by Paul Gilster | Apr 26, 2021 | Exoplanetary Science |
At some point, and probably soon, we’re going to be able to identify planets around Alpha Centauri A and B, assuming they are there and of a size sufficient for our methods. We may even be able to image one. Already we have an extremely tentative candidate around Alpha Centauri A — I hesitate even to call it a candidate, because this work is so preliminary — which could be a ‘warm Neptune’ at about 1 AU. One of the pleasures of the recent Breakthrough Discuss meeting was to hear film director James Cameron on the matter. Cameron, after all, gave us Avatar, where a habitable moon around a gas giant in this system plays the key role.
Despite his frequent protestations that he is not a scientist, Cameron was compelling. He’s obviously well-enough versed in the science to know the terminology and the issues involved in the ongoing deep dive into the Alpha Centauri system, and he’s done wonders in fixing the public’s attention not only on its possibilities but also on presenting a starship concept that, using hybrid propulsion methods, makes a bid for most realistic starship ever in film.
I imagine Kevin Wagner thinks about the Avatar scenario now and again, given that his work on Centauri A has turned up the observation he refers to as C1. Let’s put it in context (and I’ll also send you to Imaging Alpha Centauri’s Habitable Zones, which ran here in February), delighting in the fact that we have more than one habitable zone to talk about.
Wagner (University of Arizona Steward Observatory) and team run NEAR (New Earths in the Alpha Centauri Region), which thus far has been a full-on 100-hour attempt to look into the habitable zones of Centauri A and B. It’s fascinating to realize that these stars are close enough to us that with technology like Hubble, we can actually observe the habitable zones, for these are at separations we can see, at about 1 arcsecond, which is resolvable with large telescopes. Think Sagan’s ‘pale blue dot’ when you imagine the ultimate goal of actually imaging an Earth-like world, although it will take future instrumentation to get us to that level of sensitivity.
Image: This is a familiar image from Hubble showing Centauri A at left and B on the right. Kevin Wagner superimposed the circles showing the size of the habitable zones. The image was captured by the Wide-Field and Planetary Camera 2 (WFPC2), and is drawn from observations in the optical and near-infrared. Credit: Kevin Wagner/ESA/NASA.
NEAR, whose first 100-hour run is complete, used an adaptive secondary telescope mirror working in combination with light-blocking and masking technologies in the mid-infrared to suppress light from each of the binaries in sequence. The NEAR equipment is mounted on the Very Large Telescope’s Unit Telescope 4 in Chile, and the key, according to Wagner, is the deformable secondary mirror, which maximizes adaptive optics without adding warm optics downstream in a tertiary mirror that would degrade the infrared signal. About 1600 magnetic actuators zone out atmospheric distortion even as the coronograph nulls out star light.
Imaging something on the order of a pale blue dot around another star is quite a goal. We’ve only imaged a dozen or so exoplanets thus far, and all of these have been young and massive gas giants that still radiate brightly, no more than tens of millions of years old. Mature planets like those in our own Solar System are much cooler, and if we are after a planet like the Earth, we have to look in areas where the infrared signal is swamped by our own atmosphere. Adds Wagner:
“The earth is a 300 K black body. Here the primary radiation is at 10 microns, which is where we have to look at more mature exoplanets. And the problem is that the atmosphere of our own planet is what we have to look through, and it also radiates at about 10 microns. The sky, the telescope, the camera, everything is glowing at us.”
I ran the figure below in February, but I want to introduce it again, as it shows not only the C1 observation but also, on the left, the systematic artifacts that have to be removed to come up with what the astronomers hope is a clean image. Remember, we are in early days here, and when discussing C1 as a possible planet, we have to keep in mind that other explanations are possible, including distortion in a not yet recognized effect within the equipment itself.
