by Paul Gilster | Apr 22, 2011 | Exoplanetary Science |
Although we’re finding more and more exoplanets, we can always use another technique besides radial velocity, transit searches, direct imaging and microlensing. And now Jonathan Nichols (University of Leicester) has proposed one at the Royal Astronomical Society’s meeting in Llandudno, Wales, which concluded its proceedings yesterday. Nichols has the notion of looking for the radio emissions generated by the aurorae of planets like Jupiter, believing that these could be detected by radar telescopes like the soon to be completed LOFAR.
Now LOFAR (Low Frequency Array) is quite a story in itself, being the largest radio telescope ever constructed. The idea here is to create a vast array of some 7000 small antennae, distributed among some 77 larger stations across the Netherlands, Germany, Great Britain, France and Sweden. You wind up with a total collecting area whose interferometric data can be processed by a supercomputer at the University of Groningen in the Netherlands. The key here is to bring huge new sensitivity to radio frequencies below 250 MHz. In fact, LOFAR will reach down to about 10 MHz.
And that takes me right back to my childhood, working with an old Hallicrafters shortwave receiver. I had read that Jupiter could outshine the Sun at the wavelengths my receiver could detect, and today we know that interactions between Jupiter and Io can provide a power source for the radio emissions that move away from the planet’s magnetic poles in cone-shaped beams. It was possible to pick up oddball noises — people always describe them as staccato sounds like woodpeckers banging on the side of a house, and sometimes slow, swelling sounds that evoke waves rushing in to a shore — and pulling them in on a desktop receiver was a thrill. The article Detecting Jupiter’s Radio Emissions is a good introduction to Jovian radio possibilities.
Image: The Hubble telescope’s view of the rapid, spectacular dance of luminescent gases high in Jupiter’s atmosphere is allowing astronomers to map Jupiter’s immense magnetic field and better understand how it generates such phenomena. The ultraviolet-light images [bottom frames] show how the auroral emissions change in brightness and shape as Jupiter rotates. The aurorae are the bright, circular features at the top and bottom of the planet. The top panel illustrates the effects of emissions from Io, one of Jupiter’s moons. Io ejects an invisible electrical current of charged particles that flow along the planet’s magnetic field lines. Credit: ESA.
But back to Nichols, who told the RAS meeting in Llandudno that the LOFAR antennae will be sufficiently sensitive to detect the kind of emissions Jupiter makes in our own Solar System, even when they occur in systems many light years away:
“This is the first study to predict the radio emissions by exoplanetary systems similar to those we find at Jupiter or Saturn. At both planets, we see radio waves associated with auroras generated by interactions with ionized gas escaping from the volcanic moons, Io and Enceladus. Our study shows that we could detect emissions from radio auroras from Jupiter-like systems orbiting at distances as far out as Pluto,” said Nichols.
This is useful stuff, for we’d like to find more solar systems something like our own, thinking that these might be prime candidates for terrestrial-class worlds in inner orbits. But finding a Jupiter or a Saturn using transit or radial velocity methods would be a long process, given their distance from the central star. Nichols believes that planets orbiting UV-bright stars at distances between 1 and 50 astronomical units would generate enough radio power to be detectable from Earth. In the best case scenario, we should be able to detect such planets 150 light years away.
The paper is Nichols, “Magnetosphere-ionosphere coupling at Jupiter-like exoplanets with internal plasma sources: implications for detectability of auroral radio emissions,” accepted by Monthly Notices of the Royal Astronomical Society (preprint).
by Paul Gilster | Apr 21, 2011 | Outer Solar System
The study of carbon monoxide found in the atmosphere of Pluto — a strong signal rendered in data from the James Clerk Maxwell Telescope in Hawaii — gives us insight into the significant changes happening to the dwarf planet on its 248-year orbital path around the Sun. Pluto is one of a kind, offering us a cold planetary atmosphere that shows marked changes over time, an atmosphere that may at times freeze out and settle to the surface. No other dwarf planet is known to have an atmosphere, and Pluto’s is now known to vary in pressure and probably in composition.
