Astronomers use astro-comb to seek Earth-like exoplanets

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The astro-comb will try to "rediscover" Venus by measuring its gravitational effects on the Sun (Imge: NASA)

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Astronomers looking for exoplanets are using a fine-toothed comb – a fine-toothed astro-comb, to be precise. And just to make sure it works, the first planet they’ll be looking for is Venus. Developed by astronomers Chih-Hao Li and David Phillips of the Harvard-Smithsonian Center for Astrophysics, the astro-comb uses a new spectroscopic device installed in the Italian Telescopio Nazionale Galileo (TNG) in the Canary Islands that will detect the beclouded planet by its gravitational effect on the Sun as a test of a potentially valuable tool in the hunt for Earth-like planets beyond our Solar System.

So far, astronomers have detected over 1,700 exoplanets with many more candidates awaiting verification. Most of these have been detected by NASA’s Kepler space telescope using the transit method, where changes in the brightness of a star caused by a planet passing in front of it provide clues to an exoplanet’s presence and characteristics. Considering the number of planets this method has discovered, it can’t be called anything but successful, but it has its limits.

The astro-comb provides another string in the exoplanet hunter's bow. According to the Harvard-Smithsonian team, its a form of frequency comb that detects exoplanets using the "radial velocity method." This is based on a common misunderstanding about how planets orbit their stars.

We talk about the Earth going around the Sun, but if we want to be tiresomely pedantic, it would be more accurate to say that the Earth and Sun orbit around a common center of gravity. The only reason we say the Earth orbits the Sun is because, due to the Sun’s mass, that common center is located so close to the Sun’s center that it makes no difference. The important thing, however is that, though that though the distance between the common center and the Sun’s center is minuscule, it is measurable.

So what does this have to do with hunting exoplanets? The answer is that as a possible exoplanet orbits its star, its mass is tugging its star away from that common center the same as the Earth tugs at the Sun. That means that as the star is moving through space, the planet is regularly causing the star to speed up and slow down as the planet travels around it.

This tug of war affects the the familiar red-shift phenomenon, where the speed of an object pushes its light spectrum toward the red or blue end, depending on whether its coming toward or going away from the observer. Traditionally, astronomers have used this to calculate the speed of galaxies and quasars as they hurtle away from the Earth as the universe expands, but it can also be used to detect exoplanets thanks to the astro-comb.

The red-shift is determined by looking at a spectrograph of a star and measuring how much properties like emission lines have shifted due to changes in speed. If this shift can be measured accurately, then the changes in the speed of the star can be calculated, and the presence of an invisible planet determined. Unfortunately, conventional spectronomy can only measure speed to within one meter per second. That’s not good enough for finding anything smaller than Jupiter. The astro-comb allows for much more precise measurements, and so smaller planets.

The Harvard-Smithsonian team are using what is called a "green astro-comb," which sets it apart from previous versions that worked with infrared and blue light. The team says green light is preferable because the candidate stars are brightest in the green band of the spectrum, and the astro-comb can measure speeds of 10 centimeters per second, which can detect an Earth-size planet orbiting in the habitable zone of a star hundreds of light years away.

The astro-comb works by means of a femtosecond laser calibrated using GPS time to generate thousands of optical lines across a complete band of the visible spectrum. This laser passes through a short piece of optical fiber called a Photonic Crystal Fiber that shifts the laser from the blue to the green spectrum band, while a filter cavity matches the line spacing to the resolution of the spectrograph.

"The astro-comb works by injecting 8,000 lines of laser light into the spectrograph. They hit the same pixels as starlight of the same wavelength," says Phillips. “This creates a comb-like set of lines that lets us map the spectrograph down to 1/10,000 of a pixel. So if I have light on this section of the pixel, I can tell you the precise wavelength. 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."

The team says that, as a bonus, the astro-comb reveals information on the planet's mass with the ability to tell if a planet is a super-Earth or a mini-Neptune.

In order to test the astro-comb, the team will install it in the High-Accuracy Radial Velocity Planet Searcher-North (HARPS-N) of the TNG. Instead of looking for exoplanets straightaway, it will run a test on an easier target – "rediscovering" the planet Venus by looking at the Sun's spectrum and from that trying to deduce Venus’ revolution, size, mass, and composition.

The astro-comb device will be presented in a paper to the Optical Society's 98th Annual Meeting this month.

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