Project Blue: Looking for Terrestrial Worlds at Alpha Centauri

Eduardo Bendek’s ACEsat, conceived at NASA Ames by Bendek and Ruslan Belikov, seemed to change the paradigm for planet discovery around the nearest stellar system. The beauty of Alpha Centauri is that the two primary stars present large habitable zones as seen from Earth, simply because the system is so close to us. The downside, in terms of G-class Centauri A and K-class Centauri B, is that their binary nature makes filtering out starlight a major challenge.

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.

If we attack the problem from the ground, ever bigger instruments seem called for, like the European Southern Observatory’s Very Large Telescope in conjunction with the VISIR instrument (VLT Imager and Spectrometer for mid-Infrared) that Breakthrough Initiatives is now working with the ESO to enhance. Or perhaps one of the extremely large telescopes now in the works, like the Thirty Meter Telescope in Hawaii, or the Giant Magellan Telescope in Chile.

And if we did this from space, surely it would be an expensive platform. Except that ACEsat wasn’t expensive, nor was it large. It was designed to do just one thing and do it well.

While NASA turned down Bendek and Belikov’s idea for Small Explorer funding, the striking thing is that it would have fit that category’s definition. ACEsat was designed as a 30 to 45 cm space telescope (you can see a Belikov presentation on the instrument here, or for that matter, read Ashley Baldwin’s ACEsat: Alpha Centauri and Direct Imaging). The small instrument now being proposed by an initiative called Project Blue builds on many of the ACEsat concepts. It would run perhaps $50 million even though the original ACEsat was a $175 million design.

In other words, compared to the $8 billion James Webb Space Telescope, Project Blue’s instrument is almost inexpensive enough to be a rounding error. A privately funded initiative out of the Boldly Go Institute, in partnership with the SETI Institute, Mission Centaur, and UMass Lowell, the telescope shows its pedigree both in its low cost and big scientific return. It seems the ACEsat concept is just too good to go away.

So now we have Project Blue, which is all about seeing the blue of an Earth-like world around one or even both of the Sun-like stars of the Alpha Centauri system. No one discounts the value of the planet already discovered around Proxima Centauri, but the project hopes to find an Earth 2.0, a rocky planet in a habitable zone orbit around a star like our own. That would mean no tidal locking, no small red dwarf primary, and a year measured in months rather than days.

Image: An Earth-like planet around one of the primary Alpha Centauri stars, as simulated by Project Blue.

The project’s new Indiegogo campaign has been set up to raise $175,000 to help establish mission requirements, including the design of an initial system architecture to which computer simulations can be applied by way of testing ideas and simulating outcomes. The launch goal of 2021 is ambitious indeed, as is the low $50 million budget profile, but the project’s backers believe their work can leverage advances in the small satellite industry and imaging systems to pull it off. An explicit goal is to engage the public while tapping the original NASA work.

The project’s connection to NASA is in the form of a cooperative agreement explained on the Indiegogo site:

The BoldlyGo Institute and NASA have signed a Space Act Agreement to cooperate on Project Blue, a mission to search for potentially habitable Earth-size planets in the Alpha Centauri system using a specially designed space telescope. The agreement allows NASA employees – scientists and engineers – to interact with the Project Blue team through its mission development phases to help review mission design plans and to share scientific results on Alpha Centauri and exoplanets along with the latest technology tests being undertaken at NASA facilities. The agreement also calls for the raw and processed data from Project Blue to be made available to NASA within one year of its acquisition on orbit via a publicly accessible online data archive. The Project Blue team has been planning such an archive for broadly sharing the data with the global astronomical community and for enabling citizen scientist participation.

And I notice that Eduardo Bendek is among the ranks of an advisory committee (available here) that includes the likes of exoplanet hunters Olivier Guyon, Debra Fischer, Jim Kasting and Maggie Turnbull. But have a look at the advisor page; every one of these scientists is playing a significant role in our discovery and evaluation of new exoplanetary systems.

Thus we can say that ACEsat lives on in this new incarnation that will benefit from the input of its original designers. The spacecraft would spend two years in low Earth orbit accumulating thousands of images with the help of an onboard coronagraph to remove light from the twin stars, along with a deformable mirror, low-order wavefront sensors, and control algorithms to manage incoming light, enhancing image contrast with software processing methods.