Jane Greaves (University of St. Andrews) led the team doing the new Pluto work, and she’s the first to admit that what the data revealed was a surprise. Pluto reached perihelion in 1989, its closest approach to the Sun, so astronomers assumed that as it began to recede, the atmosphere would contract. But as measured by the occultation of background stars, the atmospheric pressure and size actually increased between 1988 and 2002, even as the surface ice reddened and brightened around the northern pole as it began to experience more sunlight.
We’re looking at a world where the atmosphere and the surface are strongly coupled, with the sublimation of ices pumping gas into the atmosphere. That means that seasonal changes on Pluto drive changes to the gas abundances, and it has been observed that temperatures in the atmosphere (around 100 K) are much higher than at the surface (roughly 40 K). This is a fragile atmosphere, and one that is easy prey to the solar wind, which may be driving some of it into space. Some of these gases may in fact be forming a tail in the manner of a comet.
We’ve found through spectroscopy of surface ice that frozen nitrogen is readily available, and it is assumed that nitrogen dominates the atmosphere, which also includes methane and the aforementioned carbon monoxide. Greaves’ team believes the latter acts as a thermostat, being the strongest coolant. The new data show that an atmosphere previously measured as a hundred kilometers thick now extends to a height of more than 3000 kilometers, and the carbon monoxide signature detected is twice that of an earlier study in 2000. Says Greaves:
“The change in brightness over the last decade is startling. We think the atmosphere may have grown in size, or the carbon monoxide abundance may have been boosted.”
Image: Artist’s impression of Pluto’s huge atmosphere of carbon monoxide. The source of this gas is erratic evaporation from the mottled icy surface of the dwarf planet. The Sun appears at the top, as seen in the ultra-violet radiation that is thought to force some of the dramatic atmospheric changes. Pluto’s largest moon, Charon, is seen at the lower right. Credit: P.A.S. Cruickshank.
The presence of both methane and carbon monoxide in this atmosphere poses interesting issues. We can assume that light from the Sun heats surface ices that in turn evaporate. Carbon monoxide, acting as a coolant, has the opposite effect of methane, which absorbs sunlight and thus produces heating. But both of these gases are merely trace elements in an atmosphere that is thought to be dominated by nitrogen. The carbon monoxide, then, depending on the balance between it and the methane, could help cool the atmosphere, but it could also lead to all the atmospheric gases freezing out. Pluto’s is a fragile and tenuous atmosphere indeed, with its highest layers, at least at times, being blown away by the solar wind.
Observation in the decades ahead will help us understand how this delicate balance plays out as Pluto moves inexorably away from the Sun. From the paper:
Since surface changes… and the gaseous methane abundance… are seen to be temporally variable, time-dependent models will be needed to balance the effects of heating, cooling, sublimation and solar irradiation. Further CO line monitoring is underway, and N2 gas can be detected for the first time with ultraviolet spectroscopy…by the ALICE spectrograph on New Horizons… Hence atmospheric models can be tested from data taken before and during the 2015 flyby.
Is Pluto forming a comet-like tail, as study of the carbon monoxide line’s redshift seems to indicate? That’s one interpretation of the new data, but much work lies ahead. Greaves and company note that the recent increase in the brightness of the carbon monoxide line could be caused by the sublimation of surface ice, but add that the effect could also be caused by increases in atmospheric pressure of the kind that have been previously detected. And marked surface changes have occurred over intervals as short as a year on the distant world, so we’re seeing an atmosphere in movement as gas densities and abundances change, driven by heat.
“Seeing such an example of extra-terrestrial climate-change is fascinating”, says Greaves. “This cold simple atmosphere that is strongly driven by the heat from the Sun could give us important clues to how some of the basic physics works, and act as a contrasting test-bed to help us better understand the Earth’s atmosphere.”