Image: This is Figure 2 from the paper. Caption: a high-pass filtered image without PSF subtraction or artifact removal. The α Centauri B on-coronagraph images have been subtracted from the α Centauri A on-coronagraph images, resulting in a central residual and two off-axis PSFs to the SE and NW of α Centauri A and B, respectively. Systematic artifacts labeled 1-3 correspond to detector persistence from α Centauri A, α Centauri B, and an optical ghost of α Centauri A. b Zoom-in on the inner regions following artifact removal and PSF subtraction. Regions impacted by detector persistence are masked for clarity. The approximate inner edge of the habitable zone of α Centauri A13 is indicated by the dashed circle. A candidate detection is labeled as ‘C1’. Credit: Wagner et al.
The C1 candidate looks, says Wagner, like what the team’s simulated planetary sources look like, but it could also represent, in addition to a systematic error, dust in the habitable zone, bearing in mind that while the Sun has its own zodiacal light from such dust, the Alpha Centauri system is known to have 50 times more dust. We could be looking, in other words, at dust that is off-center simply because of the orbital perturbations within the binary. “We can’t attribute this to any of the known systematics,” says Wagner, “but we don’t know all the systematics in this new system.”
What’s truly newsworthy in the NEAR work is the sensitivity of the dataset, which demonstrates that a habitable zone planet somewhere between Neptune and Saturn in size is detectable around the Alpha Centauri stars. NEAR is, in other words, sensitive to planets smaller than Jupiter at about 1 AU, and thus we can expect further work to find out whether C1 can be verified as a planet. This could be done through imaging using the James Webb Space Telescope, or through another observing run with NEAR, or via astrometry (about which more in a day or so) or even time-tested radial velocity using the hugely sensitive ESPRESSO.
The current limit on radial velocity detection around Alpha Centauri is on the order of 50 Earth masses in the habitable zone, Wagner added. NEAR itself is not currently in operation but could be reinstalled at UT-4 on the VLT, and of course on top of the other options, we have the next generation of ground-based telescopes coming, extremely large instruments that could accomplish within a single hour what it took the NEAR instrumentation 100 hours to do.
NEAR has demonstrated a technology, then, that is apparently capable of imaging mature Neptune-class planets in this system. Ramp its sensitivity up four times and we get to ‘super-Earth’ detection capability. We’re not yet at Earth-like planet imaging, but within decades, the ELTs should make it possible. We can consider NEAR a pathfinder experiment that has demonstrated the limits of the possible and shown us the way forward as, step by step, Alpha Centauri yields its secrets.
For more, see Wagner et al., “Imaging low-mass planets within the habitable zone of α Centauri,” Nature Communications 12: 922 (2021). Abstract / full text.
by Paul Gilster | Jan 10, 2017 | Exoplanetary Science |
Breakthrough Starshot, the research and engineering effort to lay the groundwork for the launch of nanocraft to Alpha Centauri within a generation, is now investing in an attempt to learn a great deal more about possible planets around these stars. We already know about Proxima b, the highly interesting world orbiting the red dwarf in the system, but we also have a K- and G-class star here, either of which might have planets of its own.
Image: The Alpha Centauri system. The combined light of Centauri A (G-class) and Centauri B (K-class) appears here as a single overwhelmingly bright ‘star.’ Proxima Centauri can be seen circled at bottom right. Credit: European Southern Observatory.
To learn more, Breakthrough Initiatives is working with the European Southern Observatory on modifications to the VISIR instrument (VLT Imager and Spectrometer for mid-Infrared) mounted at ESO’s Very Large Telescope (VLT). Observing in the infrared has advantages for detecting an exoplanet because the contrast between the light of the star and the light of the planet is diminished at these wavelengths, although the star is still millions of times brighter.
To surmount the problem, VISIR will be fitted out for adaptive optics. In addition, Kampf Telescope Optics of Munich will deliver a wavefront sensor and calibration device, while the University of Liège (Belgium) and Uppsala University (Sweden) will jointly develop a coronagraph that will mask the light of the star enough to reveal terrestrial planets.
Image: Paranal at sunset. This panoramic photograph captures the ESO Very Large Telescope (VLT) as twilight comes to Cerro Paranal. The enclosures of the VLT stand out in the picture as the telescopes in them are readied for the night. The VLT is the world’s most powerful advanced optical telescope, consisting of four Unit Telescopes with primary mirrors 8.2 metres in diameter and four movable 1.8-metre Auxiliary Telescopes (ATs), which can be seen in the left corner of the image. Credit: ESO.
