by Paul Gilster | Sep 8, 2023 | Sail Concepts |
One of the great problems of lightsail concepts for interstellar flight is the need to decelerate. Here I’m using lightsail as opposed to ‘solar sail’ in the emerging consensus that a solar sail is one that reflects light from our star, and is thus usable within the Solar System out to about 5 AU, where we deal with the diminishment of photon pressure with distance. Or we could use the Sun with a close solar pass to sling a solar sail outbound on an interstellar trajectory, acknowledging that once our trajectory has been altered and cruise velocity obtained, we might as well stow the now useless sail. Perhaps we could use it for shielding in the interstellar medium or some such.
A lightsail in today’s parlance defines a sail that is assumed to work with a beamed power source, as with the laser array envisioned by Breakthrough Starshot. With such an array, whether on Earth or in space, we can forgo the perihelion pass and simply bring our beam to bear on the sail, reaching much higher velocities. Of the various materials suggested for sails in recent times, graphene and aerographite have emerged as prime candidates, both under discussion at the recent Montreal symposium of the Interstellar Research Group. And that problem of deceleration remains.
Is a flyby sufficient when the target is not a nearby planet but a distant star? We accepted flybys of the gas giants as part of the Voyager package because we had never seen these worlds close up, and were rewarded with images and data that were huge steps forward in our understanding of the local planetary environment. But an interstellar flyby is challenging because at the speeds we need to reach to make the crossing in a reasonable amount of time, we would blow through our destination system in a matter of hours, and past any planet of interest in perhaps a matter of minutes.
Robert Forward’s ingenious ‘staged’ lightsail got around the problem by using an Earth-based laser to illuminate one part of the now separated sail ring, beaming that energy back to the trailing part of the sail affixed to the payload and allowing it to decelerate. Similar contortions could divide the sail again to make it possible to establish a return trajectory to Earth once exploration of the distant stellar system was complete. We can also consider using magsail concepts to decelerate, or perhaps the incident light from a bright target star could allow sufficient energy to brake against.
Image: Forward’s lightsail separating at the beginning of its deceleration phase. Laser sailing may turn out to be the best way to the stars, provided we can work out the enormous technical challenges of managing the outbound beam. Or will we master fusion first? Credit: R.L. Forward.
But time is ever a factor, because you want to reach your target quickly, while at the same time, if you approach it too fast, you’re incapable of creating the needed deceleration. Moreover, what is your target? A bright star gives you options for deceleration if you approach at high velocity that are lacking from, say, a red dwarf star like Proxima Centauri, where the closest terrestrial-class world we know is in what appears to be a habitable zone orbit. In Montreal, René Heller (Max Planck Institute for Solar System Research), a familiar name in these pages, laid out the equations for a concept he has been developing for several years, a mission that could use not only the light of Proxima itself but from Centauri A and B to create a deceleration opportunity. You can follow Heller’s presentation at Montreal here.
Remember what we’re dealing with here. We have two stars in the central binary, Centauri A (G-class) and Centauri B (K-class), with the M-class dwarf Proxima Centauri about 13000 AU distant. Centauri A and B are close – their distance as they orbit around a common barycenter varies from 35.6 AU to 11.2 AU. These are distances in Solar System range, meaning that 35.6 AU is roughly the orbit of Neptune, while 11.2 AU is close to Saturn distance. Interesting visual effects in the skies of any planet there.
Image: Orbital plot of Proxima Centauri showing its position with respect to Alpha Centauri over the coming millennia (graduations are in thousands of years). The large number of background stars is due to the fact that Proxima Cen is located very close to the plane of the Milky Way. Proxima’s orbital relation to the central stars becomes profoundly important in the calculations Heller and team make here. Credit: P. Kervella (CNRS/U. of Chile/Observatoire de Paris/LESIA), ESO/Digitized Sky Survey 2, D. De Martin/M. Zamani.
Using a target star for deceleration by braking against incident photons has been studied extensively, especially in recent years by the Breakthrough Starship team, where the question of how its tiny sailcraft could slow from 20 percent of the speed of light to allow longer time at target is obviously significant. Deceleration into a bound orbit at Proxima would be, of course, ideal but it turns out to be impossible given the faint photon pressure Proxima can produce. Investing decades of research and 20 years of travel time is hardly efficient if time in the system is measured in minutes.
