New BEER for planet-hunters

BEER is not the solution to life’s problems, but it might help astronomers characterize new exoplanets.

As you probably surmised, BEER isn’t the beverage: it’s an observational technique standing for BEaming, Ellipsoidal, and Reflection/emission modulation. (That wins the award for the most awkward acronym I’ve seen in some time. As Mary Roach would say, it’s “an example of PLEASE—Pretty Lame Excuse for an Acronym, Scientists and Experimenters.”) The method looks for a fluctuation in the host star’s light due to motion caused by the gravitational pull of its planet, along with the tiny additional effects due to distortions in the star’s shape and the reflection off the exoplanet.

The BEER technique scored its first major victory: astronomers used it to determine a signal from Kepler observatory data was due to a planet and not some other source. (Kepler is probably dead, alas, but we’ll see results from its observations for many months or even years to come.) Previously BEER was used on previously detected exoplanets, but the sample is small — mainly because the signals BEER is designed to detect are themselves very small. The promise of BEER isn’t necessarily as a detection method, though. Instead, it opens up a bottle-full of possible physical properties of planets and stars unavailable to other techniques: direct mass measurement, rotation rates, and the distortion of the shape of the star.

The exoplanet — labeled Kepler-76b, since it was found in Kepler data — was first hinted as a fluctuation in the light of its host star. Using BEER, astronomers in Israel, the United States, and Denmark determined not only that this was a planet, but found evidence of its rotation and an estimate of its mass.

We can drive it home with one headlight

Simulation of relativistic beaming, for a star moving to the right (relative to us) at 55% of light-speed. [Credit: moi]
Simulation of relativistic beaming, for a star moving to the right (relative to us) at 55% of light-speed. [Credit: moi]
Much of the power of BEER comes from the theory of relativity, so of course it piqued my interest. Specifically, the BE refers to relativistic BEaming, also known as the “headlight effect”. If you observe the star in its own frame of reference — where it doesn’t appear to move from your perspective — its light will be emitted more or less equally in all directions. However, if the star is moving rapidly from your point of view, the star’s light will appear to be focused in the direction the star is moving. If you’re directly in the line of the star’s motion, you’ll see the light appear brighter, while if you’re “behind” the star, it’ll seem fainter.

Beaming is in addition to the Doppler effect, where the star's light will be pushed to shorter wavelengths — blueshifted — if it's moving toward you, and stretched longer — redshifted — if it's moving away.
Beaming is in addition to the Doppler effect, where the star’s light will be pushed to shorter wavelengths — blueshifted — if it’s moving toward you, and stretched longer — redshifted — if it’s moving away. [Credit: moi]
As you can see from the video, the motion has to be pretty fast before the beaming effect is large. The animation doesn’t begin to show much noticeable beaming until the star is moving about 10% of the speed of light (1 second into the video) — 30 thousand meters per second. A star with an exoplanet won’t move that fast relative to us, much less show the strong beaming effects you get from motion 50% of light-speed or higher. However, even small periodic fluctuations in the light output from a star can be measured, and could be a sign of beaming.

An exoplanet or other companion will cause a star to move slightly as they mutually orbit. (All planets have this effect, though only Jupiter in our Solar System is massive enough to produce a measurable result on the Sun.) Thanks to beaming, whichever angle we look at a star system, we should see a small variation in the star’s brightness. It will appear slightly brighter when the star is moving toward us, and slightly fainter when moving away, with the pattern repeating itself every time the companion object completes an orbit. The effect will obviously be strongest if we observe the system in the plane of the companion’s orbit (since we’d be directly in line of the “beam”). However, as you can see from the movie, beaming has an effect even if we’re seeing the orbit from a steep angle, where the Kepler method of observing a transit isn’t possible.

In Heaven, there is no BEER; that is why we use it here

Periodic fluctuations of light alone won’t tell us if the star has a companion, so beaming by itself isn’t enough. This is the reason for the Ellipsoidal and Reflection/emission portion of BEER.

If the companion is close enough to produce orbital wobbling in the host star, then it’s also close enough to squeeze the star via tidal forces. The change in shape from spherical to very slightly ellipsoidal means the star’s light will no longer be emitted in the same way in all directions. Measuring the variations in light output is a means of measuring the shape of the star; the amount of ellipsoidal deformation provides a way of determining the mass of the companion object.

Finally, the star’s light will heat up its companion, producing a hot spot on the surface closest to the star. That hot spot itself emits light (in the infrared), which modifies the spectrum and variation in the light we see from the system. If the companion is gaseous — the most likely possibility — then the hot spot will probably migrate as atmospheric effects carry gases around the object via jet streams. In other words, the reflection/emission part of BEER can be used to measure the rotation of the companion.

Like nearly all other exoplanet observation techniques, BEER works best for high-mass exoplanets orbiting very close to their host stars — a class known as the “hot Jupiters”. Kepler-76b is between 1.74 and 2.26 times the mass of Jupiter and orbits once every 1.54 days, so it certainly is a hot Jupiter. It’s also close enough to be tidally locked: presenting one face to its star the same way the Moon always presents the same face to Earth. However, a planet that large is certainly gaseous (as Jupiter is), which means processes in the atmosphere keep things churning and turbulent. As the planet rotates to keep facing the star, the jet stream — flow of gas in the atmosphere — will push the hottest spot away from its position nearest to the star. This is evidence that the atmosphere rotates faster than the planet itself, something known as superrotation (no doubt one of Superman’s weird powers from the embarrassing era of superhero comics).

BEER is a very sophisticated algorithm, relying on measuring small variations in the light output form a star system and cross-correlating them to make sure they aren’t due to something else. However, independent observations of Kepler-76b indicate the method is reliable — and that it can give us information unavailable any other way. Hot Jupiters are relatively common objects in our galaxy, we’d like to understand their structure and interactions with their host stars, not to mention how they end up so close. BEER is a promising means to that end.

Now for some reason, I’m thirsty.

  • S. Faigler et al., BEER analysis of Kepler and CoRoT light curves: I. Discovery of Kepler-76b: A hot Jupiter with evidence for superrotation. ArXiV 1304.6841, accepted to Astrophysical Journal.

One response to “New BEER for planet-hunters”

  1. […] of the light. If you observe the photon while moving relative to the source that emitted it, it will exhibit the Doppler effect and the beaming effect (though both effects are going to be small unless the relative speed is pretty big). In the Doppler […]

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