic milestones Kepler makes with each new discovery will determine the course of every exoplanet mission to follow," said Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington.

The Kepler space telescope looks for planet signatures by measuring tiny decreases in the brightness of stars caused by planets crossing in front of them. This is known as a transit. Since transits of planets in the habitable zone of sun-like stars occur about once a year and require three transits for verification, it is expected to take three years to locate and verify Earth-size planets orbiting sun-like stars.
The Kepler science team uses ground-based telescopes and the Spitzer Space Telescope to review observations on planetary candidates and other objects of interest the spacecraft finds. The star field that Kepler observes in the constellations Cygnus and Lyra can only be seen from ground-based observatories in spring through early fall. The data from these other observations help determine which candidates can be validated as planets.

Unlike the eight major planets in our solar system, which circle the sun along slightly tilted paths, the Kepler-11 planets’ orbits are surprisingly flat and circular: “flatter than a CD case,” according to Lissauer.

These six small planets are packed so snugly together that they actually drag each other back and forth, causing their orbits (which range from 10 to 47 days for the five inner planets) to vary by as much as 20 minutes, Seager says. Earth’s year, by comparison, “doesn’t even vary by a nanosecond.” That strange dance was a boon to scientists, who—by measuring each planet’s gravitational tug on its neighbors—could calculate their masses.

 In September, U.S. astronomers announced they’d found the first potentially habitable planet outside our solar system, Gliese-581g. Astronomers believe there could be six planets orbiting that same red dwarf star, and they’ve been observing it for 11 years. 

But excitement about Gliese-581g was quickly dampened after other scientists looked over the data and said they doubted the planet even existed. A distant star’s winks and wobbles might suggest it’s hosting a planet, but confirming it is “very time consuming,” says Natalie Batalha, Kepler's deputy science team lead at NASA's Ames Research Center in Moffett Field, California, “and telescope time is hard to come by.”

Confirming all 1,235 of the possible planets Kepler has found will be a monumental job, but experts estimate that over 80 per cent of these candidates will turn out to be real planets. For William Borucki, Kepler team leader, one of the most exciting tasks will be checking out the five potential planets that are close in size to Earth, and orbit in the habitable zone of stars that are smaller and cooler than our sun.

“In the coming year, we expect to go through them, and determine which we can confirm,” Borucki says. Some of the 54 candidates in the habitable zone might even have moons with liquid water, he suggests. As the Kepler mission progresses, “we’ll start discovering planets with longer orbital periods,” he says: planets that travel around their suns in 100 days, then 200 days. And, eventually, maybe some that take about 365 days to orbit their own sun—just like Earth.

“Very importantly,” Sasselov adds, “Kepler is not finished yet.” To find an Earth-size planet orbiting a star like our sun in a one-year orbit would take three years, since three different sightings are needed to confirm it isn’t a fluke. Their signals are incredibly faint. But Kepler, these scientists believe, can find them. It might seem myopic to hunt for other forms of life by seeking out planets that look exactly like our own. After all, we still don’t understand how life sprung up here on Earth, and we’re just beginning to learn all the surprising forms it can take on our own home planet.

"We can’t even begin to imagine what the possibilities are out there in the universe. But we have earthling eyes,” Batalha says. “We look around here on Earth and ask ourselves the question, ‘Where does life exist?’ It exists in every nook and cranny, but they all require liquid water.” So we’ll continue hunting for planets that could support water, like Earth, and maybe even life.

It’s impossible to predict what we could find. Kepler space telescope is just watching 156,000 stars “out of a couple hundred billion in the Milky Way galaxy,” Jayawardhana says. How many galaxies are out there? “Many billions and billions.”


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