Time Out

Dave Brubeck’s Time Out album was the first jazz LP I ever bought, just after it came out in 1959, the same year that Miles Davis released Kind of Blue. Watershed moments both. Paul Desmond once said of his alto sax work that he was trying to create the sound of a dry martini, a description I certainly can’t top.

Last night, while listening to Desmond and Brubeck, I realized that the Time Out album would be emblematic for today’s post. For it’s that time of year, and I am indeed taking time out for a much needed break. Centauri Dreams will be back in the first week of August, but until then, my break will include a good bit of jazz, much catch-up reading, a lot of long walks and, perhaps, a few of those martinis Desmond talks about. I’ll keep an eye on the site to handle comment moderation as well. Meanwhile, I hope all of you are having a splendid summer.


Unusual Companion for a Brown Dwarf Binary

A cluster of stars sharing a common origin, now gravitationally unbound, is referred to as a stellar association. I’ve written before about how useful some of these groupings can be. In the form of so-called moving groups — a stellar association that is still somewhat coherent — they help us identify stars of similar age, an aid as we discover new objects. Now we have word of an object called 2MASS 0249 c, found in the Beta Pictoris moving group, that has striking similarities to the most famous member of that group, Beta Pictoris b.

2MASS 0249 c, like Beta Pictoris b, was found by direct imaging, meaning we’re actually looking at the object under discussion in the image below. The two objects are all but identical in mass, brightness and spectrum. Images from the Canada-France-Hawaii Telescope (CFHT) showed an object moving at a large distance from its host, which turned out to be a pair of closely spaced brown dwarfs.

Follow-up observations with the Keck instrument allowed that determination, while spectroscopy at the NASA Infrared Telescope Facility and the Astrophysical Research Consortium 3.5-meter Telescope at Apache Point Telescope completed the data.

“To date, exoplanets found by direct imaging have basically been individuals, each distinct from the other in their appearance and age. Finding two exoplanets with almost identical appearances and yet having formed so differently opens a new window for understanding these objects,” said Michael Liu, astronomer at the University of Hawai`i Institute for Astronomy, and a collaborator on this work.

Image: Image of the 2MASS 0249 system taken with Canada-France-Hawaii Telescope’s infrared camera WIRCam. 2MASS 0249 c is located 2000 astronomical units from its host brown dwarfs, which are unresolved in this image. The area of sky covered by this image is approximately one thousandth the area of the full moon. Credits: T. Dupuy, M. Liu.

The difference in formation scenarios that Liu talks about is instructive. Here we have two host stars, one of them being a binary system of brown dwarfs, that formed in the same stellar nursery, as per their membership in the moving group. But while Beta Pictoris b, a gas giant of about 13 Jupiter masses, orbits a star 10 times brighter than the Sun, 2MASS 0249 c orbits a brown dwarf pair 2000 times fainter. At a distance of 9 AU, Beta Pictoris b is relatively close to its star, while 2MASS 0249 c is a whopping 2000 AU from its brown dwarf hosts.

These distances imply different formation scenarios. Beta Pictoris b likely formed from the accumulation of dust grains in the circumstellar disk surrounding its star. 2MASS 0249 c could not have done so, given that the two brown dwarfs it orbits would not have had sufficient disk material to produce it. That implies the new planet took form through the gravitational collapse of a gas cloud found in the original stellar birth cluster.

“2MASS 0249 c and beta Pictoris b show us that nature has more than one way to make very similar looking exoplanets,” says Kaitlin Kratter, astronomer at the University of Arizona and a collaborator on this work. “They’re both considered exoplanets, but 2MASS 0249 c illustrates that such a simple classification can obscure a complicated reality.”

Image: The infrared spectra of 2MASS 0249 c (top) and beta Pictoris b (bottom) are similar, as expected for two objects of comparable mass that formed in the same stellar nursery. Unlike 2MASS 0249 c, beta Pictoris b orbits much closer to its massive host star and is embedded in a bright circumstellar disk. Credit: T. Dupuy, ESO/A.-M. Lagrange et al.