Unlike the major observatories we’re soon to be launching — not just the James Webb Space Telescope but the Transiting Exoplanet Survey Satellite (TESS) — the Project Blue observatory will be dedicated to a single target, with no other observational duties.

A photograph of an Earth-like planet 40 trillion kilometers away gives us a sense of the changes in scale that have occurred since Voyager 1’s ‘pale blue dot’ photograph. But we already knew that Earth was inhabited. Now, gaining spectral information about a blue and green world around a nearby star would allow us to determine whether biosignature gases could be found in its atmosphere, potential signs of life that would mark a breakthrough in our science. The degree of public involvement assumed in the project makes the quest all the more tantalizing.


On the GW170814 Gravitational Wave Detection

What we get with yesterday’s gravitational wave announcement isn’t a breakthrough in itself. After all, this is not the first but the fourth detection of a black hole merger, so as we enter the era of gravitational wave astronomy, we’re beginning to build our catalog of exotic objects.

But the gravitational wave known as GW170814 is significant because of the addition of the Virgo Gravitational-Wave Observatory to our toolkit. We ramp up our capabilities at locating the objects we detect in the sky when we factor in this new detector. Thus Chad Hanna (Penn State), who served as co-chair of the group within LIGO (Laser Interferometer Gravitational-Wave Observatory) that made all previous detections:

“It is our hope to one day detect gravitational waves and to simultaneously observe the source of the gravitational waves with conventional telescopes so that we might learn even more about what causes the gravitational waves. In order to do that, we need to know where to look. LIGO and Virgo together allow us to pinpoint the gravitational wave source in the sky far better than before, which will dramatically improve our chances of capturing the gravitational wave source with other telescopes.”

Image: Top row: Signal-to-noise ratio as a function of time. The peaks occur at different times in different detectors because gravitational waves propagate at the finite speed of light; this causes the signal to reach the detectors at different times. GW170814 arrived first in LIGO-Livingston, then 8 ms later in LIGO-Hanford and 6 ms after that in Virgo. Middle row: Time-frequency representation of the strain data. The brighter a given pixel in any of the three 2D-maps, the larger the signal at this particular time and frequency with respect to the expected noise level. Note the characteristic “chirp” pattern of increasing frequency with time. Bottom row: Strain time series with the best waveforms selected by the matched filtering (black solid curves) and unmodeled search methods (gray bands) superimposed. Credit and copyright: LIGO Scientific Collaboration and Virgo Collaboration.

Gravitational wave astronomy is less than two years old, but we’re adding substantial resources to the investigation with the addition of the Virgo detector. Located near Pisa, the Italian effort involves more than 280 physicists and engineers in 20 different European research groups. The Virgo detector took data jointly with the two LIGO observatories, one in Livingston, Louisiana and the other at Hanford in Washington state, in a network that also included contributions from the Anglo-German GEO600 instrument near Hanover.

The network observed GW170814 on August 14, only two weeks after the Virgo detector began taking data. Subsequent analysis showed that the event marked the merger of two black holes of 31 and 25 solar masses respectively, occurring at a distance of 1.8 billion light years. The newly produced black hole is thought to have 53 times the mass of the Sun, with three solar masses being converted into gravitational wave energy during the coalescence of the constituent black holes.

And here is where the triangulation comes in. The gravitational wave arrived at the Livingston detector some 8 milliseconds before the LIGO detector at Hanford, and some 14 milliseconds before reaching the Virgo detector. Combined arrival time delays allows the direction toward the source to be determined. Researchers are saying they can trace it down to a patch of 60 square degrees in the southern sky between the constellations Eridanus and Horologium. Moving from a two- to three-detector network shrinks the volume of sky likely to contain a source by more than a factor of 20, according to this LIGO Scientific Collaboration news release.

Image: The Virgo Observatory. Credit: The Virgo collaboration/CCO 1.0.