The paper is Greaves et al., “Discovery of carbon monoxide in the upper atmosphere of Pluto,” discussed at the Royal Astronomical Society’s ongoing meeting in Llandudno, Wales and in press at Monthly Notices of the Royal Astronomical Society (preprint).
by Paul Gilster | Apr 20, 2011 | Astrobiology and SETI |
Space writer Keith Cooper, the editor of the UK’s Astronomy Now, is currently attending the Royal Astronomical Society’s meeting in Llandudno, Wales — in fact, the photo of him just below was taken the other day in Llandudno. But the frantic round of presentations hasn’t slowed Keith down. When last spotted in these pages, he was engaged in a dialogue with me about SETI issues. That exchange got me thinking about having Keith talk to Michael Michaud, considering their common interests and realizing that they had already met at last year’s Royal Society meeting where so many of these issues were discussed. Michael was kind enough to agree, and what follows is an exchange of views that enriches the SETI debate.
Centauri Dreams readers know Michael Michaud to be the author of the essential Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials (Springer, 2006), but he’s also the author of over one hundred published works, many of them on space exploration and SETI. Michaud was a U.S. Foreign Service Officer for 32 years before turning full-time to research and writing. He had a wide variety of assignments during his diplomatic career, including Counselor for Science, Technology and Environment at the U.S. embassies in Paris and Tokyo, and Director of the State Department’s Office of Advanced Technology.
Michael was directly involved in the negotiation of international science and technology cooperation agreements, represented the Department of State in interagency space policy discussions, and testified before Congressional committees on space-related issues. It’s particularly germane to mention that he was actively involved in international discussions on contact issues within the International Academy of Astronautics for more than twenty years, and was the principal author of the so-called First SETI Protocol (actually entitled ‘Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence’). His recent work has focused on the importance of bringing more historians and other social scientists into debates about the possible consequences of contact.
A few years ago I interviewed Andy Sawyer, the chief librarian and administrator of the Science Fiction Foundation at the University of Liverpool, and asked him about our predilection for alien invasion stories. “Because alien invasion stories are cool, and there are all sorts of anxieties that can be set in an invasion story,” he said. And I agree – I enjoy watching the latest alien invasion movie as much as the next person. But for many people their only exposure to ‘aliens’ is through the lazy stereotypes of Hollywood flicks, or perhaps the mono-cultural depictions in Star Trek, and this is driving how they envision real alien civilisations to be. Michael, in your work raising awareness of the consequences of METI you’ve helped bring home the fact that our ideas about aliens are just too simplistic, and consequently our visions of contact are equally superficial.
At the Royal Society last October you spoke of binary stereotypes – the wise old aliens happy to pass on their knowledge to us and teach us to become better beings, and the nasty, snarling conquerors hell-bent on wiping us off the face of the planet and stealing all our women and water (as the B-movie SF tropes usually go). What seems a shame is that some scientists in the SETI community have slipped into taking the first of these stereotypes for granted; the notion of an Encyclopedia Galactica is wholly expected by many, but this requires an enormous act of altruism on behalf of the sender.
I’ve seen lots of arguments suggesting that advanced extraterrestrials will inevitably be altruistic, yet it seems to me to be mainly the astronomers, speaking outside of their field of expertise, who are advocating such optimistic assumptions (without even defining what they mean by altruism – kin, or nepotistic altruism, is very different from reciprocal altruism, which is probably what they are referring to). Whenever I speak to evolutionary biologists, anthropologists, philosophers and other social scientists – people whose job it is to study social behaviour – they are much more cautious. For instance Professor Jerome Barkow, a sociocultural anthropologist at Dalhousie University in Halifax, Canada, described to me one possible example of “a fear-filled species, hypersensitive to danger, who have evolved intelligence selected for fear because it was the only way they could manage to win in competition against related species.”