What we get out of all this is opportunity. The formation of gas giants is a key phase in the emergence of new planetary systems, and being able to use direct imaging to study such worlds means we can probe their atmospheres directly, examining the composition, surface temperature, chemistry and other physical properties of the exoplanet. Moreover, direct imaging is most effective when working with planets far from their star, as this object is. If we are after insights into gas giant formation, the different formation pathways for Beta Pictoris b and 2MASS 0249 c may provide evidence both orbital and spectral. The paper notes:

As directly imaged objects, ? Pic b and 2MASS J0249?0557 c provide a new opportunity to test atmospheric compositions and angular momentum evolution for a close-in planet and a very wide companion that share a common mass and age and that formed from the same material.

And on the issue of spectra:

If different formation mechanisms produced these objects, then their spectra could contain evidence of their divergent pasts. As noted above, we suspect that 2MASS J0249?0557 c arose from a star-formation-like process of global, top-down gravitational collapse in the same way as the freefloating object 2MASS J2208+2921. On the other hand, ? Pic b bears architectural resemblance to planetary systems and thus may have formed via core accretion. Core accretion models and observations of solar system gas giants show substantial metal enrichment (e.g., Stevenson 1982; Bolton et al. 2017). Thus, if ? Pic b is a scaled-up gas giant (?13 MJup), then we may expect to see substantial metal enrichment in its atmosphere.

What an interesting find 2MASS 0249c turns out to be. Is it a planet or actually a brown dwarf? One thing is for sure: The discovery puts a spotlight on the boundary between planet and brown dwarf, both in terms of composition and in terms of formation history. Maybe it’s best to fall back on how today’s paper describes the object: a ‘planetary-mass companion.’ Now we can go to work, via direct imaging, on issues like variability, rotation and atmospheric composition. How these data vary across different formation histories should occupy astronomers for some time.

The paper is Dupuy et al., “The Hawaii Infrared Parallax Program. III. 2MASS J0249-0557 c: A Wide Planetary-mass Companion to a Low-mass Binary in the beta Pic Moving Group,” accepted at the Astronomical Journal (preprint).


An Unusually Interesting Asteroid

We learned late last week that the near-Earth asteroid 2017 YE5, discovered just last December, is what is described as an ‘equal mass’ binary. This would make it the fourth near-Earth asteroid binary ever detected in which the two objects are nearly identical in size, both about 900 meters. The binary’s closest approach to Earth was on June 21, 2017, when it came to within 6 million kilometers, some 16 times the distance between the Earth and the Moon. It won’t be that close again for at least another 170 years.

Image: Artist’s concept of what binary asteroid 2017 YE5 might look like. The two objects show striking differences in radar reflectivity, which could indicate that they have different surface properties. Credit: NASA/JPL-Caltech.

What you have above is an artist’s impression of how 2017 YE5 appears, but have a look at the radar imagery below. This comes from NASA’s Goldstone Solar System Radar (GSSR, observations conducted on June 23, 2018), and shows the presence of two lobes. We don’t yet see a binary, but these radar images were enough for Goldstone scientists to alert astronomers at Arecibo Observatory, who had already inserted 2017 YE5 into their observing list.

Image: Radar images of the binary asteroid 2017 YE5 from NASA’s Goldstone Solar System Radar (GSSR). The observations, conducted on June 23, 2018, show two lobes, but do not yet show two separate objects. Credit: NASA/JPL-Caltech/GSSR.

Working with researchers at the Green Bank Observatory in West Virginia, the Arecibo scientists linked the two observatories in a bi-static radar configuration, meaning that Arecibo transmits the radar signal while Green Bank receives the return signal. It was the combination of data from the two observatories that allowed 2017 YE5 to be confirmed as two separated objects.

Image: Bi-static radar images of the binary asteroid 2017 YE5 from the Arecibo Observatory and the Green Bank Observatory on June 25. The observations show that the asteroid consists of two separate objects in orbit around each other. Credit: Arecibo/GBO/NSF/NASA/JPL-Caltech.

A surprising number of near-Earth asteroids may be binaries, according to this JPL news release, which tells us that among near-Earth asteroids larger than 200 meters in size, about 15 percent are binaries with one larger object and a much smaller asteroid satellite. While equal-mass binaries are apparently rare, contact binaries (two equally sized objects in contact with each other) make up another 15 percent of the population in this size range.