The search for an analog to the gravitational wave event produced no detection at electromagnetic wavelengths, although 25 observatories searched at wavelengths ranging from gamma, optical, infrared, x-ray, and radio as well as neutrino emissions. The lack of an electromagnetic detection was not surprising, because although collisions of neutron stars are thought likely to produce light emissions as well as gravitational waves, black hole mergers produce gravitational waves but not light.

Scientists at the Albert Einstein Institute in Potsdam and Hanover ruled out random noise fluctuations, finding the signal to be real with a probability of more than 99 percent. The Hanover team developed many of the software algorithms used in the analysis of LIGO data.

A third observatory ensures that future detections will be accompanied by a search for the source at all these wavelengths as we begin to extend gravitational wave astronomy into events beyond black hole collisions.

“This is just the beginning of observations with the network enabled by Virgo and LIGO working together,” says David Shoemaker of MIT, LIGO Scientific Collaboration spokesperson. “With the next observing run planned for Fall 2018 we can expect such detections weekly or even more often.”

The paper on this event, accepted at Physical Review Letters, is LIGO Scientific Collaboration and The Virgo Collaboration, “GW170814 : A three-detector observation of gravitational waves from a binary black hole coalescence” (available online).


The Milky Way as an Outlier

How ‘normal’ is the Milky Way? It’s an interesting question because as we look out into a visible universe filled with perhaps 100 billion galaxies, we base many of our models for their behavior on what we know of our own. That this may not be the best way to proceed is brought home by a much smaller study, the comparison between our Solar System and what we’ve been finding around other stars. Finding Solar System analogs has proven surprisingly difficult, although older models assumed outer gas giants and inner rocky worlds as a common pattern.

Thus I am keeping an eye on a survey called Satellites Around Galactic Analogs (SAGA), which is looking into galaxies with smaller satellite galaxies. We’re only in the early days of this survey, with eight galaxies now examined in a new paper from lead author Marla Geha (Yale University). But the goal is 100 galaxies, with 25 of these studied within the next two years.

Image: A three-color optical image of a Milky Way sibling. Credit: Sloan Digital Sky Survey.

Even now, however, the results are intriguing. It turns out that the satellite galaxies of the Milky Way are far more sedate than those in other galactic systems comparable in luminosity and environment. It’s not uncommon for ‘sibling’ galaxy satellites to be producing new stars, but the Milky Way’s satellites are generally inert. Like our Solar System, our galaxy too may have its quirks.

“We use the Milky Way and its surroundings to study absolutely everything,” said Geha, “Hundreds of studies come out every year about dark matter, cosmology, star formation, and galaxy formation, using the Milky Way as a guide. But it’s possible that the Milky Way is an outlier.”

Like the study of exoplanet atmospheres we looked at yesterday, comparative surveys like these are essential for placing what we see around us in a much broader, if not universal context. Thus far SAGA has generated complete spectroscopic coverage within 300 kpc, counting eight Milky Way analogs. The process of choosing ‘analogs’ is detailed and painstakingly recounted in the paper, but the gist of it is that the team looks at a galaxy’s K-band infrared luminosity as a proxy for stellar mass and considers a host of factors related to the galaxy’s halo and its large-scale environment including other nearby galaxies.

Thus far, SAGA has uncovered 25 new satellite galaxies, 14 of which meet the survey’s formal criteria, plus an additional 11 that remain incompletely surveyed. Given that the Sloan Digital Sky Survey had already found 13 satellites among these galaxies, we thus far have 27 satellites around 8 Milky Way analog galaxies that have been subjected to exhaustive analysis.

As to the Milky Way itself, we continue to find what the paper considers ‘faint satellites’ as large-area imaging surveys continue, but the number of bright satellites has remained fixed since the discovery of the Sagittarius dwarf spheroidal galaxy about twenty years ago. Geha and team consider the catalog of bright Milky Way satellites to be largely complete.

The SAGA survey is in its early days, but it is striking that 26 out of the 27 satellite galaxies considered are actively forming stars, unlike both the Milky Way and M31. As the paper notes:

The above results suggest that the satellite population of the Milky Way may not be representative of satellite populations in the larger Universe. Expanding the number of Milky Way analog galaxies with known satellites is required to use these objects as meaningful probes of both cosmology and galaxy formation.