Such a fearful species may constantly be on the lookout for danger, and in the worst case scenario may even be xenophobic. Simon Conway Morris, a paleontologist at the University of Cambridge who champions the idea of convergent evolution, is even more blunt: “If intelligent aliens call, don’t pick up the phone!” Meanwhile, Professor Nick Bostrom, Director of the Future of Humanity Institute at the University of Oxford, told me that “even if we could detect a pattern of increasing moral enlightenment in human history, it would be hazardous to extrapolate from that into our future, but I imagine that is what would underpin some people’s optimism about technologically advanced civilisations. As for altruism, we just have no idea about that.”
There are so many questions I would like to ask. My main question to you Michael is why are we not listening to those with the knowledge base of human behaviour who are best-placed to make judgments about the possible actions of extraterrestrials? Does SETI run the risk of becoming too exclusive? How can we expand the discussion to include people from all disciplines all around the globe? Furthermore, at the Royal Society, you said that it was time to make the most objective analysis that we can of the benefits and risks of METI. How do we escape the cliche of invasion to investigate the subtler pros and cons of contact?
Keith, I agree with much of what you say. Escaping the stereotypes that bedevil discussions about contact with ETI will not be easy. Because we have no confirmed information about the nature or behavior of extraterrestrial intelligence, our speculations about contact rest on belief, preference, or analogies with ourselves. Many people are impatient with the resulting ambiguity and want binary, either-or answers.
Early SETI advocates promoted the image of aliens as altruistic philosopher-kings not only because of personal preference, but also because they were trying to gain public support for a highly speculative endeavor. An idealistic and hopeful vision was a better sales pitch. For SETI advocates, that became the default position in discussions about the consequences of contact. Media treatments of contact issues, including documentaries, also are driven by what producers think will sell. Alien invasions — or wise, harmless extraterrestrials — overpower sober analyses based on what we know of human behavior. My responses to questions from three companies producing TV shows on the alien invasion theme were politely rejected not because they were wrong, but because they were less exciting than a war game.
Historians and other social scientists could offer more grounded visions of contact based on their studies of the only technological intelligent species we know – ourselves. For example, the human experience suggests that civilizations are not by nature either hostile or peaceful; their actions depend on the circumstances of the moment. Unfortunately, there is very little research funding or career reward for doing such work.
Many physical and biological scientists are not listening because they are skeptical of social science findings. Papers on social science issues such as the consequences of contact were pushed into the last hour of the last day of the June 2010 Astrobiology Conference. Yet some physical and biological scientists make sweeping statements about the behavior of intelligent beings that are not empirically grounded in the human experience. Imagine the reaction if a group of social scientists published conclusions about physical or biological processes without confirmed evidence.
How can we expand the dialogue about the consequences of contact? One starting point might be for a worthy organization interested in policy or social science issues to invite physical, biological, and social scientists to a themed conference on the benefits and risks of contact, including examples from human history. The Royal Society has set a useful precedent. I have some additional thoughts on this issue, but will hold them until I have your response.
Michael, I’m intrigued by your comment that a hopeful and optimistic vision of contact has been adopted because it makes a better sales pitch. It strongly echoes something that James Benford commented on following my previous SETI dialogue with Paul, namely the claim that Arecibo could detect a signal from another Arecibo 500 light years away is a myth perpetuated as part of SETI’s sales pitch.
[PG comment: Excuse me for interjecting, but I want to add that Benford has added material on the Arecibo signal in a revised edition of his paper “Costs and Difficulties of Large-Scale ‘Messaging’, and the Need for International Debate on Potential Risks,” written with John Billingham. This latest version is not the one currently up on the arXiv server, so let me quote from what Benford says:
A similar argument quantifies the ‘Arecibo Myth’, i.e., that that Arecibo (Earth’s largest radio telescope) would be able to detect its hypothetical twin across the Galaxy (Shuch, 1996). Arecibo can’t realistically communicate with another Arecibo over such distances. Those who so claim do not state their assumption that the bandwidth will be extremely narrow (0.01 Hz), that both the receiver and the transmitter will stare exactly at the right very small part of the sky, tracking each other and that the receiver will track for hours in order to integrate a very weak signal and that no information will be sent. This last point is that, because the bandwidth is so small, the bit rate is glacial, far less than the slowest modem, maybe a bit per hour in the best case.