Thus at 2017 YE5 we have two objects that revolve around each other every 20 to 24 hours, as confirmed through brightness variations at visible light wavelengths at the Center for Solar System Studies in Rancho Cucamonga, California. As to composition, the two components do not reflect as much sunlight as a typical rocky asteroid, making it likely that 2017 YE5 has a surface as dark as charcoal. Differences in reflectivity of the two objects suggest that they have different composition at the surface or perhaps different surface features.

I was startled to learn that more than 50 binary asteroid systems have turned up in radar studies since 2000, with the majority consisting of one large object and a much smaller satellite. The differences in radar reflectivity found at 2017 YE5 have not appeared in this population. That makes this binary a useful system for the study of binary formation. Further study of combined radar and optical observations may allow tighter constraints on the density of the 2017 YE5 objects, which should give us a window into their composition and structure.


Ross 128b: Analyzing a Planet by the Light of its Star

Red dwarfs have a lot of things going for them when it comes to finding possibly habitable planets. A planet of Earth size in the HZ will produce a substantial transit signal because of the small size of the star (‘transit depth’ refers to the amount of the star’s light that is blocked by the planet), and the tight orbit the planet must follow increases the geometric probability of observing a transit. But planets that do not transit are also more readily detected because of the large size of the planet compared to the star, gravitational interactions producing a strong radial velocity signature, which is what we have in the case of Ross 128b.

About 11 light years from Earth, the planet was culled out of more than a decade of radial velocity data in 2017 using the European Southern Observatory’s HARPS spectrograph (High Accuracy Radial velocity Planet Searcher) at the La Silla Observatory in Chile. The location of the planet near the inner edge of its star’s habitable zone excited interest, as did the fact that Ross 128 is much less subject to flares of ultraviolet and X-ray radiation than our nearest neighbor, Proxima Centauri, which also hosts a planet in a potentially habitable orbit.

Image: Artist’s impression of the exoplanet Ross 128b. Credit: ESO.

What we know about Ross 128b is that it orbits 20 times closer to its star than the Earth orbits the Sun, but receives only 1.38 times more irradiation than the Earth, with an equilibrium temperature estimated anywhere between -60 degrees Celsius and 20°C, the host star being small and relatively cool. But bear in mind that what we get from radial velocity is a minimum mass, because we don’t know at what angle this system presents itself in our sky. Now a team led by Diogo Souto (Observatório Nacional, Brazil) is attempting to deduce more about the planet’s composition using an unusual method: Analyzing the composition of the host star.

If we learn the chemical abundances found in the star Ross 128, the thinking goes, we should be able to make reasonable estimates about the composition of any planets that orbit it. Souto and team are presenting new techniques for making these measurements, using data from the Sloan Digital Sky Survey’s APOGEE spectroscope. Measuring the star’s near-infrared light, where Ross 128 shines the brightest, the researchers have been able to derive abundances for carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium and iron.

“The ability of APOGEE to measure near-infrared light, where Ross 128 is brightest, was key for this study,” says co-author Johanna Teske (Carnegie Institution for Science). “It allowed us to address some fundamental questions about Ross 128 b’s `Earth-like-ness.’”

APOGEE is the Apache Point Galactic Evolution Experiment, an investigation using high-resolution spectroscopy to probe the dust that obscures the inner Milky Way. The project surveyed 100,000 red giant stars across the galactic bulge, but also observed M-dwarfs in the neighborhood of the Sun as a secondary study. Tightening up our knowledge of stellar parameters, the paper notes, offers an indirect route to studying exoplanet composition.

The assumption in this work is that the chemistry of a host star influences the contents of the disk from which planets form around it, which in turn affects the interior structure of any planet. Thus we can hope to tell from the amount of magnesium, iron and silicon available something about the exoplanet. This is the first detailed abundance analysis for Ross 128, and it shows that the star has iron levels similar to the Sun. The silicon level could not be measured, but the ratio of iron to magnesium points to a large core for the planet, larger than Earth’s.