And this is also interesting:

We have characterized complete satellite luminosity functions for 8 Milky Way analog hosts. We find a wide distribution in the number of satellites, from 1 to 9, in the luminosity range for which there are five satellites around the Milky Way. We see no statistically significant correlations between satellite number and host properties, although any correlation would be hard to detect robustly with our small sample size of hosts.

Bear in mind as the SAGA Survey continues that until now, we have based most of our information about satellite galaxies on what we see right here in the Milky Way and in M31. We’re now developing the larger picture that can help us place galaxy formation in context. Finding that even the galaxy we live in is not typical would fit the pattern of recent exoplanet discoveries in suggesting that galaxy as well as planet formation is a deeply stochastic process.

The paper is Geha et al., “The SAGA Survey: I. Satellite Galaxy Populations Around Eight Milky Way Analogs,” accepted at the Astrophysical Journal (preprint).


A Statistical Look at Exoplanet Atmospheres

Comparative exoplanetology? That’s the striking term that Angelos Tsiaras, lead author of a new paper on exoplanet atmospheres, uses to describe the field today. Kepler’s valuable statistical look at a crowded starfield has given us insights into the sheer range of outcomes around other stars, but we’re already moving into the next phase, studying planetary atmospheres. And as the Tsiaras paper shows, constructing the first atmospheric surveys.

Tsiaras (University College, London) assembled a team of European researchers that examined 30 exoplanets, constructing their spectral profiles and analyzing them to uncover the characteristic signatures of the gases present. The study found atmospheres around 16 ‘hot Jupiters,’ learning that water vapor was present in each of them. Says Tsiaras:

“More than 3,000 exoplanets have been discovered but, so far, we’ve studied their atmospheres largely on an individual, case-by-case basis. Here, we’ve developed tools to assess the significance of atmospheric detections in catalogues of exoplanets. This kind of consistent study is essential for understanding the global population and potential classifications of these foreign worlds.”

Image: An artist’s impression of the kind of systems studied by the UCL team. Credit: Alexaldo.

Presented at the European Planetary Science Congress (EPSC) 2017 in Riga, the study used archival data from the Hubble telescope’s Wide Field Camera 3 (WFC3), finding that most of the detected atmospheres show evidence for clouds, although the two hottest planets, with temperatures exceeding 1700 degrees Celsius, evidently have clear skies at least at high altitudes. Both of the latter show indications of water vapor, titanium oxide and vanadium oxide.

The authors have defined a metric they call the Atmospheric Detectability Index (ADI) to measure the statistical significance of an atmospheric detection, meaning that while we have 16 planets with atmospheres the metric finds significant, other less detectable atmospheres are present in the rest of the sample. The paper explains the 14 spectra without significant atmosphere detection as the result of opaque, high-altitude clouds or low water abundances. It is highly unlikely, in other words, that gas giant planets will fit any no-atmosphere models.

What jumps out of this work is the fact that the detectability of ‘hot Jupiter’ atmospheres through the ADI metric appears to be dependent on planetary radius rather than planetary mass.

“These results,” the paper adds, “indicate that planetary surface gravity is a secondary factor in identifying inflated atmospheres,” though we should also note that the paper identifies an outlying group of five planets with large radii and no detectable atmospheres. The other planets show the correlation between atmosphere and planetary radius. And it turns out that very hot planets produce strong results with this method. From the paper:

Very hot and highly irradiated planets, with atmospheric temperatures above 1800 K feature high ADI atmospheres. Our quantitative retrievals suggest that the cloud top-pressures in these planets are significantly high, meaning clouds are deep in the atmosphere, if present at all…, while retrieved water abundances are constant within the errors… We can conclude that planets with temperatures higher than 1800 K feature clear atmospheres, confirming that most of the element-carriers are present in a gaseous form at such hot temperatures.

We’ll see how the Atmospheric Detectability Index fares as it is applied to future, larger-scale surveys. For we’ll certainly need such surveys as we enter the era of extremely large telescopes on the ground and new missions that will produce huge numbers of new planet detections. The Tsiaras team’s work is important because it shows we are developing the tools and models that will be applied in the future to much larger samples of planetary atmospheres.

