The first confirmation I had of the discovery of Proxima Centauri b came from James Nicoll, who shared the European Southern Observatory’s announcement that the Pale Red Dot search program bore spectacular fruit.
Phil Plait at Bad Astronomy, Jennifer Ouellette at Gizmodo, and Franck Marchis at the Planetary Society Blog provided among the first blogged reactions I saw on my Facebook feed. Marchis’ summary of what led to the discovery deserves reproduction.
We now know of 3,374 exoplanets, an enormously large number, given that we discovered the first one only in 1995. Like the cartographers of the seventeenth century, who slowly build a map of our world, astronomers are drawing a map of our galactic neighborhood. We think we have a good handle on the location of nearby stars—that is, ones that are less than 50 light-years away. We know their distance, size, temperature, and if they are multiple systems or single stars, for example; but ultimately what we would really like to add to this 3D map of the galaxy are the planets in orbit around these stars.
The Pale Red Dot group was particularly interested in finding planets around Proxima Centauri, the star closest to the Sun. Proxima Centauri is only 4.25 light-years away, so it’s in our cosmic backyard. Because of its small mass, it’s too faint to be seen with the naked eye, and was discovered only in 1915. At the end of the 1990s, astronomers tried to detect potential large planets in orbit around this star using the radial-velocity method and came back empty-handed.
In the article published today in Nature, a group of modern astronomers reported on what they learned by using two high-precision radial-velocity instruments: HARPS at the 3.6m telescope of La Silla and UVES at the VLT 8m class telescope, both part of the European Southern Observatory. Several of these observations were done as part of other programs that took place between 2000 and 2016, but from January 2016 to March 2016, the team collected what we call high-cadence data, a fancy way to state that the star was observed once per night to increase its chance of detecting a tiny variation in its motion (about a meter per second, or the speed of a human walking) that might be caused by the presence of a small planet.
This ambitious program has paid off beyond our wildest dreams in that we have now unambiguously detected a planet with a minimum mass 1.3 times that of Earth orbiting the star right in the middle of the goldilocks zone (0.05 AU). I am not a specialist in radial-velocity measurement, but this detection seems quite convincing in that it has a false-alarm probability of less than 0.1% and uses a careful comparison of star activity (done by using additional small telescopes during the survey) that are known to mimic the signal of a planet. That is a very significant new data point to add in our cosmic map.
This world, Marchis notes, is not necessarily an Earth analog. Its tidal locking to Proxima aside, as is Proxima Centauri’s nature as a very active flare star, we know only basic data about Proxima Centauri b: “The planet’s MINIMUM mass is 1.3 Earths because we don’t really know the orientation of the orbital plane with respect to the observer. (The radial-velocity method provides a measurement of m sin i, with i being the inclination of the system with respect to us.) Assuming random orientations of orbital planes, we have a 90% probability that the true mass is less than 2.3 times the minimum mass, so 3 Earths. In short, this could be a super-Earth or something more exotic, like a baby-Neptune.”
Even so, this is huge. The nearest star to our own hosts a potentially Earth-like world? The Dragon’s Gaze was quick to link to the discovery paper, but it was when I saw the news appear on Joe. My. God. that I knew this was big.
Centauri Dreams’ Paul Gilster reacted at length, going back to Proxima’s first appearance in science fiction in 1935 and noting the many potential issues with Proxima Centauri b being truly habitable.
We have a long way to go before knowing whether a planet around a red dwarf like this can truly be habitable. Tidal locking is always an issue because a planet this close to its host (Proxima Centauri b is on an 11.2-day orbit) is probably going to have one side fixed facing the star, the other in permanent night. There are papers arguing, however, that tidal lock does not prevent a stable atmosphere with global circulation and heat distribution from occurring.
And what about Proxima’s magnetic field? The average global magnetic flux is high compared to the Sun’s (600±150 Gauss vs. the Sun’s 1 G). Couple this with flare activity and there are scenarios where a planet gradually has its atmosphere stripped away. A strong planetary magnetic field could, however, prevent this erosion. Nor would X-rays (400 times the flux the Earth receives) necessarily destroy the planet’s ability to keep an atmosphere.
And then there’s the matter of the planet’s origins, and how that could affect what is found there. From the paper:
…forming Proxima b from in-situ disk material is implausible because disk models for small stars would contain less than 1 M Earth of solids within the central AU. Instead, either 1) the planet migrated in via type I migration, 2) planetary embryos migrated in and coalesced at the current planet’s orbit, or 3) pebbles/small planetesimals migrated via aerodynamic drag and later coagulated into a larger body. While migrated planets and embryos originating beyond the ice-line would be volatile rich, pebble migration would produce much drier worlds.
Discover‘s blogs provided good coverage, D-Brief looking up the Alpha Centauri system’s more notable appearances in science fiction and Crux summing up the data.
The question of habitability has been coming up. The Pale Red Dot team engaged in a Reddit AMA about their discovery, while co-discover Ignas Ribisi analyses the potential for habitability, and liquid water, at length. (Much depends on how this world is tidally locked, it turns out.) In a charming poetic analysis, Sean Raymond also examines the question of how the planet is in synchronous orbit with its sun. Gizmodo, meanwhile, published an article suggesting that Proxima’s flares need not pose a challenge for life on Proxima b, that the phenomenon of biofluorescence–briefly, using proteins to absorb high-energy light and retransmit it in less harmful forms–could well be present.
New Scientist has an enlightening article that, among other things, looks at the background to the planet’s discovery and hints at more.
Astronomers will still want to turn their scopes towards Proxima Centauri – to confirm that the planet is real, and avoid a repeat of an earlier embarrassment. Despite initial excitement, the claimed discovery in 2012 of a planet orbiting neighbouring Alpha Centauri B now looks to have been a mistake.
[Mikko] Tuomi and his colleagues have done everything they can to avoid that happening again. He first saw signs of Proxima b in 2013, when looking at data taken by the Very Large Telescope at Paranal Observatory in Chile between 2003 and 2009. “I spent weeks trying to make the signal go away, trying to show that it was caused by the star’s activity or pure measurement noise rather than a planet,” he says. But the team became increasingly convinced.
To confirm the find, the group examined data from other telescopes and in January this year began the Pale Red Dot campaign, using another instrument in Chile – the HARPS planet-searcher at the La Silla Observatory. The observations lasted 60 nights, but the team was confident of a discovery after just 10 nights of data, says Tuomi. “It was as predicted by the previous observations. We knew this was going to become a year to remember for exoplanet science.”
“I think this is a very solid thing,” says Snellen. “For me personally, this is the scientific discovery of the year, maybe of the decade.”
The team also saw signs of a second potential planet around Proxima Centauri, a super-Earth with an orbit of between 60 and 500 days. If such an outer planet exists, it might be possible to observe it, says Tuomi.
What can be said but that we need–want–so much more data? What is Proxima Centauri b actually like? Could it be Earth-like? Are there oceans, life?
I, and the Earth, await more.