Souto and team believe that knowledge of Ross 128b’s minimum mass (from the radial velocity data), coupled with their data on stellar abundances, can provide a broad estimate of the planet’s radius, a key factor because it would allow a calculation of its density. From the paper:

While both mass and radius are not available for Ross 128b, we can estimate its radius given its observed minimum mass and assuming the stellar composition of the host star is a proxy for that of the planet. We calculate the range of radii possible for Ross 128b using the ExoPlex software package (Unterborn et al. 2018) for all masses above the minimum mass of Ross 128b (1.35M?; Bonfils et al. 2017). Models were run assuming a two-layer model with a liquid core and silicate mantle (no atmosphere). We increase the input mass until a likely radius of 1.5R? was achieved, roughly the point where planets are not expected be gas-rich mini-Neptunes as opposed to rock and iron-dominated super-Earths…

Measurements of the temperature of Ross 128 coupled with the estimated radius of the exoplanet and its inferred composition allow the team to calculate Ross 128b’s albedo, the amount of light reflecting off its surface. These estimates allow the possibility of a temperate climate, taking into account the insolation flux (energy received from the host star) and equilibrium temperature. “Our results,” the authors write, “support the claim of Bonfils et al. (2017) that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.”

But the paper urges caution in the interpretation:

However, this is not to say that Ross 128b is a “Exo-Earth.” Geologic factors unexplored in Bonfils et al. (2017) such as the planet’s likelihood to produce continental crust, the weathering rates of key nutrients into ocean basins or the presence of a long-term magnetic field could produce a planet decidedly not at all “Earth-like” or habitable due to differences in its composition and thermal history. Furthermore, other aspects of the M-dwarf’s stellar activity and its effect on the retention of any atmosphere and potential habitability should be studied, although we find no evidence of activity in the Ross 128 spectra.

Indeed. The number of variables affecting ‘habitability’ is striking. So let’s say this: We have a planet for which mass-radius modeling based on the composition of its host star indicates a mixture of rock and iron, the relative amounts of each being set by the ratio between iron and magnesium. The derived values for insolation and equilibrium temperature are not inconsistent with previous studies indicating a temperate planet at the inner edge of its star’s habitable zone.

The work hinges on modeling of an exoplanet based on a deeper analysis of its host star than has previously been available for an M-dwarf. Tuning up such modeling will demand further data, in particular applying these methods to the host stars of transiting worlds (think TRAPPIST-1) to test their accuracy and reliability in characterizing planets we cannot see.

The paper is Souto et al., “Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra,” Astrophysical Journal Letters Vol. 860, No. 1 (13 June 2018). Abstract / preprint.


Pluto Maps Inspire Thoughts of Bradbury

Something happens when we start making maps of hitherto unknown terrain. A sense of familiarity begins to settle in, a pre- and post-visit linearity, even when the landscape is billions of miles away. To put a name on a place and put that name on a map is a focusing that turns a bleary imagined place into a surface of mountains and valleys, a place that from now on will carry a human perspective. It can’t be undone; a kind of wave function has already collapsed.

And what place more remote than Pluto? At the dwarf planet’s Tenzing Montes, we find striking peaks, some of them running up to 6 kilometers in height, and all this on a world that, until 2015, we weren’t sure even had mountains. Certainly we weren’t expecting mountains this tall, or a terrain this rugged. Given how many years may pass before we have another chance to visit Pluto/Charon, these first official validated topographic maps of the dwarf planet and its moon, just released, will carry our science — and our imaginations — for a long time to come.

Image: Perspective view of Pluto’s highest mountains, Tenzing Montes, along the western margins of Sputnik Planitia, which rise 3-6 kilometers above the smooth nitrogen-ice plains in the foreground. The mounded area behind the mountains at upper left is the Wright Mons edifice interpreted to be a volcanic feature composed of ices. Area shown is approximately 500 kilometers across. Image credit: Lunar and Planetary Institute/Paul Schenk.