According to the agreement signed by Breakthrough Initiatives executive director Pete Worden and European Southern Observatory director general Tim de Zeeuw, Breakthrough Initiatives will pay for a large part of the technology and development costs for the VISIR modifications. Meanwhile, the ESO will provide the necessary telescope time for a search program that will be conducted in 2019. The VISIR work, according to this ESO news release, should provide a proof of concept for the METIS instrument (Mid-infrared E-ELT Imager and Spectrograph), the third instrument on the upcoming European Extremely Large Telescope.
by Paul Gilster | Nov 15, 2016 | Exoplanetary Science |
We know about an extremely interesting planet around Proxima Centauri, and there are even plans afoot (Breakthrough Starshot) to get probes into the Alpha Centauri system later in this century. But last April, when Breakthrough Initiatives held a conference at Stanford to talk about this and numerous other matters, the question of what we could see came up. For in Alpha Centauri, we’re dealing with three stars that are closer to us than any other. If there are planets around Centauri A and/or Centauri B, are there ways we could image them?
This gets interesting in the context of Project Blue, a consortium of space organizations looking into exoplanetary imaging technologies. This morning Project Blue drew on the work of some of those present at Stanford, launching a campaign to fund a telescope that could obtain the first image of an Earth-like planet outside our Solar System, perhaps by as early as the end of the decade. The idea here is to ignite a Kickstarter effort aimed at raising $1 million to support needed telescope design studies. A $4 million ‘stretch goal’ would allow testing of the coronagraph, completion of telescope design and the beginning of manufacturing.
Project Blue thinks it can bring this mission home — i.e., launch the telescope and carry out its mission — at a final cost of $50 million (the original ACEsat was a $175 million design). The figure is modest enough when you consider that Kepler, which has transformed our view of exoplanets, cost $600 million, while the James Webb Space Telescope weighs in at $8 billion. About a quarter of the total cost, according to the project, goes into getting the telescope into orbit, which will involve partnering with various providers to lower costs.
But Project Blue also hopes to build a public community around the mission to support design and research activities. Jon Morse is mission executive for the project:
“We’re at an incredible moment in history, where for the first time, we have the technology to actually find another Earth,” said Morse. “Just as exciting — thanks to the power of crowdfunding — we can open this mission to everyone. With the Project Blue consortium, we are bringing together the technical experts who can build and launch this telescope. Now we want to bring along everyone else as well. This is a new kind of space initiative — to achieve cutting-edge science for low cost in just a few years, and it empowers us all to participate in this moment of human discovery.”
I go back to last April because it was at Stanford that I saw Eduardo Bendek’s model of a small space telescope called ACEsat, which was conceived at NASA Ames by Ruslan Belikov and Eduardo Bendek and submitted (unsuccessfully) for NASA Small Explorer funding. Belikov had gone on to present the work at the American Astronomical Society meeting in 2015 (see “How to Directly Image a Habitable Planet Around Alpha Centauri with a ~30-45cm Space Telescope,” available here). You’ll recall that Ashley Baldwin wrote up the concept in superb detail on this site in December of that year as ACEsat: Alpha Centauri and Direct Imaging.
Now we have Project Blue, which has connections to the BoldlyGo Institute, its offshoot Mission Centaur, the SETI Institute and the University of Massachusetts Lowell. The aim is to launch a space telescope with a 45-50 centimeter aperture, looking for potentially habitable planets from 0.5 to 1.5 AU within the habitable zones of both Centauri A and B. The ultimate hope, then, is to ‘see blue’ — meaning oceans and atmosphere, a world on which life could emerge. This is Sagan’s ‘pale blue dot,’ only now it’s not our own planet but an Earth 2.0.
The Project Blue space telescope would spend two years in low Earth orbit accumulating image after image — hundreds, thousands, tens of thousands — as a way of teasing out its faint targets. When it comes to ‘another Earth,’ Centauri A and B up the ante on Proxima Centauri. The Proxima planet may well be habitable, but a true Earth analog is not going to be tidally locked to its star, as Proxima b probably is, and it’s not going to orbit a red dwarf.
