Centauri Dreams

Luhman 16AB (otherwise known as WISE J104915.57-531906) holds out quite an allure for those of us hoping to see future exploratory missions to nearby interstellar space. As recounted here in December (see Possible Planet in Nearby Brown Dwarf System), the European Southern Observatory’s Henri Boffin has found that this brown dwarf pair is likely home to a previously undetected companion. Bear in mind that we know of no closer brown dwarf; indeed, Luhman 16AB is no more than 6.6 light years out, making it the third closest system to our Sun after Barnard’s Star and, of course, Alpha Centauri.

We know very little about this putative companion other than Boffin’s estimate that its likely mass is between a few Jupiter masses and perhaps as many as 30, but the good news is that the high end of this mass range should offer us an object that can be detected by adaptive optics, given the size of the apparent separation. In any case, radial velocity methods should work nicely here as well, given that as seen from Earth, the brown dwarf pairing appears nearly edge-on.

Now comes further news from the European Southern Observatory, with the announcement that one of the two brown dwarfs, Luhman 16B (WISE J104915.57-531906.1B) shows variations in brightness as it rotates, giving us strong hints of detectable features in its atmosphere. Using ESO’s Very Large Telescope, a team of researchers has mapped out the light and dark areas on Luhman 16B, observing the brown dwarf pair with the CRIRES instrument on the VLT (CRIRES is a high-resolution infrared spectrograph). The resulting ‘map’ appears below.

Image: ESO’s Very Large Telescope has been used to create the first ever map of the weather on the surface of the nearest brown dwarf to Earth. An international team has made a chart of the dark and light features on WISE J104915.57-531906.1B, which is informally known as Luhman 16B and is one of two recently discovered brown dwarfs forming a pair only six light-years from the Sun. The figure shows the object at six equally spaced times as it rotates once on its axis. Credit: ESO/I. Crossfield.

Ian Crossfield (Max Planck Institute for Astronomy) is lead author of the paper on this work, which will appear in Nature:

“Previous observations suggested that brown dwarfs might have mottled surfaces, but now we can actually map them. Soon, we will be able to watch cloud patterns form, evolve, and dissipate on this brown dwarf — eventually, exometeorologists may be able to predict whether a visitor to Luhman 16B could expect clear or cloudy skies.”

‘Exometeorologist’ — yet another new job description spawned by this golden age of exoplanetary exploration!

You can see where this is heading. As we learn more about the weather patterns on brown dwarfs, we are moving toward theoretical models which can be tested against future observations. When next-generation telescopes like the European Extremely Large Telescope come online, we can anticipate eventually creating similar maps for young gas giants. In none of these worlds should we expect clement weather: The clouds Crossfield’s team detected show temperatures in the range of 1100 degrees Celsius and are likely carrying droplets of molten iron and various minerals adrift in an atmosphere made up largely of hydrogen.

Crossfield’s map is complemented by a second study led by Beth Biller (University of Edinburgh), which observed brightness variations at different wavelengths in an attempt to understand what happens in various layers of the atmosphere on both Luhman 16A and 16B. This team drew its data from the 2.2 meter telescope at ESO’s La Silla observatory in Chile. Measurements were made in seven different filter bands, where emissions are correlated with the temperature of the emitting gas. Thus the different wavelengths probably represent layers at different depths within the brown dwarf’s atmosphere, constituting a kind of atmosphere probe.

The resulting paper appears in Astrophysical Journal Letters. According to this MPIA news release, this investigation marks the first simultaneous monitoring of a brown dwarf’s brightness variability in more than two wavelength ranges. Says Biller:

“We’ve learned that the weather patterns on these brown dwarfs are quite complex. The cloud structure of the brown dwarf varies quite strongly as a function of atmospheric depth and cannot be explained with a single layer of clouds.”

So we wind up with brown dwarfs with multiple cloud layers and/or temperature variations as we push ahead with the effort to understand weather patterns in distant solar systems. Keep in mind we are dealing with a place that is only slightly farther from the Earth than Barnard’s Star. If the companion object is eventually confirmed as a planet, Luhman 16AB will have the second closest exoplanet to Earth yet detected (assuming Centauri Bb is also confirmed). It will be an exciting period ahead as we try to learn what other objects may circle the nearest stars.

The mapping of Luhman 16B is described in Crossfield et al., “A Global Cloud Map of the Nearest Known Brown Dwarf,” about to appear in Nature (30 January 2014). See this ESO news release for more on that finding. Beth Biller and team’s work is found in Biller et al., “Weather on the Nearest Brown Dwarfs: Resolved Simultaneous Multi-Wavelength Variability Monitoring of WISE J104915.57-531906.1AB” in Astrophysical Journal Letters, Volume 778, Issue 1, L10.