Sorry for the interruption, and now back to Keith].
Of course we want the public, and those who would finance SETI, to feel hopeful about the possibility of contact. The detection of a message from another intelligent civilisation would be profound in ways that we are only beginning to perceive, both scientifically and culturally, and we can only explore those consequences with the aid of social science. Despite having an astronomical background, I do find the social science aspects of contact equally as fascinating as the physical science of the search itself. I’ve learnt so much more about the human experience through my reading as I try to understand the nature of contact than I otherwise would have. SETI is a rare merging of the physical and social sciences and is all the richer for it – those that push cultural and sociological debates on contact to the end of conferences, to footnotes in papers and wave them away in vague, sweeping gestures on alien altruism do SETI a great disservice.
The social sciences do not necessarily come down against METI and the hope for peaceful and rewarding contact – there are compelling arguments both for and against (as an example of the former, I’d cite Professor Steven Pinker of Harvard University championing the ‘myth of violence’ and the notion that we are becoming more peaceful as we become culturally and technologically more advanced – see his TED talk on the subject – the idea is that we could use this as a template for alien civilisations). What the social sciences do show is that the consequences of contact are more complex than the threat of invasion or dreams of uplift by wise philosopher kings.
This dialogue is about alien motivations; fear seems to be a standout candidate, as Jerome Barkow alluded to. Adrian Kent at the Perimeter Institute has taken this to extremes to explain the Fermi Paradox in his recent paper ‘Too Damned Quiet.’ Here he argues the Galaxy is quiet because natural selection chooses civilisations that remain inconspicuous in the face of galactic predators. He writes:
“Advertising our existence in such an environment would be risky: a predator species might decide it could afford to predate on us, and even a reticent neighbour species might decide it could not afford to leave us attracting the attention of predators to the neighbourhood.”
While Kent’s fear-filled, predator-dominated Galaxy is at the extreme end of possible contact scenarios, it is something to be aware of. So too are the more subtle aspects that are often dismissed in the face of sensational stories of alien invasion. But we have to be able to discuss all the possibilities without fear of prejudice. No one can say what alien motivations will be, and I’m not trying to argue for any particular viewpoint other than all possibilities are currently fair game.
SETI’s current sales pitch seems to be holding back the wider debate. We have both commented on the often abysmal reporting of the consequences of contact in the media. I’d like to begin to change this, here and now. We need a new sales pitch. Through SETI we’re trying to find our place in a Universe that could be equally full of exquisite wonders and terrifying dangers, and everything in between. If we cannot guarantee a vision of contact that is as idealistic as the one that went before, then Michael what should our new message be?
So, where do we go from here? First, it is time to free ourselves of the binary stereotypes that Hollywood, the Cold War, and the political reaction to it have imposed on us. We should be wary of both utopian and apocalyptic predictions of what contact might bring. We have no basis for assuming that other technological civilizations would welcome contact with us, nor do we have any basis for assuming that they would be hostile.
Second, it is time to fill in the middle ground between the extremes of a helpful altruistic message and an alien invasion. There are lots of other potential scenarios of contact, many of which have appeared in science fiction. For example, what if we detect a one-time alien signal not meant for us and are unable to find it again? What if that signal is nothing more than a dial tone? That would be contact without either communication or danger.
What if we find evidence of astroengineering on a scale far beyond our own abilities? That would imply a civilization so technologically powerful that it might consider us irrelevant, if it even noticed our existence. (In Arthur Clarke’s Rendezvous with Rama, a giant alien space craft using our sun for a gravitational assist passes through our solar system without stopping, or communicating.) What if we find an alien probe in our solar system that is doing nothing we can detect – no messages, no firing of weapons, not even a friendly glow? Such a probe might have been another civilization’s response to detecting signs of biochemistry in earth’s spectrum a billion years ago. (Would they be looking for the same kinds of evidence we seek? If not, they might miss Earth life.)