In fact, to use photon pressure from Proxima Centauri, whose luminosity is 0.0017 that of the Sun, would require approaching the star so slowly to decelerate into a bound orbit that the journey would take thousands of years. Hence Heller’s notion of using the combined photon pressure and gravitational influences of Centauri A and B to work deceleration through a carefully chosen trajectory. In other words, approach A, begin deceleration, move to B and repeat, then emerge on course outbound to Proxima, where you’re now slow enough to use its own photons to enter the system and stay.
Working with Michael Hippke (Max Planck Institute for Solar System Research, Göttingen) and Pierre Kervella (CNRS/Universidad de Chile), Heller has refined the maximum speed that can be achieved on the approach into Alpha Centauri A to make all this happen: 16900 kilometers per second. If we launch in 2035, we arrive at Centauri A in 2092, with arrival at Centauri B roughly six days later and, finally, arrival at Proxima Centauri for operations there in a further 46 years. That launch time is not arbitrary. Heller chose 2035 because he needs Centauri A and B to be in precise alignment to allow the gravitational and photon braking effects to work their magic.
So we have backed away from Starshot’s goal of 20 percent of lightspeed to a more sedate 5.6 percent, but with the advantage (if we are patient enough) of putting our payload into the Proxima Centauri system for operations there rather than simply flying through it at high velocity. We also get a glimpse of the systems at both Centauri A and B. I wrote about the original Heller and Hippke paper on this back in 2017 and followed that up with Proxima Mission: Fine-Tuning the Photogravitational Assist. I return to the concept now because Heller’s presentation contrasts nicely with the Helicity fusion work we looked at in the previous post. There, the need for fusion to fly large payloads and decelerate into a target was a key driver for work on an in-space fusion engine.
Interstellar studies works, though, through multiple channels, as it must. Pursuing fusion in a flight-capable package is obviously a worthy goal, but so is exploring the beamed energy option in all its manifestations. I note that Helicity cites a travel time to Proxima Centauri in the range of 117 years, which compares with Heller and company’s now fine-tuned transit into a bound orbit at Proxima of 121 years. The difference, of course, is that Helicity can envision launching a substantially larger payload.
Clearly the pressure is on fusion to deliver, if we can make that happen. But the fact that we have gone from interstellar flight times thought to involve thousands of years to a figure of just over a century in the past few decades of research is heartening. No one said this would be easy, but I think Robert Forward would revel in the thought that we’re driving the numbers down for a variety of intriguing propulsion options.
The paper René Heller drew from in the Montreal presentation is Heller, Hippke & Kervella, “Optimized Trajectories to the Nearest Stars Using Lightweight High-velocity Photon Sails,” Astronomical Journal Vol. 154 No. 3 (29 August 2017), 115. Full text.
by Paul Gilster | Mar 17, 2023 | Exoplanetary Science |
The problem with Alpha Centauri is that the system is too close. I don’t refer to its 4.3 light year distance from Sol, which makes these stars targets for future interstellar probes, but rather the distance of the two primary stars, Centauri A and B, from each other. The G-class Centauri A and K-class Centauri B orbit a common barycenter that takes them from a maximum of 35.6 AU to 11.2 AU during the roughly 80 year orbital period. That puts their average distance from each other at 23 AU.
So the average orbital distance here is a bit further than Uranus’ orbit of the Sun, while the closest approach takes the two stars almost as close as the Sun and Saturn. Habitable zone orbits are possible around both stars, making for interesting scenarios indeed, but finding out just how the system is populated with planets is not easy. We’ve learned a great deal about Proxima Centauri’s planets, but teasing out a planetary signature from our data on Centauri A and B has been frustrating despite many attempts. Alpha Centauri Bb, announced in 2012, is no longer considered a valid detection.
