Proxima b is our neighbor… better get used to it!

It is true. We are convinced that there is a planet orbiting Proxima now. The evidence goes as follows : a signal was spotted back in 2013 on previous surveys (UVES and HARPS). The preliminary detection was first done by Mikko Tuomi, our in-house applied mathematician and his Bayesian codes. However, the signal was not convincing as the data was really sparse and the period was ambiguous (other possible solutions at 20 and 40 days, plus a long period signal of unknown origin). We followed up Proxima in the next years but our two observing runs were 12 days, barely sufficient to secure a signal which ended up being 11.2 days. So the Pale Red Dot was designed with the sole purpose of confirming or refuting its strict periodicity, plus carefully monitor the star for activity induced variability. We got very lucky with the weather so we obtained 54 out of 60 observations. The photometric monitoring telescopes (ASH2 and several units of Las Cumbres Observatory Global Telescope network), worked flawlessly so we could see the effect of spots, flares and rotation of the star, which also had a footprint on the spectra. However, nothing indicated that spurious variability would be happening at 11.2 days.

This plot shows how the motion of Proxima Centauri towards and away from Earth is changing with time over the first half of 2016. Sometimes Proxima Centauri is approaching Earth at about 5 kilometres per hour — normal human walking pace — and at times receding at the same speed. This regular pattern of changing radial velocities repeats with a period of 11.2 days. Careful analysis of the resulting tiny Doppler shifts showed that they indicated the presence of a planet with a mass at least 1.3 times that of the Earth, orbiting about 7 million kilometres from Proxima Centauri — only 5% of the Earth-Sun distance.
This plot shows how the motion of Proxima Centauri towards and away from Earth is changing with time over the first half of 2016. Sometimes Proxima Centauri is approaching Earth at about 5 kilometres per hour — normal human walking pace — and at times receding at the same speed. This regular pattern of changing radial velocities repeats with a period of 11.2 days. Careful analysis of the resulting tiny Doppler shifts showed that they indicated the presence of a planet with a mass at least 1.3 times that of the Earth, orbiting about 7 million kilometres from Proxima Centauri — only 5% of the Earth-Sun distance.

So that’s basically it : the Pale Red Dot campaign also detects the same period, and confirms that the signal has been in phase for the 16 years of accumulated observations. This is a requirement for a proper Keplerian orbit. Features like starspots are more short lived plus affect the velocities in the time-scales of the rotation of the star, which is now confirmed at ~83 days.

The combination of all the data produces this periodogram

detection_periodogram

which leaves little doubt to the reality of the signal. The peaks in a periodogram tells us where a significant period is spotted, plus give us information about its significant. The horizontal lines correspond to False Alarm Probabilities of 10%, 1% and 0.1%. Our signal is now well beyond that. The probability of a statistical false positive is smaller than one of ten millions!

So what we know? We know the period and the size of the radial velocity wobble. From that we derive a minimum mass of 1.3 masses of the Earth. With the period and the mass of the star, we know it orbits at 5% of an astronomical unit (this is 20 times closer than Earth is from the Sun), which combined with the luminosity of the star tells us that the planet is warm and can currently support liquid water on its surface. Beyond this, all is mostly speculative. But one can do simulations and educated guesses. If you want to learn more about them, follow forthcoming articles at

www.proximacentauri.info

and a contributed one to this website by Rory Barnes.

We had a press release event at ESO today. We want to thank everyone for the passion and effort shared in this project, including the Breakthrough Starshot foundation and its chair Pete Worden for giving us their support. We hope to reach the stars, there is a foundation to promote technological advancements, and now we have a target. The sky is the limit!