Friday’s look at habitable zones, and the possibilities of life below the surface or in the atmosphere of an exoplanet, segues naturally into the fascinating notion of ‘superhabitable’ worlds. René Heller (McMaster University) and John Armstrong (Weber State University) ponder the possibilities in a recent paper for Astrobiology. What if, the scientists ask, our notions of habitability are too closely crafted to our own anthropocentric viewpoint? Could there be planets that are actually more habitable than the Earth? Should the Earth itself be considered, with respect to a broader view of biology, only marginally habitable?

The question has important ramifications for how we approach the search for other habitable worlds. We study extremophilic life forms on Earth and question whether conditions even more bizarre than these could still produce life. But Heller and Armstrong reframe the issue:

The word ‘bizarre’ is here to be understood from an anthropocentric point of view. From a potpourri of habitable worlds that may exist, Earth might well turn out as one that is marginally habitable, eventually bizarre from a biocentric standpoint. In other words, it is not clear why Earth should offer the most suitable regions in the physicochemical parameter space that can be tolerated by living organisms. Such an anthropocentric assumption could mislead research for extrasolar habitable planets because planets could be non-Earth-like but yet offer more suitable conditions for the emergence and evolution of life than Earth did or does; that is, they could be superhabitable.

Image: A super-Earth, as depicted in this artist’s impression of HD 215497 b, could actually be more ‘habitable’ than the Earth, according to Heller and Armstrong’s new work, which re-examines our ideas about the conditions needed for life. Credit: Wikimedia Commons.

Heller and Armstrong stick with liquid water as a prerequisite for life, so their extrapolations are incremental, but the proposal to enter a new word into the lexicon is bold enough. So let’s consider what ‘superhabitability’ might mean. One possibility is that a larger world (though not large enough to cause plate tectonics to cease) could offer more spacious conditions for life’s development. Planets with what the duo call ‘fractionate continents and archipelagos’ should produce a wider diversity of habitats, their spacious shallow waters offering higher biodiversity than Earth’s deep oceans. The paper leans toward dry planets with a lower fractional surface coverage of water, where liquid water is abundant but distributed into many reservoirs.

The elements of superhabitability multiply. Plate tectonics allow us to speculate on super-Earths with masses up to about two Earth masses. Beyond this, high pressures in the mantle reduce the likelihood of tectonic activity (the two Earth masses used here is a deliberately conservative figure). Also needed: An intrinsic magnetic field produced by a liquid, rotating, convecting core to protect the planet from high-energy cosmic radiation. And the biodiversity of Earth seems to multiply in periods of warmer conditions, suggesting that a warmer version of Earth might have extended tropical zones and, over the course of aeons, a greater variance in biological forms.

The factors at play in Heller and Armstrong’s speculations make for interesting scenarios. Imagine a solar system with more than a single terrestrial planet or moon in the habitable zone. More even distribution between the Earth and the Moon would have resulted in a double-planet, with both objects habitable. Or imagine switching Mars and Venus, to potentially produce three habitable planets. Some systems, indeed, might be ‘multihabitable,’ with massive moons around gas giants and the possibility of panspermia within the system to spread local biology.

Or ponder moving the Earth, which some recent work describes as being at the very inner edge of the Sun’s habitable zone. A terrestrial world located closer to the center of the HZ could be considered superhabitable because it would be more resistant to runaway greenhouse states than the Earth is. Or perhaps age is itself a condition for superhabitability. Heller and Armstrong make the case that the Earth experienced greater biodiversity as it aged — thus the introduction of oxygen about 2.5 billion years ago (from oceanic algae) led to greater habitability on the surface, allowing life to move onto the continents, an increase in planetary habitability.

And what of the star the planet orbits? The authors see K stars as offering a compromise between initial and long-term high-energy radiation, making them favorable hosts for superhabitable worlds. From the paper:

Higher biodiversity made Earth more habitable in the long term. If this is a general feature of inhabited planets, that is to say, that planets tend to become more habitable once they are inhabited, a host star slightly less massive than the Sun should be favorable for superhabitability. These so-called K-dwarf stars have lifetimes that are longer than the age of the Universe. Consequently, if they are much older than the Sun, then life has had more time to emerge on their potentially habitable planets and moons, and — once occurred — it would have had more time to ‘tune’ its ecosystem to make it even more habitable.

That, of course, gets us to the K-class star we have so often discussed in these pages, Alpha Centauri B. We already have one planetary candidate around it, an Earth-mass planet in a 3.235-day orbit. In terms of age, recent work based on asteroseismology, chromospheric activity and other means shows it to be a bit older than the Sun, with estimates ranging from 4.85 billion years up to about 6.5 billion. The authors note that a planet in the habitable zone around this star, collecting water from objects beyond the snowline, could have had primitive life forms when the Earth had just collided with the Mars-sized object that would be responsible for the Moon.

Image: Artist’s impression of Alpha Centauri B, with Centauri A in the distance and the planet candidate Centauri B b also marked. Credit: Adrian Mann.