But the work continues. I was pleased to see just the other day that Peter Tuthill (University of Sydney) is continuing to advance a mission called TOLIMAN, which we’ve discussed in earlier articles (citations below). The acronym here stands for Telescope for Orbit Locus Interferometric Monitoring of our Astronomical Neighborhood, a mission designed around astrometry and a small 30cm narrow-field telescope. The project has signed a contract with Sofia-based satellite and space services company EnduroSat, whose MicroSat technology can downlink data at 125+ Mbps, and if the mission goes as planned, there will be data aplenty.
Image: Alpha Centauri is our nearest star system, best known in the Southern Hemisphere as the bottom of the two pointers to the Southern Cross. The stars are seen here in optical and x-ray spectra. Source: NASA.
The technology here is quite interesting, and a departure from other astrometry missions. Astrometry is all about tracking the minute changes in the position of stars as they are affected by the gravitational pull of planets orbiting them, a series of angular displacements that can result in calculations of the planet’s mass and orbit. Whereas both transit and radial velocity methods work best when dealing with planets close to their star, astrometry is the reverse, becoming more effective with separation.
Finding an Earth-class planet in the habitable zone around one of these two stars requires us to identify a signal in the range of 2.5 micro-arcseconds for Centauri A, an amount that is halved for a planet around Centauri B. Not an easy catch, but the ingenious TOLIMAN technology uses a ‘diffractive pupil’ to spread the starlight and increase the ability to spot and subtract systematic errors. I’ve quoted the team’s online description before but it usefully encapsulates the method, which has no need of field stars as references because it uses the binary companion to the star being examined as a reference, making a small aperture suitable for the work.
With the fortuitous presence of a bright phase reference only arcseconds away, measurements are immediately 2 – 3 orders of magnitude more precise than for a randomly chosen bright field star where many-arcminute fields (or larger) are required to find background stars for this task. Maintaining the instrument imaging distortions stable over a few arcseconds is considerably easier than requiring similar stability over arcminutes or degrees. Alpha Cen’s proximity to Earth means that the angular deviations on the sky are proportionately larger (typically a factor of ~10-100 compared to a population of comparably bright stars).
Image: Telescope design: The proposed TOLIMAN space telescope with a candidate telescope mirror pattern known as a diffractive pupil. Rather than concentrating the starlight into a tight focused beam as is usually done for optical systems, TOLIMAN has a strongly featured pattern, spreading starlight into a complex flower pattern that, paradoxically, makes it easier to register the fine detail required in the measurement to detect the small wobbles a planet would make in the star’s motion. Credit: Peter Tuthill / University of Sydney.
You can imagine the thermal and mechanical stability issues involved here. Doubtless Tuthill’s experience in the design of NIRISS (Near-Infrared Imager and Slitless Spectrograph ) and the aperture masking interferometry for the instrument on the James Webb Space Telescope will inform the evolution of the TOLIMAN hardware. As to EnduroSat, Raycho Raychev, founder and CEO, has this to say:
“We are exceptionally proud to partner in this mission. The challenges are enormous, and it will drive our engineering efforts to the extreme. The mission is a first-of-its-kind exploration science effort and will help open the doors for low-cost astronomy missions.”
A successful TOLIMAN mission could lead to what the team has referred to as TOLIMAN+, a larger instrument capable of detecting Earth-class worlds around both 61 Cygni and 70 Ophiuchi. But let’s get the Alpha Centauri results first, perhaps leading to detections around a target whose planetary signals would be much stronger than those of these other systems. We’ve seen how larger instruments like those aboard HIPPARCOS and Gaia have used astrometry to upgrade our view of vast numbers of stars, but it may be a small, dedicated mission with a unique technology that finally settles the question of planets around the two nearest Sun-like stars.
For more on TOLIMAN, see two previous posts: TOLIMAN Targets Centauri A/B Planets and TOLIMAN: Looking for Earth Mass Planets at Alpha Centauri. Also see this useful backgrounder.
by Paul Gilster | May 24, 2022 | Exoplanetary Science |
It’s not easy teasing out information about a tiny red dwarf star, even when it’s the closest star to the Sun. Robert Thorburn Ayton Innes (1861-1933), a Scottish astronomer, found Proxima using a blink comparator in 1915, noting a proper motion similar to Alpha Centauri (4.87” per year), with Proxima about two degrees away from the binary. Finding out whether the new star was actually closer than Centauri A and B involved a competition with a man with a similarly august name, Joan George Erardus Gijsbertus Voûte, a Dutch astronomer working in South Africa. Voûte’s parallax figures were more accurate, but Innes didn’t wait for debate, and proclaimed the star’s proximity, naming it Proxima Centaurus.
