by Paul Gilster | Dec 2, 2010 | Astrobiology and SETI |
As if it were news, one thing the great flap over astrobiology and yesterday afternoon’s NASA news conference tells us is that anything smacking of extraterrestrial life brings over the top commentary long before the findings are officially discussed, as should be clear from some of the Internet blogging about the GFAJ-1 bacterium found in Mono Lake. And what a shame. Despite the astrobiology teaser, GFAJ-1 does not in itself tell us anything about alien life and does not necessarily represent a ‘shadow biosphere,’ a second startup of life on Earth that indicates life launches in any available niche. But the find is remarkable in its own right.
Let’s leave the astrobiology aside for the moment and simply focus on the fact that life is fantastically adaptable in terms of biochemistry, and can pull off surprises at every turn. That’s always a result worth trumpeting, even if it leaves the wilder press speculations in the dust. After all, it’s long been assumed that the six elements that underlay the basic chemistry of life are carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. Despite persistent speculation, few thought life could exist without them.
Now we learn that the GFAJ-1 bacterium found in eastern California’s Mono Lake can, at least in the conditions of a fascinating experiment, use arsenic in its metabolism rather than being poisoned by it. Arsenic occurs in the lake in one of the highest concentrations of any site in the world. Let me quote the ever reliable Caleb Scharf (Columbia University) on arsenic and its role:
Arsenic is an insidious element. With 5 outer valence electrons the arsenic atom is chemically similar to the biologically critical element phosphorus, but only in crude terms. Life depends extensively on phosphorus – it helps form the molecular backbone of DNA, it is part of molecules like Adenosine triphosphate (ATP) that serves as a vital rechargeable chemical battery within all living cells, as well as many other biologically vital roles. Arsenic on the other hand can weasel its way in, waving its valence electrons in a come-hither fashion, and getting the best seat in the house. The problem is that once an organism takes in arsenic, replacing some of its phosphorus, it typically begins to malfunction – arsenic is is a fatter atom and biochemistry is a sensitive thing. There is good reason why arsenic has long been a poison of choice for nefarious human dealings.
Indeed. GFAJ-1 is intriguing because rather than just being tolerant of a toxin like arsenic, it’s actually able to use it. The team working under Felisa Wolfe-Simon (USGS) reported online today in Science that phosphorus is here replaced by arsenic, a case of an alternate building block for life of the kind long speculated about by science fiction writers. This from Nature:
Arsenic is positioned just below phosphorus in the periodic table, and the two elements can play a similar role in chemical reactions. For example, the arsenate ion, AsO43-, has the same tetrahedral structure and bonding sites as phosphate. It is so similar that it can get inside cells by hijacking phosphate’s transport mechanism, contributing to arsenic’s high toxicity to most organisms.
The team proceeded by collecting mud from the lake and adding samples to a salt medium that was high in arsenate, then diluting the material to wash out remaining phosphate. Steeping the cells in arsenic, the scientists discovered an organism that seemed to grow well under these conditions, even after multiple generations since their first collection more than a year ago. What phosphorus was available to the bacteria was present only in traces from the original colony of cells, and in impurities found in the growth medium. Again from Nature:
When the researchers added radio-labelled arsenate to the solution to track its distribution, they found that arsenic was present in the cellular fractions containing the bacterium’s proteins, lipids and metabolites such as ATP and glucose, as well as in the nucleic acids that made up its DNA and RNA. The amounts of arsenate detected were similar to those expected of phosphate in normal cell biochemistry, suggesting that the compound was being used in the same way by the cell.
Can these bacteria replace phosphate with arsenic naturally? Wolfe-Simon herself says thirty years of work remain to figure out exactly what’s going on, a comment on the preliminary nature of this work, which remains controversial in some quarters and is in obvious need of extensive follow-up. No shadow biosphere yet, but obviously the quest is ongoing because of its implications, and we’ve now received one very tantalizing piece of evidence that such things may be possible.
If life really did start here more than once — a finding that is not remotely demonstrated by this work — then we can talk about how likely it will have done the same thing on distant planets, upping the chances that we live in a universe where life emerges whenever given the chance. But we’re hardly there yet, as became evident in the exchanges between Wolfe-Simon and Steven Benner (Foundation for Applied Molecular Evolution) at the news conference. British science writer Ed Yong fleshes out some of the reasons for skepticism:
It’s an amazing result, but even here, there is room for doubt. As mentioned, Wolfe-Simon still found a smidgen of phosphorus in the bacteria by the end of the experiment. The levels were so low that the bacteria shouldn’t have been able to grow but it’s still not clear how important this phosphorus fraction is. Would the bacteria have genuinely been able to survive if there was no phosphorus at all?