Neither Belikov or deputy principal investigator Eduardo Bendek are formally connected to Project Blue, but their work in the form of papers and conference presentations feeds directly into the concept driving the project. The original mission now cedes the floor to the private sector, whose job it will be to raise enough cash to support the development of the needed coronagraph to filter out the light of two very close stars, along with other key flight hardware elements. The next step, though, long before building flight hardware, is to finalize the telescope design.
The new Kickstarter campaign will pay for analysis, design, and simulations, but Project Blue has an eye on other partnerships as well as wealthy donors and foundations. Usefully, the project should be able to test coronagraph technologies similar to those being considered on much larger space instruments currently under study by the major space agencies, thus providing a useful testbed for such designs. To make this work, everything must fall into place — the coronagraph for starlight suppression, a deformable mirror to feed the coronagraph and rock-solid stability. No aspect can be allowed to fail if the mission is to achieve its goal.
If the Project Blue planners are correct, we can solve the attendant problems and get this mission into space is as little as 4 to 6 years. The goal is hugely ambitious but it also opens the door to citizen-science, with private donors contributing to an instrument that will not be the result of a government program or a for-profit commercial space effort. The initial Kickstarter campaign is designed to bolster the technical groundwork needed for the telescope, but stretch goals could see publicly funded flight component manufacturing.
Looking for Earth-like planets around other stars is like looking for bioluminescent algae next to a lighthouse. But I keep coming back to that Breakthrough Discuss meeting in Stanford, because I remember Ruslan Belikov telling his audience that the key advantage of Alpha Centauri is how large the habitable zones around its component stars appear in terms of angular size. We would need a significantly larger instrument to attempt something similar around other nearby stars. The Alpha Centauri stars are nature’s gift, and it’s one we would do well to exploit. Check the Kickstarter page for more on this low cost, high impact idea.
For more on the technical background of the ACEsat concept, see Belikov et al., “How to Directly Image a Habitable Planet Around Alpha Centauri with a ~30-45cm Space Telescope” (preprint) and Bendek et al., “Space telescope design to directly image the habitable zone of Alpha Centauri” (preprint).
by Paul Gilster | Nov 9, 2015 | Exoplanetary Science |
Finding a habitable world around any one of the three Alpha Centauri stars would be huge. If the closest of all stellar systems offered a blue and green target with an atmosphere showing biosignatures, interest in finding a way to get there would be intense. Draw in the general public and there is a good chance that funding levels for exoplanet research as well as the myriad issues involving deep space technologies would increase. Alpha Centauri planets are a big deal.
The problem is, we have yet to confirm one. Proxima Centauri continues to be under scrutiny, but the best we can do at this point is rule out certain configurations. It appears unlikely, as per the work of Michael Endl (UT-Austin) and Martin Kürster (Max-Planck-Institut für Astronomie), that any planet of Neptune mass or above exists within 1 AU of the star. Moreover, no ‘super-Earths’ have been detected in orbits with a period of less than 100 days. This doesn’t rule out planets around Proxima, but if they are there, so far we don’t see them.
Image: The Alpha Centauri stellar system, consisting of the red dwarf Proxima Centauri and the two bright stars forming a close binary, Centauri A and B. Credit: NASA.
Centauri B, the K-class star in close proximity to G-class Centauri A, was much in the news a few years back with the announcement of Centauri Bb, a candidate world announced by Swiss planet hunters. This is radial velocity work based on data gathered by the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. The signal that Xavier Dumusque and team drew out of the data was 0.5 meters per second, a fine catch if confirmed.
What we thought we had in Centauri Bb was a mass just a little over the Earth’s and an orbit of a scant 3.24 days. As the blistering first planet detected around one of the Centauri stars, it would be a significant find even if it’s a long way from the temperate, life-sustaining world we’d like to find further out. The putative Centauri Bb supported the idea that there might be other planets there, and we’ve known since the work of Paul Wiegert and Matt Holman back in the 1990s that sustainable habitable zone orbits are possible around both the primary Alpha Centauri stars.
