2.1  Our closest stellar neighbour

The closest star to the Sun is an M dwarf – a small, cool, red star called Proxima Centauri, or Proxima for short (Figure 1). It is very dim – despite its proximity to us it appears one hundred times fainter than could be seen with the naked eye. It is part of a triple-star system with the binary stars Alpha Centauri A and B. In recent years astronomers suspected that they’d found planets orbiting Alpha Centauri B, although unfortunately further studies have questioned their existence.

Figure 1  The size of Proxima Centauri compared with Alpha Centauri A and B, the Sun, and the planets Jupiter and Saturn

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Proxima Centauri is a low mass M dwarf star. It is not much bigger than Jupiter. Stars like the Sun are much bigger and brighter than Proxima Centauri.

Figure 1  The size of Proxima Centauri compared with Alpha Centauri A and B, the Sun, and the planets Jupiter and Saturn...

But what about Proxima Centauri? It’s a very favourable target for a planetary search. You’ve looked in previous weeks at the things that make it easier to detect a planet around a star, so now it’s time for a quick refresher of them.

Because Proxima Centauri is small but close by, it is a good target for a radial velocity (RV) search. The transit method only works if the system is lined up perfectly, so the planet passes between us and the parent star. The RV method works for almost all orbital orientations, as you saw in Week 3. So, the Pale Red Dot project was born. Starting in January 2016, astronomers used the HARPS instrument mounted on the La Silla telescope in Chile to observe Proxima Centauri (Figure 2) and measure any tiny shifts in the positions of absorption lines in its spectrum (Figure 3).

Figure 2  Proxima Centauri rising above La Silla

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This is a view of the sky from La Silla observatory, with the location of Proxima Centauri shown. If you’re lucky enough to travel in the Southern Hemisphere you might be able to spot Alpha Centauri while you’re there, although Proxima Centauri itself is probably not bright enough for you to pick it out.

Figure 2  Proxima Centauri rising above La Silla

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Figure 3  First spectrum from the Pale Red Dot project, which was broadcast live by the astronomers at the telescope

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This is a snapshot of the first spectrum of Proxima Centauri taken with the Pale Red Dot project. It would have needed quite a bit of cleaning up before it looked like the ones you have already seen, but this was a very exciting time for the project astronomers.

Figure 3  First spectrum from the Pale Red Dot project, which was broadcast live by the astronomers at the telescope