Nor is it clear if the arsenic-based molecules are part of the bacteria’s natural portfolio. Bear in mind that Wolfe-Simon cultured these extreme microbes using ever-increasing levels of arsenic. In doing so, she might have artificially selected for bacteria that can use arsenic in place of phosphorus, causing the denizens of Mono Lake to evolve new abilities (or overplay existing ones) under the extreme conditions of the experiment.
And I should also return to Caleb Scharf, who notes that while phosphorus is relatively rare — in terms of cosmic abundance — compared to other major bio-chemically important elements, it is a thousand times more abundant than arsenic, which is little more than a trace by comparison. So much for the idea of entire biospheres crowded with life forms drawing on the stuff, at least in terms of the odds. The GFAJ-1 experiments make for a fascinating story, one that was upstaged by a media circus but remains notable news for all that. Paul Davies, one of the authors of the paper, calls this work ‘the beginning of what promises to be a whole new field of microbiology.’
But again, note the caveat, as Davies explains:
“This organism has dual capability. It can grow with either phosphorous or arsenic. That makes it very peculiar, though it falls short of being some form of truly ‘alien’ life belonging to a different tree of life with a separate origin. However, GFAJ-1 may be a pointer to even weirder organisms. The holy grail would be a microbe that contained no phosphorus at all.”
The paper is Wolfe-Simon et al., “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus,” published online by Science (2 December 2010). This article in Astrobiology Magazine provides an excellent backgrounder on the Wolfe-Simon team’s methods. Wolfe-Simon’s own Web site is impressive and well worth checking re her ongoing work.
by Paul Gilster | Dec 2, 2010 | Exoplanetary Science |
There’s so much we still don’t know about GJ 1214b. What we do know is this: The planet is 2.7 times the size of Earth and about 6.5 times as massive, orbiting its star at a distance of 0.014 AU. That’s far too close to the primary to make this a habitable world, but a planet hardly has to be habitable to be interesting, and GJ 1214b becomes interesting indeed now that Jacob Bean (Harvard-Smithsonian Center for Astrophysics) has announced the first analysis of its atmosphere. Until now, we’ve been looking at exoplanet atmospheres only around larger worlds.
Super-Earths are generally considered to range from two to ten Earth masses. To study this one, the researchers used the Very Large Telescope at Paranal Observatory (Chile) to examine the near-infrared (780 to 1000 nanometers) region of the spectrum. More work at different wavelengths will be needed to tease out GJ 1214b’s secrets, but the progression in atmospheric observations to smaller planets is satisfying to see, as David Charbonneau (CfA) notes:
“In less than 10 years, we’ve gone from studying the atmospheres of alien worlds like Jupiter, to Neptunes, to super-Earths. Earth-sized worlds are next, although they’ll be the most difficult.”
That last goes under the category of heroic understatement, but there’s no question that studying the spectrum of a true Earth analog is one of the great goals of exoplanetary science, one that may tell us whether or not life exists on the world under study. Until then, we can take heart from this first analysis of a super-Earth atmosphere, which turned up a basically featureless spectrum. That ruled out a cloud-free atmosphere dominated by hydrogen, but it leaves open a hydrogen-rich atmosphere cloaked by a thick blanket of clouds or haze.
Image: The extrasolar planet GJ 1214b, shown here in an artist’s conception with two hypothetical moons, orbits a red dwarf star 40 light-years from Earth. Astronomers have confirmed that the planet has a thick atmosphere, but can’t yet determine whether the atmosphere is primarily hydrogen or a steamy soup of water vapor. Credit: David A. Aguilar (CfA).
A possible alternative: A dense and steamy atmosphere loaded with water vapor. Says Bean:
“A steamy atmosphere would have to be very dense — about one-fifth water vapor by volume — compared to our Earth, with an atmosphere that’s four-fifths nitrogen and one-fifth oxygen with only a touch of water vapor. During the next year, we should have some solid answers about what this planet is truly like.”
The CfA team looks to have produced rock-solid work — Drake Deming (NASA GSFC) is quoted in Nature as saying he was ‘stunned by the quality of their data.’ Further observations in the mid- or far-infrared are needed to differentiate between the two models, which means this exciting research leaves open as many questions as it answers. Bean says the planet is ‘veiling its true nature from us,’ an accurate assessment that only energizes the search for more data.
We’ve found gases like hydrogen and sodium vapor in the atmospheres of ‘hot Jupiters’ by studying transits as the planet passes between the star and Earth, allowing us to see at what wavelengths the star’s light is absorbed. The resulting chemical signature lets us break down the atmosphere of a planet we cannot directly see. GJ 1214b, which transits its star for fifty minutes out of every 38-hour orbit, has now opened a passage into super-Earth atmosphere studies that will soon be well traveled.