The back and forth over parallax and the subsequent careers of both Innes and Voûte make for interesting reading. I wrote both astronomers up back in 2013 in Finding Proxima Centauri, but I’ll send you to my source for that article, Ian Glass (South African Astronomical Observatory), who published the details in the magazine African Skies (Vol. 11 (2007), p. 39). You can find the abstract here.
Image: Shining brightly in this Hubble image is our closest stellar neighbour: Proxima Centauri. Although it looks bright through the eye of Hubble, as you might expect from the nearest star to the Solar System, the star is not visible to the naked eye. Its average luminosity is very low, and it is quite small compared to other stars, at only about an eighth of the mass of the Sun. However, on occasion, its brightness increases. Proxima is what is known as 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 on the main sequence 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). Credit: NASA/ESA.
It’s a long way from blink comparators to radial velocity measurements, the latter of which enabled our first exoplanet discoveries back in the 1990s, measuring how the gravitational pull of an orbiting planet could pull its parent star away from us, then towards us on the other side of the orbit, with all the uncertainties that implies. We’re still drilling into the details of Proxima Centauri, and radial velocity occupies us again today. The method depends on the mass of the star, for if we know that, we can then make inferences about the mass of the planets we find around it.
Thus the discovery of Proxima Centauri’s habitable zone planet, Proxima b, a planet we’d like to know much more about given its enticing minimum mass of about 1.3 Earths and an orbital period of just over 11 days. Radial velocity methods at exquisite levels of precision rooted out Proxima b and continue to yield new discoveries.
We’re learning a lot about Alpha Centauri itself – the triple system of Proxima and the central binary Centauri A and B. Just a few years ago, Pierre Kervella and team were able to demonstrate what had previously been only a conjecture, that Proxima Centauri was indeed gravitationally bound to Centauri A and B. The work was done using high-precision radial velocity measurements from the HARPS spectrograph. But we still had uncertainty about the precise value of Proxima’s mass, which had in the past been extrapolated from its luminosity.
This mass-luminosity relation is useful when we have nowhere else to turn, but as a paper from Alice Zurlo (Universidad Diego Portales, Chile) explains, there are significant uncertainties in these values, which point to higher error bars the smaller the star in question. As we learn more about not just other planets but warm dust belts around Proxima Centauri, we need a better read on the star’s mass, and this leads to the intriguing use to which Zurlo and team have put gravitational microlensing.
Here we’re in new terrain. The gravitational deflection of starlight is well demonstrated, but to use it, we need to have a background object move close enough to Proxima Centauri so that the latter can deflect its light. A measurement of this kind was recently made on the star Stein 3051 B, a white dwarf, using data from the Hubble instrument, the first use of gravitational lensing to measure the mass of a star beyond our Solar System. Zurlo and team have taken advantage of microlensing events at Proxima involving two background stars, one in 2014 (source 1), the other two years later (source 2), but the primary focus of their work is with the second event.
Using the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) at the Very Large Telescope at Cerro Paranal in Chile, the researchers observed Proxima Centauri and the background stars from March of 2015 to June of 2017. You can see Proxima in the image below, with the two background stars. In the caption, IRDIS refers to the near-infrared imager and spectrograph which is a part of the SPHERE/VLT installation.
Image: This is Figure 1 from the paper. Caption: IRDIS FoV for the April 2016 epoch. The image is derotated, median combined, and cleaned with a spatial filter. At the center of the image, inside the inner working angle (IWA), the speckle pattern dominates, in the outer part of the image our reduction method prevents the elongation of the stars’ point spread functions (PSFs). The bars in the lower right provide the spatial scale. North is up and East is to the left. Credit: Zurlo et al.
