‘Astro-Comb’ Rediscovers Venus to Find Exoplanets

Examining Venus’ influence on the solar spectrum could help astronomers discover Earth-like planets orbiting distant stars.

A team from the Harvard-Smithsonian Center for Astrophysics is developing a laser-based frequency comb to measure this stellar relationship. The data gathered could form a template for detecting minute Doppler shifts in other stars’ spectra that indicate the presence of small, rocky planets like our own.

“We are building a telescope that will let us see the sun the way we would see other stars,” said staff scientist Dr. David Phillips.

A composite image of the surface of Venus. Courtesy of NASA/JPL.

The team’s “astro-comb” measures green light, which they said is useful for finding exoplanets. Phillips said it works “by injecting 8,000 lines of laser light into a spectrograph. These hit the same pixels as starlight of the same wavelength, creating a comb-like set of lines that can map the spectrograph down to 1/10,000 of a pixel.”

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

The astro-comb would be used to enhance radial velocity measurements, which indicate how an exoplanet’s gravity changes the light emitted from its star by gauging Doppler shifts.

Today's best spectrographs are only capable of measuring Doppler shifts caused by velocity changes of 1 m/s or more. Only large gas giants close to their host stars have enough gravity to cause such changes.

The astro-comb, on the other hand, will enable detection of Doppler shifts representing star movements as small as 10 cm/s. This should be small enough to find habitable-zone, Earth-like planets from hundreds of light-years away.

The technology would allow astronomers to “compare data we take tonight with data from the same star five years from now and find those very small Doppler shifts,” Phillips said.

After recently presenting their work at OSA's Frontiers in Optics conference, the astrophysicists plan to test the astro-comb by first pointing it at Earth’s sun, analyzing its spectrum to see if they can find Venus and rediscover its characteristic period of revolution, size, mass and composition.

The astro-comb will be installed on the High-Accuracy Radial Velocity Planet Searcher-North (HARPS-N), a new spectrograph designed to search for exoplanets using the Italian National Telescope.

“We know a lot about Venus, and we can compare our answers to what we already know, so we are more confident about our answers when we point our spectrographs at distant stars,” said research associate Dr. Chih-Hao Li.

For more information, visit www.cfa.harvard.edu.