Could A Frequency Comb Help Find Earth-Like Exoplanets?

Astronomers Chih-Hao Li and David Phillips of the Harvard-Smithsonian Center for Astrophysics want to rediscover Venus—that familiar, nearby planet stargazers can see with the naked eye much of the year.

"We are building a telescope that will let us see the sun the way we would see other stars," said Phillips, who is a staff scientist at the Harvard-Smithsonian Center for Astrophysics.

Li and Phillips are developing a new laser-based technology known as the green astro-comb for use with the "radial velocity method," which offers complementary information about the mass of the distant planet. From this information, astronomers will be able to determine whether distant exoplanets they discover are rocky worlds like Earth or less dense gas giants like Jupiter. The method is precise enough to help astronomers identify Earth-like planets in the "habitable zone," the orbital distance “sweet-spot” where water exists as a liquid.

Better Precision with a Laser

The radial velocity method works by measuring how exoplanet gravity changes the light emitted from its star. As exoplanets circle a star, their gravitation tugs at the star changing the speed with which it moves toward or away from Earth by a small amount. The star speeds up slightly as it approaches Earth, with each light wave taking a fraction of a second less time to arrive than the wave before it. To an observer on Earth, the crests of these waves look closer together than they should, so they appear to have a higher frequency and look bluer. As the star recedes, the crests move further apart and the frequencies seem lower and redder.

The new astro-comb Li, Phillips and their colleagues are developing, however, will be able to detect Doppler shifts as small as 10 centimeters per second—small enough to find habitable zone Earth-like planets, even from hundreds of light years away.

"By calibrating the spectrograph this way, we can take into account very small changes in temperature or humidity that affect the performance of the spectrograph. This way, we can compare data we take tonight with data from the same star five years from now and find those very small Doppler shifts," he said.

Seeing Green

Li and his co-researchers pioneered the astro-comb several years ago, but it only worked with infrared and blue light. Their new version of the astro-comb lets astronomers measure green light—which is better for finding exoplanets.

"The stars we look at are brightest in the green visible range, and this is the range spectrographs are built to handle," Phillips said.

"Red light goes in and green light comes out," Phillips said. "Even though I see it every day and understand the physics, it looks like magic."

The researchers plan to test the green astro-comb by pointing it at our sun, analyzing its spectrum to see if they can find Venus and rediscover its characteristic period of revolution, its size, its mass and its composition.

The Harvard-Smithsonian team is installing this device on the High-Accuracy Radial Velocity Planet Searcher-North (HARPS-N), a new spectrograph designed to search for exoplanets using the Italian National Telescope.

And, because he will have already discovered Venus, he will be more certain of the answers.

Source: The Optics Society