The paper is Tsiaras et al., “A population study of hot Jupiter atmospheres,” submitted to the Astrophysical Journal (preprint).


A Binary Main-Belt Comet

The paper in Nature covering an object known as 288P lays out the case in its title: “A Main Belt Comet.” But what makes this story stand out is the fact that 288P is also a binary.

A team of scientists led by Jessica Agarwal (Max Planck Institute for Solar System Research) discovered when 288P neared perihelion in September of 2016 that it was not one but two objects, asteroids of roughly the same mass and size, in a binary separated by about 100 kilometers. Moreover, they have verified that the small system is not quiescent.

Using the Hubble instrument, Agarwal and colleagues discovered that the increased solar heating due to perihelion was producing sublimation of water ice, in much the same way that the tail of a comet is created. Here’s how the paper describes the process on 288P:

Repeated activity near perihelion is a strong indicator of the sublimation of water ice due to increased solar heating. A model of the motion of the dust under the influence of solar gravity and radiation pressure suggests that the activity began with a brief release of comparatively large (millimetre-sized) grains in July, while from mid-September until at least the end of January 2017 (the last of our observations), the dominant grain size fell to ?10 µm… This indicates that the developing gas production first lifted a layer of large, loosely connected grains, possibly deposited around the end of the previous period of activity in 2011/12. After their removal and with decreasing heliocentric distance, the gas drag became sufficiently strong to lift also smaller particles.

As a main-belt comet, 288P may give us further insights into how water came to Earth. It is also the first known binary asteroid that can be classified as a main-belt comet.

Image: This set of images from the ESA/NASA Hubble Space Telescope reveals two asteroids with comet-like features orbiting each other. These include a bright halo of material, called a coma, and a long tail of dust. The asteroid pair, called 288P, was observed in September 2016 just before the asteroid made its closest approach to the Sun. These images reveal ongoing activity in the binary system. The apparent movement of the tail is a projection effect due to the relative alignment between the Sun, Earth, and 288P changing between observations. The tail orientation is also affected by a change in the particle size. Initially, the tail was pointing towards the direction where comparatively large dust particles (about 1 millimeter in size) were emitted in late July. However, from 20 September 2016 onwards, the tail began to point in the opposite direction from the Sun where small particles (about 10 microns in size) are blown away from the nucleus by radiation pressure. Credit: NASA, ESA, and J. Agarwal (Max Planck Institute for Solar System Research).

288P’s activity gives us clues to its history. For surface ice cannot survive for billions of years in the main belt. As Agarwal notes, it would have to be protected by several meters of dust mantle, which makes 288P as a binary a relatively recent system, perhaps one that has existed for no more than 5000 years. We then factor in the growing interest in asteroids as delivery vehicles for water to the inner system to see why this system may become something of a benchmark.

Although an impact could have caused the breakup of the original asteroid, the researchers argue in the paper that breakup due to fast rotation is the most likely cause. Later torques caused by sublimation — the breakup would have exposed the ice for subsequent sublimation — could have caused the objects to move further apart:

A decisive factor for the subsequent development of the system is whether the sublimation will last longer than the time required to tidally synchronise the spin and binary orbital periods, which is 5000 years for equal-mass components but orders of magnitude longer for lower mass ratios. Sublimation-driven activity can last longer than 5000 years, such that for high-mass ratio systems it is conceivable that activity prevails after tidal synchronisation. In this case, the recoil force from the local sublimation of water ice can drive binary evolution.

288P is unusual in another way. Asteroid binaries are not uncommon, but most are discovered by radar (when close to Earth) or by analysis of the mutual eclipses revealed in their light curves, assuming a relatively small separation between the two components. Wide binaries like 288P rarely align to produce such eclipses and are too distant to be revealed by radar.

Indeed, it was the evident sublimation of water ice that drew the attention of the researchers. Although a larger sample of wide binaries is needed, Agarwal and team believe the sublimation activity played a decisive role in the evolution of this young binary, affected by the fact that the two components were of nearly equal size, distinguishing 288P from other asteroid pairs.

The paper is Agarwal et al., “A binary main-belt comet,” Naure 549 (21 September 2017), 357–359 (abstract).