In the Atacama Desert of southern Chile is the European Space Agency’s 3.6-metre telescope.
What does HARPS
really do to detect these planets? It is all matter of perspective. As we are so
far from the stars, we cannot see their exoplanets directly. Instead HARPS
detects minute wobbles in the stars’ motion. Stars and their exoplanets are
bound together by gravity, so an exoplanet orbits its distant parent star, just
as the planets of the Solar System orbit the Sun. But a planet in orbit around
a star exerts its own gentle pull, so that the orbital centre of the system is
a little away from the centre of the star and the star itself orbits about this
point — which we can detect as a small regular movement of the star to and fro
along our line of sight. This tug of war between any star and its exoplanets
can be seen (or rather, measured) by HARPS, with an incredible precision. HARPS
picks up small changes in the star’s radial velocity (i.e. along the line of
sight), which can be as little as a gentle walking pace of 3.5 km/hour
Due to the Doppler
effect, this radial velocity change induces a shift of the star’s spectrum
towards longer wavelengths as it moves away (called a redshift) and a shift towards
shorter wavelengths (blueshift) as it approaches. This tiny shift in the star’s
spectral lines can be measured with a high-precision spectrograph such as HARPS
and used to infer the presence of a planet.
Astronomers who used the telescope in July 2015 have now made a new significant discovery .... !
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