There are hopeful signs. Recent frictions among those interested in this issue may reflect a broadening and maturing of the debate. While some astronomers still dismiss the possibility of interstellar flight even by uninhabited machines, Seth Shostak of the SETI Institute has recognized in print that we humans could launch robotic interstellar probes by the end of this century. That puts the possibility of direct contact back on the agenda. No Star Trek aliens would walk down the ramp; we would be dealing with smart machines.
Third, those of us active in this field should evaluate the implications of alternate scenarios of contact as best we can. One starting point might be the Rio Scale proposed eleven years ago by Ivan Almar and Jill Tarter.
Fourth, we need to get more high-profile figures from the social sciences to speak or publish on this question. To me, the most relevant discipline is the history of contacts between human civilizations. I would love to hear what iconoclastic historians like Niall Ferguson and Felipe Fernandez-Armesto would say about the consequences of contact with extraterrestrials in a diverse set of scenarios. Prominent historian William McNeill warned many years ago that the human precedent is not encouraging.
Fifth, we need to bring into this debate younger people who are free of the ideological blinders of both the Cold War and the political and ideological reactions to it. That may mean people less than forty years of age. There are signs of hope. Pulitzer Prize-winning historian David Hackett Fischer put it this way: “After the delusions of political correctness, ideological rage, multiculturalism, postmodernism, historical relativism, and the more extreme forms of academic cynicism, historians today are returning to the foundations of their discipline with a new faith in the possibilities of historical knowledge.”
You ask what our message should be. That has to evolve from discussions among many thoughtful people. I can only offer a suggested formulation: Open eyes, patience and prudence. Accept what our searches tell us about the universe and about the behavior of intelligent beings, whether we like those findings or not. Recognize how new we are on the interstellar scene, and how much we have to learn. In trying to foresee the consequences of contact, keep in mind the only data base we have: our own history. I hope that you will use your journalistic skills to keep this discussion lively, and tolerant.
by Paul Gilster | Apr 19, 2011 | Astrobiology and SETI, Exoplanetary Science |
If you study ‘earthshine,’ the light of our planet reflected off the unlit part of the Moon, you can discover much about how life leaves an imprint upon a spectrum. It’s a useful exercise because one of these days we’ll have the tools in place to be examining the spectrum of a terrestrial world around another star. In Earth’s case, what two different teams have found is that water vapor, oxygen and ozone can be traced, just the kind of biosignatures we’d hope to find on a terrestrial world elsewhere.
Careful study of the spectrum of earthshine also turns up a tentative detection of the so-called ‘red edge’ signature of chlorophyll. What’s happening is that plants on our planet absorb visible light as part of the process of converting sunlight into energy. Beyond about 0.7 microns, just a bit longer in wavelength than the frequencies we can see, the same plants become highly reflective. This increase in reflectivity shows up as a sharp rise in the red part of the spectrum, hence the ‘red edge.’
The Uses of ‘Earthshine’
Ray Jayawardhana talks about this in his new book Strange New Worlds (Princeton, 2011), noting how useful such reflectivity is even now:
…Earth-observing satellites like Landsat use [the red edge] to map changes of the forest cover in the Amazon, for example, by taking images in two bands that fall on either side of the red edge. In the hemisphere-averaged spectrum of Earthshine, the red-edge signature is diluted by those vast areas on the planet either devoid of vegetation (e.g. oceans, deserts) or hidden beneath clouds, making it tougher to detect.
And this is important for our future observing plans:
Giovanna Tinetti of University College London has estimated that at least 20 percent of a planet’s surface must be covered by plants and free from clouds for the vegetation’s imprint to show up in the global spectrum.
