Origins of a Planet Hunter

What does it take to be a successful planet hunter? According to Caltech astronomer John Johnson, it requires a combination of "painstaking care in data analysis, and a lot of waiting -- patience, patience, patience!"

For Johnson, patience has paid off. He was one among a team of Caltech astronomers who recently announced they had spotted two star systems with pairs of gas giant planets "locked in an orbital embrace," almost like a synchronized dance. It's called orbital resonance, and it occurs infrequently in the universe, most notably between Titan and Hyperion, two of Saturn's moons.

Johnson, who co-authored the paper that appeared in the Astronomical Journal, described the find as "the tightest system that's ever been discovered, and we're at a loss to explain why this happened."

Johnson is quite the promising young planet hunter, having made news headlines earlier this year with his discovery of the second smallest exoplanet, with a mass just four times that of Earth, orbiting the star HD156668 about once every four days. (He used the Keck I telescope sitting on top of Hawaii's Mauna Kea for those observations.) Bad Astronomer Phil Plait playfully dubbed him "John-John" at a public event this past spring, but there's nothing diminutive about his accomplishments to date. Solar_system

I chatted with Johnson last week, just to get a sense of the person behind the planet hunting. He came to astronomy via a more indirect route than most, having never been one of those kids who begged their parents for a telescope. He had a more practical bent: "I loved Legos, taking things apart, and sometimes successfully put them back together," he said.

Everyone told him he should be an engineer, so he initially intended to major in aerospace or mechanical engineering in college. But the subject matter failed to hold his interest, "and my grades reflected my boredom," he admits. (Johnson has high standards: many parents would be delighted to have a "straight B student" in engineering.)

What did hold Johnson's interest? Physics! He liked solving real physics problems, and also loved cosmology and astrophysics, devouring popular science books like "A Brief History of Time." He was fascinated by the Big Bang, in part because of his fundamentalist Christian upbringing. Physics demonstrated "hard scientific evidence" to counter the Biblical accounts he'd learned as a child, and he realized that "one could, in fact, remove god from the equation."

Johnson ended up earning a BS in physics in 1999 from the University of Missouri-Rolla. He then went to Caltech to work as a research assistant on LIGO (Large Interferometer Gravitational Observatory), hunting for gravitational waves, but found his role involved more engineering than grappling with physics questions. So when he applied to graduate school at the University of California, Berkeley, he switched fields once again, this time to astronomy. This was a field where Johnson felt he could make more of an individual mark with his science.

That instinct proved correct, given his success to date. He particularly liked the idea of finding other planets, "imagining what it might look like, and what it might be like to one day go there."

And, fortunately, he has patience to spare, given all those hours spent building things with Legos as a child. It's all about detecting the tiniest of changes in the behavior of stars, using a broad range of complementary techniques -- such as the "Doppler wobble" technique. If there is a planet exerting a gravitational force on its parent star, that star will show a Doppler shift (a "wobble") in the light it emits because of the motion of an orbiting planet (or two). Most stars don't "wobble."

Of course, these are just candidate planets. It takes a lot more time and subsequent analysis before a bona fide "discovery" can be announced. Johnson compares his work to the task of meticulously tending a garden. There are "crops" of stars that must be observed, or "tended," regularly, and "we do a lot of wedding," i.e., tossing out candidate stars that are unlikely to have planets.

"After you tend a garden for a few seasons, you start to see planets cropping up," he said.

Currently, Johnson and his colleagues are monitoring a "crop" of 450 massive stars (those 40 to 100 percent larger than our sun) known as subgiants, and finding "swarms of planets." Apparently, the more massive the star, the higher the likelihood of finding orbiting planets, "because the amount of raw material available to build planets scales with the mass of the star," Johnson explains.

Merely finding the planets is insufficient: Johnson studies the relationship between planets and their parent stars, because mapping out these characteristics in great detail can shed light on the many things we have yet to learn about the planet formation process.

Fans of the Planet Formerly Known as Pluto will be happy to hear that Johnson thinks that -- as we learn more about the planet formation processes -- astronomers may one day reclassify "planets" not by mass, but by how they formed. The complicated process that formed Earth is different from the one that formed Pluto, and both in turn differ from the process that formed a gas giant like Jupiter.

Ultimately, of course, the goal is to find exoplanets that might be "habitable" -- capable of supporting some kind of life. The Kepler mission (see video of its launch, below, for example, is focused on finding planets with small radii by studying microlensing effects, in which the presence of a planet may be detected by the way light bends or warps in a star's vicinity. But there are lots of other approaches being employed as well.

So there will be plenty of opportunities for ambitious planet hunters like Johnson to add to the catalog of 450+ exoplanets in the years to come. And who knows? One of those might even be habitable.