How Einstein's theory of special relativity helped find a new planet

To find the planet, astronomers used Einstein's theory as it pertains to the intensity of a beam of light. The method could add more exoplanets to a growing list, no 'wobble' or 'transit' required.

Courtesy of NASA / AP
Kepler space telescope is designed to search for Earth-like planets in the Milky Way galaxy. The telescope has been in space since 2009, but scientists keep finding new ways to use it — even using special relativity — to find extra-solar planets.

With a little help from Einstein's theory of special relativity, astronomers have discovered a planet orbiting a star some 2,000 light-years away using a new approach that was barely a gleam in its proposers' eyes a decade ago.

The planet is a bit larger and about twice as massive as Jupiter. It orbits its sun-like star once every 1.5 days. The team making the discovery estimates the planet's temperature at a searing 3,600 degrees Fahrenheit.

On one level, such "hot Jupiters" are a dime a dozen these days. Because they are massive and close to their host stars, they are the easiest planets to spot with virtually every planet-hunting technique astronomers have used to date.

What sets this discovery apart, however, is that the planet is the first to have been found through a process that in some ways could simplify planet hunting, researchers say. Its effectiveness is limited to big planets orbiting close to their stars, the team reporting the discovery acknowledges.

But it also holds out the hope of finding such planets when the parent stars may be too faint for other, currently used techniques. This opens the possibility of adding many more extra-solar planets to a catalog that now tops 800 of them.

No need to hunt for the wobble a planet's gravity imparts to its star's spectrum. No need to wait for a planet to pass in front of its star, known as a transit.

Instead, the team looked for a combination of three relatively small effects that wax and wane throughout a planet's orbit around a star. This delivers a different signal to a planet-hunting device like NASA's Kepler spacecraft than the eclipsing planet, or transit method, delivers, notes David Latham, an astronomer at the Harvard-Smithsonian Center for Astrophysics and a member of the team discovering the planet.

"The transits last just a short time, just a couple of hours," Dr. Latham writes in an e-mail. But the effects the team tracked "rise and fall continuously through the entire orbital period of the planet, roughly 36 hours, so it’s not hard to distinguish these phenomena."

And it can detect planets that don't transit their stars.

The approach was conceived 10 years ago by Harvard University astrophysicist Avi Loeb and Scott Gaudi, now an assistant professor of astronomy at The Ohio State University in Columbus, who took a cue from Albert Einstein.

One prediction of Einstein's theory of special relativity is that when an object is moving at a pace close to the speed of light, any light it emits appears more intense along the object's line of motion, forming a beam. To an observer watching the object approach, the light looks brighter than it would if the object were stationary.

The effect is most pronounced in powerful astronomical events such as gamma-ray bursts, in which matter emitting the gamma rays is accelerated to 99.9 percent of the speed of light, Dr. Loeb explains.

Indeed, to an astronomer looking directly into the beam, the effect can lead to the illusion that the light is traveling faster than its 186,000-mile a second speed limit. Such beams emanate from the poles of supermassive black holes that have gone on feeding binges. Researchers call them superluminal jets.

Loeb says he wondered if the effect were noticeable enough at slower velocities to use beaming to detect planets orbiting other stars. As a planet orbits, its gravity would tug the star to and fro. Perhaps the starlight would intensify slightly as the planet reaches the points along its orbit where it's pulling the star toward Earth.

After some back-of-the-envelope calculations to see if the question was worth pursing beyond the "I wonder if" stage, Loeb approached Gaudi, who calculated the intensity of the beaming effect for the types of stars that missions such as Kepler would examine.

"We estimated that it should be possible to detect it," Loeb says, although others were unconvinced.

Since then, researchers have refined the approach by looking for two other effects that change during a planet's orbit. A star can appear to brighten as gravity from a close-in giant planet gives the star more of a rugby-ball shape as seen from Earth. And a giant planet can go through phases, much like the moon's, as an observer watches it swing around its star.

A star's natural variations in light could make detecting the beaming effect more difficult, so having the additional tests provides a more reliable indicator.

The team discovering the planet, led by Simchon Faigler at Israel's Tel Aviv University, picked its target star from one of the stars in Kepler's catalog – one for which no planet had yet been discovered.

After designing a computer program that hunts for the three features, the team analyzed 26 stars that looked like they'd give the new approach the best chance of succeeding. One of these, known as Kepler 76, yielded the evidence for the planet, Kepler 76b. The discovery was confirmed when additional observations detected the wobble in the star's spectrum, the so-called radial-velocity method.

One side benefit from using the new technique, which bears the whimsical name BEER (BEaming Ellipsoidal Reflection), is that it appears to have given the discovery team a window on the planet's atmosphere. The intensity of the tug implied by the BEER's beam was higher than the intensity of the tug the radial-velocity measurement provided. Mass estimates for the planet based on the approaches BEER uses also were inconsistent.

Additional calculations suggested that the difference could be attributed to a wide belt of superfast air circulating around the planet – a jet stream that has downed one shot of espresso too many.

Dr. Faigler and colleagues note that the approach so far can be used to detect exoplanets whose masses are at least half of Jupiter's mass and orbit their stars in 30 days or less. But with additional refinements, the team hopes to improve the approach's ability to detect planets with weaker light that it now requires – a development they say could add yet more objects to the burgeoning catalog of extrasolar planets.


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