A joint Fermilab/SLAC publication

Particle physicist lends skills to planet hunt

Artist's illustration of Kepler spacecraft. Credit: NASA/ Kepler Mission/ Wendy Stenzel.

Artist's illustration of Kepler spacecraft. Credit: NASA/ Kepler Mission/ Wendy Stenzel.

You don’t normally think of high-energy physicists working with NASA to find planets that humans could live on. Working on the Large Hadron Collider or dark-energy-seeking telescopes, yeah, but, planet hunting? Not so much.

Yet, Jason Steffen, an astrophysicist at Fermilab, is a long-time member of NASA's Kepler Mission and its only practicing  particle physicist. He helped make possible the mission’s discoveries announced Wednesday of a six-planet solar system 2,000 light years away, tits first Earth-sized planet candidate, and the first such candidate that potentially could support human life.

It's one small step for Steffen and his Kepler collaborators and one giant step for dreamers everywhere.

“In one generation we have gone from extraterrestrial planets being a mainstay of science fiction, to the present, where Kepler has helped turn science fiction into today’s reality,” says NASA administrator Charles Bolden upon announcing the data release.

The Kepler spacecraft-mounted telescope, 11 million miles from Earth,  scans the sky to find, for the first time, distant life-sustaining planets the size of Earth.  Telescopes can't directly spot planets smaller than Jupiter, but Kepler uses starlight to indirectly see smaller planets. Planets that could potentially sustain life fall into a Goldilocks-like “habitable zone,” orbiting the perfect distance from a star like our sun so as to not be too hot or too cold. Often these planets’ orbits cross close in front of a star, or "transit," making them visible through the blinking out of the stars’ light.  By measuring the brightness change of a star as a planet passes in front of it, as well as  the time between these transits, scientists can tell the planet’s size, orbit, and estimated temperature.

But the closeness to the star that allows Kepler to “see” the planet also often makes it too hot for life. The more distant planets outside Kepler’s view hold a greater chance of being just right to sustain life. Kepler has difficulty spotting these planets because of orbit cycles that are longer than the time frame of the released data or because they do not transit stars.

That’s where Steffen comes in.

Jason Steffen

Jason Steffen

“We are sensitive to planets that Kepler can’t see directly,” says Steffen of the analysis team he leads. “That is where it gets interesting.”

He helped pioneer a search method that can detect distant planets more than 600 times smaller than Jupiter and out of range of Kepler‘s telescope. He uses computerized mathematical procedures, called algorithms, including many that are commonplace to particle physics, to probe deeper into space in the area around a planet Kepler sees to find planets in that often cooler, more habitable zone.

This is done by studying the amount of time it takes a planet to complete its orbit past a star. Deviations from a constant orbit time  indicate the presence of some additional unseen planet whose gravitational pull is changing the orbit speed of the observed planet. This technique is used to confirm that distant images seen by the Kepler telescope are planets and not pairs eclipsing binary stars blurred by the telescope to resemble an object of planet size.

Steffen expects to look at hundreds of planetary systems during the mission’s 3 ½ years. By looking at patterns in the times it takes planets to transit, scientists could fill in some blanks about how planetary systems form with relation to their distance from a sun.

The discoveries announced Wednesday are part of several hundred planet candidates identified in new Kepler mission science data release. The findings increase the number of planet candidates identified by Kepler to date to 1,235. Of these, 68 are approximately Earth-size; 288 are super-Earth-size; 662 are Neptune-size; 165 are the size of Jupiter; and 19 are larger than Jupiter. Of the 54 new planet candidates found in the habitable zone, five are near Earth-sized. The remaining 49 habitable-zone candidates range from super-Earth size -- up to twice the size of Earth -- to larger than Jupiter.

The findings are based on the results of observations conducted from May 12 to Sept. 17, 2009, of more than 156,000 stars in Kepler’s field of view, which covers approximately 1/400th of the sky.

Based on this large data sample “….it turns out that close to 20 percent of all stars are orbited by
planets, meaning that a significant fraction of the stars in the sky are orbited by alien worlds,” says Tim Brown, Kepler co-investigator and physics professor at the University of California Santa Barbara, in a press release.

Just as Kepler collaborators look  for a planet that is just right for habitation, the group also needed just the right skill set to expand its search reach. Steffen happened to be one of the only people in the world versed in that area of research because of his graduate degree work in transit timing variations.

NASA to note and asked him to join the Kepler mission as a participating scientist, collaborators drawn from outside the normal NASA research field to to enable the team to more effectively execute the mission's science program.

Steffen and his thesis advisor Eric Agol, associate professor of astronomy at the University of Washington, fine-tuned this method of tracking fluctuations in the orbits of planets, making it unexpectedly useful for short-time mission such as Kepler’s planet hunting.

Scientists had tracked orbit fluctuations before but always on the time scale of comparing  many thousands of orbit cycles during the course of many decades. Using smaller data sets taken during shorter periods of time seemed pointless because they generated such small effects--until Steffen and Agol came along.

By introducing a  new tracking method, they reduced the time needed to identify these hard-to-find planets to a year with only a dozen or two orbit cycles.  Matt Holman, an astronomer with the Harvard-Smithsonian Center for Astrophysics, had also been focusing on the same problem. The two joined together to adapt their tracking methods, along with help from colleagues across the country, for the Kepler exoplanet hunt.

This work with exoplanets doesn’t have direct applications to high-energy physics or Steffen’s other work at Fermilab on chameleons, axion particles, and holographic noise. However, particle physics uses many of the same mathematical algorithms in experiments and there is no telling whether Steffen’s technique could become useful in that field in the future.

“It’s fair to say I can cannibalize the components of the algorithm for future projects,” Steffen says.

For more information:

Kepler mission website

Kepler discovers new planetary system press release

Kepler finds Earth-size planet in habitable zone press release

Kepler public data website

Kepler search range. Credit: NASA.

Kepler search range. Credit: NASA.

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