None of this is easy, of course, because earthshine gets mixed up with the spectra of both the Moon and the Sun, but careful subtraction of light from the lunar crescent makes it possible to isolate the Earth’s spectrum. The earthshine spectrum rises toward the blue, Jayawardhana notes, because molecules in the Earth’s atmosphere scatter blue light more efficiently than red light.
The Colors of Life
We know how significant photosynthesis is for sustaining life on Earth, and it’s not unreasonable to think that some kind of light-processing pigments like chlorophyll would be present on other planets. We can add, then, the spectral signature of such pigments to the other biosignatures of note, because the more such signatures we find on a given world, the greater the chances we’re looking at life and not some other process at work.
It’s interesting to ponder what color pigments would dominate on worlds circling different kinds of stars than our Sun. I see the question is being discussed at the Royal Astronomical Society’s National Astronomy Meeting in Llandudno, Wales through the work of Jack O’Malley-James (University of St. Andrews). O’Malley-James is asking how the light of multiple star systems would affect the life evolving on the planets around them, and specifically, what plants might be like on an Earth-like planet with more than a single sun in the sky.
The results thus far: Life may have adapted to use the combined light of all the suns, but it’s also possible that some forms of life may develop using the light from only one of the suns. Plants might, then, develop differently depending on the color of the starlight they use. Says O’Malley James:
“Our simulations suggest that planets in multi-star systems may host exotic forms of the more familiar plants we see on Earth. Plants with dim red dwarf suns, for example, may appear black to our eyes, absorbing across the entire visible wavelength range in order to use as much of the available light as possible. They may also be able to use infrared or ultraviolet radiation to drive photosynthesis. For planets orbiting two stars like our own, harmful radiation from intense solar flares could lead to plants that develop their own UV-blocking sun-screens, or photosynthesizing microorganisms that can move in response to a sudden flare.”
The combinations would be endless, given the apparent multitudes of planets in our galaxy and the fact that 50 percent of red dwarfs, and 25 percent of G-class stars, are thought to occur in multiple systems. We then have to factor in the configuration of planetary orbits, which might circle two closely spaced stars or orbit one of two more widely separated stars. If life really is widespread, then foliage on such worlds should come in a profusion of different colors.
Image: Black plants on a world with two suns. Credit: Jack O’Malley James/University of St. Andrews.
Nancy Kiang (NASA Goddard Institute for Space Studies) has also been studying the spectral signature of photosynthesis, noting how the color of the pigments varies depending on the spectral type of the host star. Planets circling F-class stars, for example, would probably have plant pigments that absorb primarily in the blue, with the result that these plants would appear red or orange to our eyes. Work with the spectrum of an F-class star’s terrestrial planet and you might find a ‘blue edge’ rather than a red one.
The red dwarf scenario is as O’Malley-James describes. Most of the photons reaching the surface here are in the near-infrared. Plant pigments would have adapted to use the full range of visible and infrared light, while reflecting little back, hence their black appearance. But Kiang notes the difficulty in getting photosynthetic organisms to evolve in such settings, given the UV flares often produced by young M-dwarfs.
Life Goes Deep
In fact, Kiang’s team has estimated that early microbes would need to be as much as nine meters under water to escape the effects of such flares while still receiving the needed light for photosynthesis. Here I want to quote Ray Jayawardhana again, on how this scenario parallels the early Earth, when the atmosphere lacked oxygen and ozone:
The first photosynthetic organisms lived under water, which acted as a solvent for biochemical reactions and provided protection from UV rays in the absence of the ozone layer. These bacteria had to use infrared light, filtered through the ocean, so their pigments had to be different too. Later, as oxygen and ozone levels built up, green algae emerged, first in shallow water and eventually on land. Their plant descendants have adapted to the atmosphere’s changing composition, itself primarily the result of photosynthesis. Chlorophyll is customized for present-day conditions on Earth.