But Centauri Bb has remained controversial since Artie Hatzes (Thuringian State Observatory, Germany), using different data processing strategies, looked at the same data and found a signal he considered too noisy, indicating that what might be a planet might also be stellar activity on Centauri B itself. Debra Fischer’s team at Cerro Tololo Inter-American Observatory has also been studying Centauri Bb using the CHIRON spectrometer but has not been able to confirm it. And while a transit search using the Hubble Space Telescope did find a promising lightcurve (about which more in a moment), it couldn’t confirm Centauri Bb.
Image: Of the three stars of Alpha Centauri, the dimmest, Proxima Centauri, is actually the nearest star to the Earth. The two bright stars, Alpha Centauri A and B form a close binary system; they are separated by only 23 times the Earth – Sun distance. This is slightly greater than the distance between Uranus and the Sun. The Alpha Centauri system is not visible from much of the northern hemisphere. The image above shows this star system and other objects near it in the sky. Credit/copyright: Akira Fujii / David Malin Images.
Now we have a new paper from Vinesh Rajpaul (University of Oxford) and colleagues that makes Centauri Bb look more unlikely than ever. Rajpaul praises the thorough work of Xavier Dumusque and the team at the Geneva Observatory, but notes that their attempts to filter stellar activity out of their data evidently boosted other periodic signals that had nothing to do with a planet. The signal grows out of the time sampling, or ‘window function,’ of the data.
What is left behind is what the paper calls ‘the ‘ghost’ of a signal’ that was present all along. The paper argues that when a signal is sampled at discrete times (and the Dumusque team had to use the La Silla instrument only when it was not otherwise booked), periodicities can be imposed on the signal. Rajpaul was able to simulate a star with no planets, generating synthetic data out of which the exact same 3.24-day planetary signal emerged. The problem is particularly acute when working with planetary ‘signals’ as weak as these. From the paper:
D12’s data set [i.e., the data gathered by Dumusque and team] was particularly pathological because the window function happened to contain periodicities that coincided with the stellar rotation period of α Cen B, and its first harmonic; when these signals were filtered out, the significance of the 3.24 d signal was preferentially boosted.
All this is going to be quite useful if it helps us refine our techniques for identifying small planets. Rajpaul proposes that his team will carry out a new study of the spurious but coherent signals that can emerge from noisy datasets that should help us learn how to mitigate the problem:
We alluded to a number of other tests we believe worth carrying out when considering the reliability of planet detections from noisy, discretely-sampled signals. These include using the same model used to detect the planet instead to fit synthetic, planet-free data (with realistic covariance properties, and time sampling identical to the real data), and checking whether the ‘planet’ is still detected; comparing the strength of the planetary signal with similar Keplerian signals injected into the original observations; performing Bayesian model comparisons between planet and no-planet models; and checking how robust the planetary signal is to datapoints being removed from the observations.
Xavier Dumusque praises the Rajpaul team in this story in National Geographic, saying “This is really good work… We are not 100 percent sure, but probably the planet is not there.” We’re going to get a lot out of this investigation even though we lose Centauri Bb.
But back to that HST transit study run by Brice-Olivier Demory (University of Cambridge). I mentioned that it could detect no transit of Centauri Bb, which certainly fits with what we’ve just seen, but there was an interesting lightcurve suggesting a different possible planet, this one in an orbit that might range from 12 to 20 days. If this planet exists, radial velocity confirmation would be even more challenging than for Centauri Bb. Its signal, as Andrew LePage notes in The Discovery of Alpha Centauri Bb: Three Years Later, would be only half that of Centauri Bb.
