by Paul Gilster | Jul 19, 2008 | Culture and Society |
The latest Carnival of Space is now available, with several items of particular interest to those of us fixated on deep space from the edge of the Solar System to nearby stars. Have a look, for example, at this take (from Astronomy at the CCSSC) on Makemake, a dwarf planet in the newly minted IAU sense, and also a plutoid, meaning a dwarf planet outside Neptune’s orbit. Or try Starts with a Bang, where the speculation runs to placing human crews on long-haul starships using artificial incubators and frozen embryos, a subject we recently touched on in these pages.
My attention was particularly drawn to Bruce Cordell’s piece on How Great Explorations Really Work, in an intriguing site called 21st Century Waves. Here the idea is that great exploratory projects (think Apollo, for example) do not happen at random times, but tend to cluster around a 56-year energy cycle that coincides with major economic booms. My experience with the stock market tells me that when anyone identifies a major cycle, that’s a sure sign that the cycle will not work the next time around (sort of a Heisenberg uncertainty principle for macro-scale behaviors). But the idea is interesting and pegs our human urge to explore. As in this:
In this model, the assertion of anthropologists that humans are by nature explorers — because of their 200,000 year history of exploration and expansion — is adopted. In the last 200 years, the explorer’s impulse can’t often be indulged by typical individuals because of economic and security (Maslow) pressures. However, during the twice-per-century major economic booms, widespread affluence elevates society to the higher levels of Maslow’s heirarchy. Thus for a brief period (called a “Maslow Window”), society reaches a semi-rational (almost giddy) state of “ebullience,” where Great Explorations are not just favored by most people, but seem almost irresistable.
However, ebullience rapidly decays as the economic boom slows, or as a major war (which typically occurs at these times) threatens peace and security.
Whatever my doubts about identifying such ‘windows,’ I think the observation about public fixation on exploration is exactly correct. Anyone with a passion to see our society build a space-based infrastructure anywhere beyond Earth’s orbit has to cope with today’s apparent public disinterest. That can be discouraging, but it can also be dangerous when we’re weighing the odds on our planet being struck by near-Earth objects. There are times when ebullience may not carry the day but self-interest must still kick in. Big space rocks don’t wait on human cycles.
by Paul Gilster | Jul 18, 2008 | Exoplanetary Science |
You must see new video from EPOXI, whose effect can only be suggested by the photo montage below (click the link below for the movie). EPOXI is the combined extended mission of the Deep Impact spacecraft. As we discussed in an earlier story here, EPOXI turned its cameras on the Earth to view the moon transiting the planet’s disk from a vantage point of 31 million miles. Think in terms of viewing the Earth the way we will eventually view terrestrial worlds around other stars. The idea is to build insights into how these worlds can be observed and characterized.
Earth and Moon seen from EPOXI
Image: The moon crossing the Earth, as viewed by EPOXI. Video credit: Donald J. Lindler, Sigma Space Corporation/GSFC; EPOCh/DIXI Science Teams.
Drake Deming (NASA GSFC), deputy principal investigator for EPOXI and leader of the extrasolar planet component of the mission (called EPOCh), points out how the information can be helpful:
“Our video shows some specific features that are important for observations of Earth-like planets orbiting other stars. A ‘sun glint’ can be seen in the movie, caused by light reflected from Earth’s oceans, and similar glints to be observed from extrasolar planets could indicate alien oceans. Also, we used infrared light instead of the normal red light to make the color composite images, and that makes the land masses much more visible.”
None of our current or projected planet hunter missions will be able to see a terrestrial world as close up as this, but we will be able to see such a planet as a point of light that can be studied to observe changes in its total brightness. Such changes could be telling us about continents on a planet affecting the light signature as, interspersed with water, they rotate in and out of view. So taking views of a known living world helps us understand how these light changes occur, and we’ll want to keep taking pictures of Earth at ever greater distances to continue that effort. The moon transit occurring in these frames is a priceless bonus.
Now take a look at the second movie. Whereas the first was made using a red-green-blue filter, the second was made using infrared-green-blue. With plants reflecting more strongly in the near-infrared, the land masses become more readily visible. We’re thus looking at the possibility of detecting vegetation on extrasolar planets whose marker will be changes in near-infrared light as the planet rotates. All this in the future from a tiny point of light whose properties we’re learning to parse thanks to a mission that once set out to drive an impactor into a comet.
by Paul Gilster | Jul 17, 2008 | Exoplanetary Science |
It’s easy to see why interest in planets around red dwarfs is growing. The low mass of such a star makes finding smaller planets feasible. It also produces orbits closer to the star, another aid to their detection. We know that planets can form near the habitable zone of such stars because we have the example of Gliese 581, where two planets orbit close to if not just within that region. But is a habitable planet always habitable? If not, what could make these conditions change?
