Astronomers have discovered a statistical odd-ball of a planet – one orbiting a star most planets its size typically don't call home. And they found it with what's been dubbed "the little telescope that could."
The planet, about the size of Saturn, orbits a star some 700 light-years away near the constellation Leo. Dubbed KELT-6b, the planet orbits its host star once every 7.8 days. It's the fourth of six discoveries (two of which remain works in progress) by a planet-hunting project called KELT – Kilodegree Extremely Little Telescope.
Yes, in this day and age of the mighty Keck and Very Large Telescopes, the Hubble Space Telescope, and other sizable "light buckets," there is room in the planet-hunting arsenal for "telescopes" whose optics are nothing more than high-end telephoto and wide-angle lenses typically used by professional photographers.
More on that later. First, the planet.
KELT-6b is of special interest because it has a well-studied cosmic cousin – the first extrasolar planet discovered using the transit approach – to which KELT-6b can be compared, right down to the makeup of its atmosphere.
The cousin, a planet called HD209458b, orbits a star 150 light-years away in the constellation Pegasus. It has 220 times Earth's mass and orbits its star once every 3.5 days. Its host star is so bright and close that researchers have been able to detect and begin to describe the planet's atmosphere, right down to the presence of oxygen and a super storm similar to Jupiter's Great Red Spot. Although more distant, KELT-6b's star also is bright enough to allow for similar studies of the planet.
The differences between these two systems hold the promise of providing astronomers interested in the birth and evolution of solar systems with rich insights because HD209458b orbits a metal-rich star, one with a relatively high abundance of elements heavier than hydrogen and helium. KELT-6b's star is among the poorest of known metal-poor stars.
Metal content, at least as astronomers define metal, is important since elements heavier than hydrogen and helium are vital to building the molecules needed for the emergence, care, and feeding of organic life.
The new planet was discovered by Karen Collins, an electronics engineer-turned-astronomy PhD candidate at the University of Louisville and a member of the KELT research team.
As with other extrasolar planet searches, including NASA's Kepler mission, KELT researchers hunt for a planet by detecting the faint dimming that occurs when the planet passes in front of its host star – the transit method.
In this case, the hunt was "really like an adventure," said Ms. Collins at a press briefing Tuesday during the spring meeting of the American Astronomical Society, which runs through Thursday in Indianapolis.
Initially, the data hinted at the presence of a planet. But when the team followed up the discovery using a more-capable telescope and a different planet-hunting technique in an effort to determine the mass of the purported companion, the results came back as no mass.
"We didn't know what was going on here," she says. The hint could have been natural fluctuations in the star itself, or perhaps a glitch in KELT’s detectors.
So the team members turned to another telescope larger than KELT to see if they once again could detect transits. This time, they got a stronger indication – one that pointed to an orbital period of 7.8 days and a transit time of 5.5 hours. That information was enough to earn the team time on the 10-meter Keck telescope in Hawaii, which confirmed the discovery and provided data on the planet's mass.
A planet of KELT-6b's mass orbiting a metal poor star is unusual. At Tuesday's briefing, Steve Howell, an astronomer from NASA's Ames Research Center at Moffett Field, Calif., unveiled the results of a study of stars around which the Kepler program's planetary discoveries orbit and the range of planets' masses for metal-poor and metal-rich stars.
After analyzing 268 stars for which Kepler has found planets, the team noted that the stars all were larger than previously thought, implying that the planets they host are as well. But Dr. Howell's research also found that metal-rich stars hosted planets of all sizes and compositions, from gas giants to rocky planets. Metal-poor stars, like KELT-6b's, tended to host Neptune-scale planets or smaller.
Out of all of the planet-hosting stars he's analyzed so far, only two metal-poor stars host planets more massive than Neptune, as KELT-6b is.
Understanding the differences, and the nuances within differences, between these two systems could help fill in the picture astronomers have assembled so far about making planets and the factors determining their composition.
The idea for the KELT project emerged nearly seven years ago as Joshua Pepper, an astronomer at Lehigh University in Bethlehem, Pa., was working on his PhD at Ohio State University. He was performing calculations to estimate the capabilities of a space mission to hunt for transiting planets – a mission that ultimately never left the drawing board.
Transiting planets are particularly valuable because the approach used to detect them yields enough information to get initial estimates of their masses and sizes. These features lead to estimates of density, which gives a rough idea as to a planet's bulk composition.
The 170,000 stars in a relatively narrow field of view that NASA's ailing Kepler spacecraft has been observing are too far away for detailed studies of the planets the mission has uncovered.
The goal for KELT, says Dr. Pepper, also a member of the KELT team, was to search among stars over very wide patches of sky in both hemispheres for planets orbiting stars close enough for the Hubble and Spitzer Space Telescopes to study.
"That's the niche we fill," he says. "We're not going to find tons of planets, but the few that we find will be very scientifically valuable."
That role may soon end with the coming launch of NASA's TESS mission, a space telescope that will hunt for transiting planets around nearby stars.
Even so, the KELT project, which includes researchers from several universities, as well as a small group of dedicated amateur astronomers, will continue to have scientific legs, Pepper says. And at between $50,000 and $75,000 per telescope, these will remain cheap but effective tools for astronomers studying the makeup and behavior of stars, he says.