The maps are the work of New Horizons researchers led by Paul Schenk (Lunar and Planetary Institute). The team examined all the images from New Horizons’ Long Range Reconnaissance Imager (LORRI) and Multispectral Visible Imaging Camera (MVIC) systems as the raw material for their mosaics. They aligned surface images where they overlapped and performed digital analysis of stereo images both cameras acquired, producing topographic maps for each region, then assembling these into integrated topographical charts for both Pluto and Charon.

Pluto’s mountains are likely made of water ice, because ices from volatiles like methane and nitrogen would not be strong enough to support such tall features, and as the images show, the steep peaks along the southwestern edge of Sputnik Planitia, itself a frozen sheet of nitrogen, have slopes pushing 40° or more. The topographical maps put large-scale features into perspective and help us see both Pluto’s and Charon’s surfaces in a broader context.

Sputnik Planitia is a good example. Fully 1000 kilometers wide, it contains an ice sheet that averages 2.5 kilometers below Pluto’s mean elevation, which corresponds to sea level on our own planet. The maps also show us that the outer edges of the sheet are an even deeper 3.5 kilometers below mean elevation. These are the lowest known areas on Pluto, a fact that emerges only through study of the stereo images and subsequent elevation maps they spawned. The deep ridge-and-trough system running north to south near the western edge of Sputnik Planitia is more than 3000 kilometers long, evidence for extensive fracturing, as this LPI news release explains. It is the longest known feature on the dwarf planet.

And then there’s Charon. Who would have dreamed in 1978, when astronomer James Christy discovered it, that we would ever have the kind of detail that shows below?

Image: Perspective view of mountain ridges and volcanic plains on Pluto’s large moon Charon. The ridges reach heights of 4 to 5 kilometers above the local surface and are formed when the icy outer crust of Charon fractured into large blocks. The smoother plains to the right are resurfaced by icy flows, possibly composed of ammonia-hydrate lavas that were extruded onto the surface when the older block sank into the interior. Area shown is approximately 250 kilometers across. Image credit: Lunar and Planetary Institute/Paul Schenk.

Maps and Sea Change

In my first paragraph today, I summoned quantum mechanics for inspiration, saying that producing maps created the collapse of a kind of psychological wave function. Adam Alter made the same point a few years back in an article in The New Yorker, where he talked about our association of linguistic labels with the things they denote. The effects can be subtle. Northerly movement, for example, is associated in psychological testing with going uphill, apparently a remnant of the decision of ancient Greek mapmakers to put the northern hemisphere above the southern one.

Alter goes on to speak of what he calls a ‘linguistic Heisenberg principle,” meaning that as soon as you label a concept, you change how people perceive it, and I would assume this goes for landscapes as well. So we’d better choose the place names we put on our maps with care, given the freight they carry in our imaginations. Ray Bradbury knew this as well. His ‘The Naming of Names’ takes Earth colonists on Mars to strange places indeed as they begin to name the places they see.

For in the world of The Martian Chronicles, Mars is a place with a long, long history, and pretty soon the new place names the colonists have chosen begin to morph back into their ancient forms, as spoken by the original inhabitants of the planet. Before long not just the names but the people themselves are changing, returning to existences ancient, rich and strange:

The nights were full of wind that blew down the empty moonlit sea-meadows past the little white chess cities lying for their twelve-thousandth year in the shallows. In the Earthmen’s settlement, the Bittering house shook with a feeling of change.

Lying abed, Mr.Bittering felt his bones shifted, shaped, melted like gold. His wife, lying beside him, was dark from many sunny afternoons. Dark she was, and golden, burnt almost black by the sun, sleeping, and the children metallic in their beds, and the wind roaring forlorn and changing through the old peach trees, violet grass, shaking out green rose petals.

It’s a great tale, and one worth re-reading any time mapping new landscapes comes to mind.

The papers are Schenk et al., “Basins, fractures and volcanoes: Global cartography and topography of Pluto from New Horizons,” Icarus Vol. 314 (1 November 2018). abstract; and Schenk et al., “Breaking up is hard to do: Global cartography and topography of Pluto’s mid-sized icy Moon Charon from New Horizons,” Icarus Vol. 315 (15 November 2018). Abstract. Adam Alter’s “The Power of Names” appeared in The New Yorker‘s May 29, 2013 issue.