The extraordinary precision of measurement needed here is obvious, and the mechanics of making it happen are described in painstaking detail in the paper. The authors note that the SPHERE observations will not be further refined because the background star they call Source 2 is no longer visible on the instrument’s detector. Nonetheless:
The precision of the astrometric position of this source is the highest ever reached with SPHERE, thanks to the exquisite quality of the data, and the calibration of the detector parameters with the large population of background stars in the FoV. Over the next few years, Proxima Cen will be followed up to provide a better estimation of its movement on the sky. These data will be coupled with observations from HST and Gaia to take advantage of future microlensing events.
The results of the two-year monitoring program show deflection of the background sources’ light consistent with our tightest yet constraints on the mass of Proxima Centauri. The value is 0.150 solar masses, with possible error in the range of +0.062 to -0.051, or roughly 40%. This is, the authors note, “the first and the only currently possible measurement of the gravitational mass of Proxima Centauri.”
The previous value drawn from mass-luminosity figures was 0.12 ± 0.02 M?. What next? While Source 2 may be out of the picture using the SPHERE installation, the authors add that Gaia measurements of the proper motion and parallax of that star may further refine the analysis. Future microlensing will have to wait, for no star as bright as Source 2 will pass within appropriate range of Proxima for another 20 years.
The paper is Zurlo et al., “The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event,” Monthly Notices of the Royal Astronomical Society Vol. 480, Issue 1 (October, 2018), 236-244 (full text). The paper on Proxima Centauri’s orbit in the Alpha Centauri system is Kervella et al., “Proxima’s orbit around ??Centauri,” Astronomy & Astrophysics Volume 598 (February 2017) L7 (abstract).
by Paul Gilster | Mar 15, 2022 | Exoplanetary Science |
Why is it so difficult to detect planets around Alpha Centauri? Proxima Centauri is one thing; we’ve found interesting worlds there, though this small, dim star has been a tough target, examined through decades of steadily improving equipment. But Centauri A and B, the G-class and K-class central binary here, have proven impenetrable. Given that we’ve found over 4500 planets around other stars, why the problem here?
Proximity turns out to be a challenge in itself. Centauri A and B are in an orbit around a common barycenter, angled such that the light from one will contaminate the search around the other. It’s a 79-year orbit, with the distance between A and B varying from 35.6 AU to 11.2. You can think of them as, at their furthest, separated by the Sun’s distance from Pluto (roughly), and at their closest, by about the distance to Saturn.
The good news is that we have a window from 2022 to 2035 in which, even as our observing tools continue to improve, the parameters of that orbit as seen from Earth will separate Centauri A and B enough to allow astronomers to overcome light contamination. I think we can be quite optimistic about what we’ll find within the decade, assuming there are indeed planets here. I suspect we will find planets around each, but whether we find something in the habitable zone is anyone’s guess.
Image: This is Figure 1 from today’s paper. Caption: (a) Trajectories of ?-Cen A (red) and B (blue) around their barycenter (cross). The two stars are positioned at their approximate present-day separation. The Hill spheres (dashed circles) and HZs (nested green circles) of A and B are drawn to scale at periapsis. (b) The apparent trajectory of B centered on A, with indications of their apparent separation on the sky over the period from CE 2020 to 2050. The part of trajectory in yellow indicates the coming observational window (CE 2022–2035) when the apparent separation between A and B is larger than 6 and the search for planets around A or B can be conducted without suffering significant contamination from the respective companion star. Credit: Wang et al.
If we don’t yet have a planet detection around the binary Centauri stars, we continue to explore the possibilities even as the search continues. Thus a new paper from Haiyang Wang (ETH Zurich), who along with colleagues at the university has been modeling the kind of rocky planet in the habitable zone that we hope to find there. The idea is to create the benchmarks that predict what this world should look like.
The numerical modeling involved examines the composition of the hypothetical world, drawing on what we do know, based on spectroscopic measurements, of the chemical composition of Centauri A and B. Here there is a great deal of information to work with, especially on so-called refractory elements, the iron, magnesium and silicon that go into rock formation. Centauri A and B are among the Gaia “benchmark stars” for which stellar properties have been carefully calibrated, and up to 22 elements have been found in high-quality spectra, so we know a lot about their chemical makeup.