Heller and Armstrong note that our exoplanet studies have shown us that the Solar System is anything but typical when it comes to planetary systems, so it may well be that the Earth is itself atypical when it comes to inhabited worlds. In that sense — and they make this point specifically — their work refutes the Ward and Brownlee ‘rare Earth hypothesis’ that saw life as an extremely unlikely phenomenon emerging from a wide range of precise conditions. Perhaps what Ward and Brownlee are seeing, says this new work, is the emergence of an only marginally habitable world. If so, our search for inhabited planets should take in worlds slightly older and slightly more massive than our own, preferably those orbiting K-class stars like the close-by Centauri B.

Can we create a biocentric model to use in our search for habitable worlds that replaces our anthropocentric expectations? This paper makes a spirited run at the idea. The paper is Heller and Armstrong, “Superhabitable Worlds,” Astrobiology Vol. 14, No. 1 (2014), available online. The work is thick with ideas, and I didn’t have time today to go into the ‘terrestrial menagerie’ discussed in its first part, which shows not only how planets in the habitable zone can be rendered uninhabitable but how exomoons beyond the HZ may be rendered life-bearing. I suspect, though, that we’ll be coming back to these arguments again soon. Thanks to the numerous readers who forwarded links to this paper.

I’m a great believer in what I might call the ‘conventional’ habitable zone; i.e., a habitable zone defined by the possibility of liquid water on the surface. The definition is offered not to exclude exotic possibilities like micro-organisms floating in the clouds of Venus or aquatic life deep inside an ice-covered moon like Europa. Rather, it acknowledges that finding life is hard enough without losing our focus. In terms of exoplanets and feasible near-term study, a warm planet with liquid water — the kind we live on — would command our immediate attention.

But as we look at much broader issues of how life forms, we may indeed learn that our kind of life is but one component of a vast continuum, as recent work out of the University of Aberdeen reminds us. In a new paper published in Planetary and Space Science, researchers tackle the question of life living deep underground. Now the habitable zone starts to broaden, because things get warmer as we go deep.

We know of life here on Earth that exists more than five kilometers below the surface, and given the difficulty of probing these regions, we probably have fragmentary knowledge at best of what’s down there at deeper levels. So if we’re talking underground microorganisms, maybe a place like Gliese 581 d, evidently just past the outer edge of its star’s habitable zone in terms of liquid water, would still qualify. The Aberdeen team thinks conditions less than two kilometers below the surface there could be clement.

Image: New studies are examining habitable conditions below planetary surfaces, where liquid water might exist. Internal heat might keep even a ‘rogue’ planet moving without a parent star capable of hosting some kind of life underground. This image is an artist’s impression of the rocky super-Earth HD 85512 b. Credit: European Southern Observatory.

The Aberdeen work revolves around a computer model that estimates the temperature below the surface of planets of varying sizes at varying distances from their star. Doctoral student Sean McMahon comments on the implications in this Aberdeen news release:

“Using our computer model we discovered that the habitable zone for an Earth-like planet orbiting a sun-like star is about three times bigger if we include the top five kilometres below the planet surface. The model shows that liquid water, and as such life, could survive 5km below the Earth’s surface even if the Earth was three times further away from the sun than it is just now. The results suggest life may occur much more commonly deep within planets and moons than on their surfaces. If we go deeper, and consider the top 10 km below the Earth’s surface, then the habitable zone for an Earth-like planet is 14 times wider.”

An extension of the habitable zone indeed, taking us well past the orbit of Saturn in our own Solar System. And ponder the case of a rogue ‘super-Earth,’ a planet four or five times more massive than the Earth that has for whatever reason been ejected into the interstellar deep. We’ve talked about such places before and their prospects for generating enough internal heat to make some kind of life possible even in the absence of a parent star. And when you start extending habitable zones this far, a cosmos teeming with at least primitive life seems possible.

Since we’re pushing habitable zones in new directions, let me also mention Craig Stark (University of St. Andrews), whose work has focused on alien atmospheres extending from exoplanets to brown dwarfs. Stark argues that prebiotic molecules can form in these environments, saying “The atmospheres around exoplanets and brown dwarfs form exotic clouds that, instead of being composed of water droplets, are made of dust particles made of minerals.” Add a dose of lightning and you have charged particles and interesting possibilities:

“These charged gases are called plasmas – like those found in fluorescent lights and plasma televisions. The dust can find itself immersed in the charged gases and the charged particles stick to the dust making the dust charged. The charged dust attracts onto its surface other charges from the surrounding plasma helping grow molecules on the dust surface.”

The precursors to life in the atmosphere of a brown dwarf? Now we’re really pushing the definition of ‘habitable zone.’ The Stark paper is “Electrostatic activation of prebiotic chemistry in substellar atmospheres,” accepted at The International Journal of Astrobiology (preprint). The McMahon paper is “Circumstellar habitable zones for deep terrestrial biospheres,” Planetary and Space Science Vol. 85 (September, 2013), pp. 312-318 (abstract).