Discovered in 1915, Proxima Centauri has been a subject of considerable interest ever since, as you would expect of the star nearest to our own. But I had no idea research into planets around Proxima went all the way back to the 1930s. Nonetheless, a new paper from Emily Gilbert (University of Chicago) and colleagues mentions a 1938 attempt by Swedish astronomer Erik Holmberg to use astrometric methods to search for one or more Proxima planets. The abstract of the Holmberg paper (citation below) reads in part:
Many parallax stars show periodic displacements. These effects probably are to be explained as perturbations caused by invisible companions. Since the amplitudes of the orbital motion are very small, the masses of the companions will generally be very small, too. Thus Proxima Centauri probably has a companion, the mass of which is only some few times larger than the mass of Jupiter. A preliminary investigation gives the result that 25% of the total number of parallax stars may have invisible companions.
Holmberg (1908-2000) seems to be best known for his work on galaxy interactions, but the movement of nearby stars was clearly a lively interest. I think we can assume that the Proxima ‘detection’ was due to systematic factors; i.e., noise in the data. But because I was fascinated by this early flurry of exoplanet hunting, I dug around a bit in Michael Perryman’s The Exoplanet Handbook (Cambridge University Press, 2011) to learn that Holmberg came back in 1943 to use long-term time-series photographic plates in another astrometric hunt, this one at 70 Ophiuchi (he thought he detected a gas giant), while in the same year, the Danish astronomer Kaj Aage Gunnar Strand (1907-2000) found evidence for a 16-Jupiter mass planet around 61 Cygni. Neither of these worlds turned out to be any more real than Holmberg’s putative planet at Proxima Centauri.
So exoplanet hunting is replete with false positives. We’ve talked at some length in these pages about the work of Peter van de Kamp (1901-1995), a Dutch astronomer living in the US, on possible planets at Barnard’s Star. His detections were ultimately shown to have resulted from systematic errors in his equipment (though unless I am mistaken, he never accepted this conclusion). Van de Kamp’s work in the 1960s and later made the point that a small number of astronomers have been actively searching for exoplanets long before the detection of 51 Pegasi b, but the Holmberg paper, taking us back prior to World War II, came as a surprise I wanted to share.
Searching for Transits
On to today’s work on Proxima Centauri, which as we know has no gas giant of the sort Holmberg deduced, but does host at least two planets, among which is the fascinating Proxima Centauri b, the latter in the habitable zone of the star. Habitability, however, is problematic. Proxima is a flare star, so active that the atmosphere of a habitable zone planet may be threatened by the intense radiation, with obvious implications for surface life.
Proxima’s flares can dominate observation, as this passage from today’s paper makes clear:
We see 2-3+ large flares every day in the 2-minute cadence TESS light curve (Vida et al., 2019), and even with TESS 2-minute cadence, optical photometry, it can be hard to fully resolve flare morphology in order to search for transits. Davenport et al. (2016) even suggest that the visible-light light curve of Proxima Centauri may be so dominated by flares that the time series can be thought of as primarily a superposition of many flares.
Image: Proxima Centauri is a “flare star,” meaning that convection processes within the star’s body make it prone to random and dramatic changes in brightness. The convection processes not only trigger brilliant bursts of starlight but, combined with other factors, mean that Proxima Centauri is in for a very long life. Astronomers predict that this star will remain middle-aged — or a “main sequence” star in astronomical terms — for another four trillion years, some 300 times the age of the current Universe. These observations were taken using Hubble’s Wide Field and Planetary Camera 2 (WFPC2). Its two companions, Alpha Centauri A and B, lie out of frame. Credit: NASA/ESA.
Indeed, Proxima’s flares appear nearly continuous, occurring at a range of wavelengths. Flares can induce shifts in radial velocity measurements, making observations noisy and burying a potential planetary signal in a sea of misleading data. The same problem occurs for transit detection, where flare-induced variations in the lightcurve may mask an actual planetary signature. All this makes clear what fine work Guillem Anglada-Escudé and team performed at unpacking the radial velocity data that first revealed the existence of Proxima b in 2016.
Because flare activity may be masking transits at Proxima Centauri, Gilbert and team modeled the stellar activity in their planet search algorithm, refining the result. Previous flare detection algorithms have tried to identify the flares and remove them, revealing the more stable stellar signal beneath. The authors take a different approach, using their own algorithm to first identify flares, then modeling them using a template, subtracting them from the data and running a transit search on the result. The unique flare modeling they apply to Proxima, painstakingly presented in the paper’s section on methods, involves a multi-step process of filtering and fitting the flare data. The scientists injected transits into the light curves before modeling as a way of determining how sensitive their method was, with results that boosted the planet signal.
Image: This is Figure 3 from the paper. Caption: By subtracting a flare model from the light curve of Proxima Centauri, we are able to significantly increase the probability of recovering small planets. We are able to reliably recover planets down to around the radius of Mars across the period range searched, effectively ruling out any transit of Proxima Centauri b. Credit: Gilbert et al.
A transit at Proxima Centauri would be a huge boon to astronomers, allowing a precise radius and accurate determination of the composition of Proxima Centauri b (the same is true, of course, of Proxima c). Moreover, Proxima b’s short orbital period would mean frequent transits, and thus would elevate its status as a place to look for biosignatures in the atmosphere.
Alas, we have no transits. Gilbert’s team used TESS observations of Proxima Centauri to make this determination. The conclusion seems tight. From the paper:
We find no evidence for Proxima Centauri b in TESS data. This is not surprising because previous efforts using different telescopes have been similarly fruitless.
Using the known minimum mass of Proxima Centauri b (Msini = 1.27 M⊕), we used the relationship from Chen and Kipping (2017) to derive an expected planet radius to be R = 1.08 ± .14 R⊕. A 100% Iron planet would have an expected radius of 0.88 R⊕ (Zeng et al., 2019). Therefore, given our injection and recovery tests show that no planets larger than 0.4 R⊕ transit Proxima Centauri at periods between 10–12 days, we are confident that we would recover the signal from Proxima Centauri b if it were to transit.
The team’s flare modeling is a big part of the story here, improving the sensitivity to transiting planets from 0.6 Earth radii to 0.4, and allowing the team to put rigorous limits on the probability of transits. According to the authors, this kind of flare modeling is a technique that should be applicable to all active stars. This is good news for the continuing work of TESS and the future work of Plato (ESA’s PLAnetary Transits and Oscillations of stars mission). Our sensitivity to small planets transiting low-mass, nearby active stars receives a boost from these methods, even though the search for transits at Proxima Centauri comes up empty.
The paper is Gilbert et al., “No Transits of Proxima Centauri Planets in High-Cadence TESS Data,” accepted at Frontiers in Astronomy and Space Sciences (preprint). The Holmberg paper is Holmberg, E., “Invisible Companions of parallax stars revealed by means of modern trigonometric parallax observations,” Meddelanden fran Lunds Astronomiska Observatorium, Series II 92, 5–25.