But a key issue remains. While rocky planets are known to have rock and metal chemical compositions similar to that of their host stars, there is no necessary correspondence when it comes to the readily vaporized volatile elements. The authors suggest that this is because the process of planetary formation and evolution quickly does away with key telltale volatiles.
The researchers thus develop their own ‘devolatilization model’ to project the possible composition of a supposed habitable zone planet around Centauri A and B, linking stellar composition with both volatile and refractory elements. The model grew out of Wang’s work with Charley Lineweaver and Trevor Ireland at the Australian National University in Canberra, and it continues at Wang’s current venue at ETH. This is fundamentally new ground that extends our notions of exoplanet composition.
Wang and team call their imagined world ‘a-Cen-Earth,’ delving into its internal structure, mineralogy and atmospheric composition, all factors in evolution and habitability. The findings reveal a planet that is geochemically similar to Earth, with a silicate mantle, although carbon-bearing species like graphite and diamond are enhanced. Water storage in the interior is roughly the same as Earth, but the deduced world has a somewhat larger iron core mixed with a possible lack of plate tectonics. Indeed, “…the planet may be in a Venus-like stagnant-lid regime, with sluggish mantle convection and planetary resurfacing, over most of its geological history.”
As to the atmosphere of the hypothetical world that grows out of Wang’s model, its early era shows an envelope rich in carbon dioxide, methane and water, which harks back to the Earth’s atmosphere in the Archean era, between 4 and 2.5 billion years ago. That gives life a promising start if we assume abiogenesis occurring in a similar environment.
Image: ? Centauri A (left) and ? Centauri B viewed by the Hubble Space Telescope. At a distance of 4.3 light-?years, the ? Centauri group (which includes also the red dwarf ? Centauri C) is the nearest star system to Earth. Credit: ESA/Hubble & NASA.
How far can we take a model like this? We may soon have data to measure it against, but it’s worth remembering what the paper’s authors point out. After noting that planets around the “Sun-like” Centauri A and B cannot be extrapolated from the already known planets around the red dwarf Proxima Centauri, they go on to say:
Second, although ? Cen A and B are “Sun-like” stars, their metallicities are ?72% higher than the solar metallicity (Figure 3). How this difference would affect the condensation/evaporation process, and thus the devolatilization scale, is the subject of ongoing work (Wang et al. 2020b).
That’s a big caveat and a useful pointer to the needed clarification that further work on the matter should bring – metallicity is obviously significant. The paper adds:
Third, we ignore any potential effect of the “binarity” of the stars on their surrounding planetary bulk chemistry during planet formation, even though we highlight that, dynamically, the planetary orbits in the HZ around either companion are stable. Finally, we have yet to explore a larger parameter space, e.g., in mass and radius, but have only benchmarked our analysis with an Earth-sized planet, which would otherwise have an impact on the interior modeling…
So we’re in early days with planet modeling using these methods, which are being examined and extended through the team’s collaborations at Switzerland’s National Centre of Competence in Research PlanetS. Note too that the authors do not inject any catastrophic impact into their model of the sort that could affect both a planet’s mantle and/or its atmosphere, with dramatic consequences for the outcome. We know from the Earth’s experience in the Late Heavy Bombardment that this can be a factor.
With all this in mind, it’s fascinating to see the lines of observation and theory converging on the Alpha Centauri binary pair. Finding a habitable zone planet around Proxima Centauri was exhilarating. How much more so to go beyond the many imponderables of red dwarf planet habitability to two stars much more like our Sun, each of which might have a planet in its habitable zone? The Alpha Centauri triple system may turn out to be a bonanza, showing us both red dwarf and Sun-like planetary outcomes in a single system that just happens to be the closest to us.
The paper is Wang et al,, “A Model Earth-sized Planet in the Habitable Zone of ? Centauri A/B,” The Astrophysical Journal Vol. 927, No. 2 (10 March 2022). Abstract/Full Text. Preprint also available.
by Paul Gilster | Nov 23, 2021 | Exoplanetary Science |
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.
