Astronomers have announced the discovery of Kepler-76b — a “hot Jupiter” that takes only 1.5 days to complete an orbit around its star. Unofficially, however, Kepler-76b has been nicknamed “Einstein’s planet” as it was discovered using a novel method that applies a weird relativistic effect as theorized by Albert Einstein.
Usually, NASA’s Kepler space telescope looks for the very slight dips in starlight brightness as exoplanets orbit in front of their host stars from our perspective. Kepler is revolutionizing exoplanetary studies as it is sensitive to the detection of tiny worlds, which is only possible owing to the mission’s advanced optics.
Now, using Kepler data, astronomers have taken a different tact in the hunt for exoplanets.
Kepler-76b was discovered by looking for the slight brightening of a star as an exoplanet passes in front. How does that work?
Normally, as an exoplanet passes in front of a star’s disk, Kepler will detect a dip in brightness in that star’s “light curve.” The greater the dip, the bigger the planet. But this method only works if the orbital plane of the planet is exactly “edge-on” when viewed from Earth. What if there’s an exoplanetary system with an orbital plane inclined away from our point of view? To put it bluntly, Kepler won’t see those exoplanets — or will it?
In 2003, Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA) and Scott Gaudi (now at Ohio State University) had an idea. By their reckoning, if the optics of a space telescope are sensitive enough, the Einstein “beaming effect” should be observable. And on Monday, it was announced that this strange quirk in physics revealed the presence of Kepler-76b.
So, what is this beaming effect and how can it help in the search for exoplanets?
As Kepler-76b orbits its star, it “tugs” on the stellar body. Indeed, it’s this tugging effect that allows another exoplanet-hunting technique to come in to play. The “radial velocity” method is extensively used by ground-based observatories to detect the wobble of stars — as the exoplanet pulls the star toward us, its electromagnetic spectrum is slightly blue-shifted, as it’s pulled away, the spectrum is slightly red-shifted.
Relativistic beaming works in a similar manner, but there is no requirement to analyze the star’s spectrum.
As Kepler-76b is 25 percent larger than Jupiter and twice as massive, it has a sizable tugging effect on the star. The beaming effect occurs when the orbiting exoplanet tugs its star in our direction — the motion toward us creates a focusing effect on the photons we receive from the star — the photons to “bunch up” in the direction of travel, concentrating their energy, brightening the star.
This is the first time the effect has been applied to exoplanetary detection.
“We are looking for very subtle effects,” said team member David Latham of the CfA. “We needed high quality measurements of stellar brightnesses, accurate to a few parts per million,”
“This was only possible because of the exquisite data NASA is collecting with the Kepler spacecraft,” added lead author Simchon Faigler of Tel Aviv University, Israel.
Impressive as this detection may be, the team also wanted to test out two more very subtle of ways the massive exoplanet may be detected. While tugging on the star, the exoplanet elongates the star into a football shape due to the massive tidal forces exerted it. Therefore, the Kepler light-curve should also detect a slight brightening when it views the star from the side (as the “side” will have a larger surface area to radiate light than the “end”). Also, they wanted to see if they could detect the reflected light from the planet’s atmosphere too — another very faint, but measurable effect. They succeeded on all counts.
As a bonus, they also found observational evidence for violent jet streams in the exoplanet’s atmosphere, generating “hotspots” in the atmosphere, offset from the closest point to the star’s energy. The planet is “tidally locked” with its star (i.e. the same hemisphere always faces the star), so standing jetstreams blast from “high noon” (the location of the star, directly overhead) through the atmosphere, creating hotspots 10,000 miles away from noon.
If you’ve read this far wondering what this has to do with BEER, then I apologize. As much as I’d like to say that the spectroscopic signature of a hoppy chilled beverage has also been discovered in the exoplanet’s atmosphere, sadly, it’s not the case. BEER is the (rather forced) acronym for “relativistic BEaming, Ellipsoidal, and Reflection/emission modulations” — an algorithm designed by the researchers to look for relativistic beaming, tidally warped stars and reflected exoplanetary light.
There may not be beer, but it is a very impressive bit of physics detective work that uncovered an exoplanet Kepler may otherwise have missed.
Image credit: David A. Aguilar (CfA)