LePage’s work at Drew ex Machina is definitive, and he has devoted a good deal of attention to Alpha Centauri. Here he explains why that second ‘planet’ is going to be so hard to spot:
Unfortunately with such a poorly constrained orbit, three weeks of nearly continuous photometric monitoring of α Centauri B will be required to confirm this hypothesis. HST is too busy to accommodate a dedicated search of this length and no other space telescope currently available is capable of making the needed observations. In addition, since the radial velocity signature for this planet would be expected to be maybe half that of α Centauri Bb, this method has little likelihood of providing independent confirmation of this sighting any time soon. Once again, we will have to wait for a few more years for new telescopes to become available such as NASA’s TESS (Transiting Exoplanet Survey Satellite) mission or ESA’s CHEOPS (Characterizing Exoplanets Satellite) which are both scheduled for launches in 2017 and may be capable of making the required observations of such a bright target.
Alpha Centauri is frustrating in many ways because you would expect the closest stellar system to have revealed more of its secrets by now. One of the problems, though, and a huge one, is that the angular separation (as viewed from Earth) of the primary Centauri stars has been decreasing as they move through their orbits. It won’t be until December of this year that they’ll reach minimum separation as seen from Earth. We’ll need to give Alpha Centauri a little time, in other words, before we can hope to get data on other possible worlds around Centauri B.
Image (click to enlarge): Apparent and true orbits of Alpha Centauri. The A component is held stationary and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity vs. time [10] the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. The orbit is divided here into 80 points, each step refers to a timestep of approx. 0.99888 years or 364.84 days. Credit: Wikimedia Commons.
The Rajpaul paper is Rajpaul, Aigrain & Roberts, “Ghost in the time series: no planet for Alpha Cen B,” accepted for publication at Monthly Notices of the Royal Astronomical Society (preprint). The Hatzes paper is “Radial Velocity Detection of Earth-Mass Planets in the Presence of Activity Noise: The Case of α Centauri Bb”, The Astrophysical Journal, Vol. 770, No. 2, (2013) (preprint).
by Paul Gilster | Dec 11, 2006 | Exoplanetary Science |
You would think Alpha Centauri would be a prime hunting ground for extrasolar planets simply because of its proximity. But the problem for direct imaging is the sheer brightness of Centauri A and B, creating a halo of diffuse light around the pair. Getting through the glare isn’t easy, but a search based on twin techniques — adaptive optics and CCD imaging — covering a wide-field around the Centauri system has just been completed. Results on the CCD work, using European Southern Observatory equipment, have now been made available and they’ve come up short on planetary detections.
As reported by Pierre Kervella (Observatoire de Paris-Meudon) and Frederic Thévenin (Observatoire de la Côte d’Azur), the team found no co-moving companion objects between 100 and 300 AU. And that’s useful information, because it puts some constraints on possible planets around these stars. From the paper:
Within the explored area, this negative result sets an upper mass limit of 15-30 M J to the possible companions orbiting α Cen B or the pair, for separations of 50-300 AU. When combined with existing radial velocity searches…and our adaptive optics results…this mostly excludes the presence of a 20-30 M J companion within 300 AU.
First of all, note what this is not telling us. We can draw no conclusions about possible terrestrial-sized worlds orbiting within 3-4 AU of either Centauri A or B, for the equipment is not sensitive enough to detect planets that small. Thus the scenario that continues to fire the imagination of many of us — habitable planets around one or both Centauri stars — is still viable. We’ve simply learned that we can rule out massive super-Jupiters in wide orbits.
And that gives us further insight into the Alpha Centauri system itself, for some recent work has indicated that the mass of Centauri B could be higher than what earlier models have suggested. Specifically, radial velocity studies have come up with mass estimates that differ by 28 Jupiter masses (plus or minus 9) from the results of long-baseline interferometry. If the missing mass is in the form of an unseen companion, we can now exclude at least one planetary configuration that might have accounted for it.
The paper is Kervella and Thévenin, “Deep imaging survey of the environment of α Centauri,” accepted as a research note by Astronomy & Astrophysics and available as a preprint online. The team’s earlier work using adaptive optics (which feeds directly into the present paper) is Kervella et al., “Deep imaging survey of the environment of α Centauri: I. Adaptive optics imaging of α Cen B with VLT-NACO,” available here. Centauri Dreams‘ earlier story on the latter is also available.