The paper is Bean et al., “A ground-based transmission spectrum of the super-Earth exoplanet GJ 1214b,” Nature 468 (2 December 2010), pp. 669-672 (abstract).
by Paul Gilster | Dec 1, 2010 | Astrobiology and SETI |
I never have trouble finding topics to discuss on Centauri Dreams, but this morning’s take was unusually bountiful. For the past several days I’ve had two embargoed stories to choose from, both going public this PM. Do I write about tripling the number of stars in the universe, or do I choose the first analysis of a ‘super-Earth’ atmosphere? It’s a tough choice, but I’m going with the stars, given that the story relates to what I consider the most fascinating venue for astrobiology, planets around red dwarfs. We’ll do the super-Earth atmosphere — fascinating in its own right — tomorrow.
The story comes out of Yale University, whose Pieter van Dokkum led the research using telescopes at the Keck Observatory in Hawaii. We’ve long known that because of their faintness and small size, getting a handle on the red dwarf population was problematic. Usually, I’ve seen a figure around 75 percent cited for the Milky Way, meaning most stars in our galaxy are red dwarfs (the Sun, a G-class object, turns out to be representative of only about seven percent of main sequence stars, meaning we live around a star that is not typical). And because red dwarfs are so abundant, the consequences for astrobiology are obvious if we determine habitable planets can orbit them.
The Yale team has looked at red dwarfs not in our galaxy but in eight relatively nearby elliptical galaxies, located between 50 million and 300 million light years away. What they discovered is that there are about twenty times more red dwarfs in these elliptical galaxies than in the Milky Way. Says van Dokkum:
“No one knew how many of these stars there were. Different theoretical models predicted a wide range of possibilities, so this answers a longstanding question about just how abundant these stars are.”
Elliptical galaxies make up between ten and fifteen percent of the galaxies in the local universe, and the finding triples our best guess about the total number of stars in the universe, thereby increasing the number of planets we assume to be orbiting these stars. We’ve seen robust planetary systems around stars like Gliese 581 and can assume similar systems exist in the galaxies under observation. The red dwarfs recently discovered are typically more than ten billion years old, giving life plenty of time to gain a foothold. Indeed, van Dokkum talks about ‘possibly trillions of Earths orbiting these stars,’ a notion that gives still more punch to the Fermi paradox.
Image: Galaxies in the cluster Abell S0740, over 450 million light-years away in the direction of the constellation Centaurus. The giant elliptical ESO 325-G004 looms large at the cluster’s center, as massive as 100 billion of our suns. Hubble resolves thousands of globular star clusters orbiting ESO 325-G004. Globular clusters are compact groups of hundreds of thousands of stars that are gravitationally bound together. At the galaxy’s distance they appear as pinpoints of light contained within the diffuse halo. Other fuzzy elliptical galaxies dot the image. Some have evidence of a disk or ring structure that gives them a bow-tie shape. Several spiral galaxies are also present. The starlight in these galaxies is mainly contained in a disk and follows along spiral arms. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).
If a multi-billion year old civilization existed in a typical galaxy, would it have made it all the way to Kardashev Type III status, able to put to use the entire energy resources of its galaxy? If so, wouldn’t we see some sign of its handiwork? Dick Carrigan has been studying such issues for years, looking for Dyson spheres by sifting through Infrared Astronomy Satellite (IRAS) data for objects that radiated in the infrared and carried the signature of a Dyson sphere, in which a star is surrounded by a swarm of energy-catching habitats or even completely enclosed within their shell.
No luck yet, but what Carrigan calls ‘interstellar archaeology’ gets more and more interesting when we consider a tripling of the number of potential life-giving stars in the universe (see Carrigan’s analysis of Kardashev Type II and III signatures and his Dyson sphere methodology here). What kinds of traces would a Type III civilization leave, and would we recognize it if we saw it? We do know that elliptical galaxies are made up of older, low-mass stars and show little star formation activity compared to more active spiral galaxies. Perhaps a lack of discernible Type III activity is telling us that technological civilizations have a relatively short lifetime.
Beyond these blue-sky musings, though, a tripling of the stars in our universe could have an impact on our understanding of how galaxies evolve, forcing us to take account of the mass of a much larger population of red dwarfs than we thought existed, and thus providing a new constraint on dark matter and its effects in relation to elliptical galaxies. That’s a helpful outcome, and it will be fascinating to see how the results of this paper are received and put to use.
The paper is van Dokkum and Conroy, “A substantial population of low-mass stars in luminous elliptical galaxies,” published online in Nature 1 December, 2010 (abstract)