Find the right absorption band in the spectrum of a distant exoplanet and you just might be looking, then, at the signature of alien plant life. Work like O’Malley-James’ and Kiang’s can help us understand where in the spectrum to look, taking into account the different kind of stars we’ll be finding planets around, and the multiple systems in which many of them will orbit. The further trick will be to demonstrate that the signatures we do detect do not have other possible explanations, a challenge astrobiology continues to address. For more on the colors of exobiology, see this earlier Centauri Dreams post.
by Paul Gilster | Apr 18, 2011 | Exoplanetary Science |
A number of interesting things are coming out of the Royal Astronomical Society’s now convening meeting in Llandudno, Wales, many of them still embargoed, though we’ll be able to discuss them later in the week. But among the papers now open for discussion, I was drawn to work by Aline Vidotto and colleagues at the University of St. Andrews. Vidotto has been working with the exoplanet WASP-12b, a ‘hot Jupiter’ discovered in transit by the wide-field cameras of the SuperWASP project (WASP stands for Wide Angle Search for Planets). The work focuses on how a planetary ‘bow shock’ can protect an exoplanet’s atmosphere from emissions from its host star.
For the new evidence Vidotto and team are discussing at Llandudno shows that there are signs of a magnetosphere around WASP-12b. Discovered in 2008, this ‘hot Jupiter’ is one of the largest exoplanets yet found, more than 250,000 kilometers in diameter. It’s also an extremely hot planet, orbiting the star designated WASP-12 every 26 hours, at a distance of no more than 3.4 million kilometers (0.0229 AU). Violent interactions between star and planet are inescapable. Vidotto’s team used data from the Hubble Space Telescope to analyze variations in ultraviolet and visible light that can be explained through the effects of a planetary magnetosphere.
WASP-12b bow shock simulation from Joe Llama on Vimeo.
Image: Simulation of a planet and bow shock transiting a limb-darkened star. The parameters have been tuned to be representative of the WASP-12 system. Credit: Joe Llama/University of St. Andrews.
What astronomers had thought to be a flow of material from the planet onto the star turns out to be better explained by bow shock. Close study of the planet has shown that the dip in the star’s light during a transit begins earlier in ultraviolet light than in visible light. Simulations of the planet and its bow shock can reproduce the ultraviolet dip, a window into the interactions between a planetary magnetic field and the magnetic field of the host star. The fact that we’re seeing the interactions in a magnetic field here means that WASP-12b has a conducting, rotating interior.
Bow shocks are helpful things indeed. The Earth’s magnetic bow shock protects us from the solar wind, and in WASP-12b’s case, we can assume that the bow shock offers some protection for the planet’s atmosphere from the charged, energized particles of its parent star, shielding it from erosion. And scientists wind up with a new and useful tool, one that lets us measure the strength of the magnetic field of a planet we can ‘see’ only through transit dips. These observations are anything but static. From the most recent paper on this work:
Observational follow-up suggests that the near-UV light curve presents temporal variations, which may indicate that the stand-off distance between the shock and the planet is varying. This implies that the size of the planet’s magnetosphere is adjusting itself in response to variations in the surrounding ambient medium.
Joe Llama (University of St. Andrews), the PhD student who ran the simulations of the bow shock, notes the significance of the finding:
“Our models are able to reproduce the data from the Hubble Space telescope for a range of wind speeds implying that bow shocks could be far more commonplace than had been thought… Although our model predicts a bow shock similar to that of the Earth, we are not expecting any messages from WASP-12b as it is too hot to support life. But the first hints that extrasolar planets have magnetosphere is a big step forward in understanding and identifying the habitable zones where we ultimately hope to find signs of life.”
The paper is Vidotto, “Transit Variability in Bow Shock-Hosting Planets,” Monthly Notices of the Royal Astronomical Society, published online 6 April 2011 (abstract / preprint). See also Vidotto et al., “Early UV Ingress in WASP-12b: Measuring Planetary Magnetic Fields,” Astrophysical Journal Letters Vol. 722, No. 2 (2010), L168 (abstract / preprint). A news release from the University of St. Andrews is also available.
