The Future For Velocimetry



Man’s endeavor to discover life in space has been a continuous effort since generations. Discovery of a large number of exoplanets has given a boost to space exploration. Velocimetry is one of the methods which has made a major contribution in the discovery of these exoplanets.

Modern instruments can determine stellar velocities to within a few metres per second, which means that they can be used to study some 2,000 stars of a spectral type(stars characteristics) similar to that of the Sun, out to a distance of a few tens of parsecs(1 parsec=30.9 trillion kilometres).

The HARPS spectrograph is installed on the 3.6 metre telescope at the ESO site, LA Silla, Chile. HARPS accuracy combines with the large size of the telescope to ensure important advances in the search for exoplanets using the velocimetric method.

In the not-too-distant future we will have discovered most of the giant exoplanets (those of masses at least half of Jupiter’s mass) with orbital periods of less than 1 year and within 50 parsecs of the Sun. We remarked elsewhere that after a period which saw a rapid succession of discoveries, the detection rate has slowed somewhat during the last few years; massive, short-period Pegasids(meteor showers) have been the easiest to find.

Now, recently developed instruments are opening new perspectives: for example, the High Accuracy Radial Velocity Planetary Search (HARPS) spectrometer, mounted on the 3.6-metre ESO telescope at La Silla in Chile, and in use since 2004. HARPS works on the same principle as its predecessor, simultaneously measuring the Doppler shifts of a large number of spectral lines in the visible domain.

ESO Telescope at La Silla, Chile

It is capable of observing 1,000 stars beyond 50 parsecs to an accuracy of 1 m/s. Compare the shifts in the case of the Sun, due to interactions with Jupiter and Saturn: respectively, 12 m/s and 3 m/s. HARPS’ great sensitivity has placed within our reach a whole new class of giant exoplanets: exoSaturns, or perhaps even less massive planets. A spectacular early illustration of the capabilities of HARPS has been the discovery of a planet of 17 Earth masses (the mass of Neptune) at a distance of approximately 0.1 AU from its star.

Accurate measurement of the radial velocities of nearby stars has led to the detection of the vast majority of known exoplanets, with the exception of those accompanying pulsars. Total number of exoplanets found so far by NASA as of July 2013 are 919, out of which there is a possibility that 262 planets may be habitable.

Other teams, using similar techniques, have discovered two objects of about the same mass. These new-style exoplanets have certainly intrigued the theoreticians. What of their composition and their origins? Are they the first members of a new class of object? There is no doubt that velocimetry, pushed to the limits of its detective capability, holds many more surprises in store!

This tried and trusted method surely has a lot of potential to offer us many new discoveries in the years to come.

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