I’m looking at a paper that examines tidal effects, an important factor when dealing with M dwarfs. Planets in the habitable zone around these stars experience effects that can cause both their orbital distance and orbital eccentricity to decrease [see comments below re my original misstatement of the eccentricity change, now corrected]. The paper, by Rory Barnes (University of Arizona, Tucson), Sean Raymond (University of Colorado, Boulder) and team, examines an interesting parameter: The habitable lifetime. The authors define this as the time needed for a planet to move from a habitable region to one that is not. More massive stars have habitable zones far enough away that this tidal evolution does not occur, but M dwarfs below about 0.35 solar masses can be affected.
The result is striking and potentially devastating to life: A planet with a large enough orbital eccentricity (larger than about 0.5) around a low-mass star can, because of this tidal effect, be pulled out of the habitable zone in less than a billion years. Given the fact that M dwarfs account for over 75 percent of the stars in our galaxy, and given the fraction of known exoplanets with high eccentricity, the authors suggest that tidal effects may be a noteworthy constraint on the total number of habitable planets. The definition of habitability used in this study is the classic one, based upon the presence of liquid water upon the surface.
The results for Gliese 581 c are intriguing. This is the planet that caused such a stir when researchers announced that it was within the habitable zone of its star, a finding that has since been sharply questioned. Looking at the interactions between Gliese 581 c and inner planet 581 b, the authors state:
Tidal theory suggests that planet c orbited with larger values of semi-major axis and eccentricity in the past. Therefore, it may have been habitable in the past, but tides subsequently moved the planet into an uninhabitable orbit. If planet c was the only planet in the system, plausible physical properties indicate that it was habitable. However, when constraints from the mutual interactions of the additional planets are considered, planet c has likely never been habitable.
The details of these interactions are complicated, but what I want to focus on is the deeper implication here:
The detection of a terrestrial planet around a low-mass star is insufficient to determine that planet’s past and future habitability. The tidal forces between planet and star can significantly change the orbits and hence limit the habitable lifetime. Planets detected in the HZ with large eccentricities may be bound for hotter temperatures and, ultimately, a global extinction. On the other hand, planets detected interior to the HZ may have been habitable in the past. Gliese 581 c was most likely not habitable in the past, but if its companion planets were on different orbits, past habitability would have been possible.
As if we didn’t have reason enough for caution about these matters, we’re now reminded that just finding a rocky world at a particular distance from its parent star is no guarantee of its long-term habitability, particularly when we’re dealing with M dwarfs. Because we’re talking about major orbital evolution over a span comparable to the age of the Earth, it’s clear that sustained complex life demands planets that form with low eccentricities. The good news is that most exoplanets are thought to have been formed with relatively low eccentricities.
So now we’re looking at a true science fiction scenario, a planet that once supported life but moved ultimately inside the habitable zone of its star. What might future exobiologists find among the wreckage? The authors of this paper note that we should extend our work on habitable atmospheres and their evolution to include the possibility of detecting the signatures of extinct life on planets like this around M dwarfs. In many ways, the thought is poignant, and it’s easy to agree with this statement:
Perhaps the most distressing aspect of this work (from a SETI perspective) is the prediction that planets can be habitable long enough for complex life to develop, but then that life is extinguished by tides. Yet this work suggests that such a “tidal extinction” may occur on some planets around low-mass stars.
The paper is Barnes et al., “Tides and the Evolution of Planetary Habitability,” accepted by Astrobiology and available online. Be aware as well of Jackson, Greenberg and Barnes, “Tidal Heating of Extra-Solar Planets,” accepted by the Astrophysical Journal (abstract). An earlier Centauri Dreams story on that paper is here. Thanks to Adam Crowl for his assistance on this story.
by Paul Gilster | Jul 16, 2008 | Outer Solar System |
Now in press at Astrobiology is a look at the possibilities of life on Enceladus that holds out hope for detecting biomarkers with data gathered during a Cassini flyby. That’s an exciting possibility, depending as it does not on an orbiter or lander mission from an indefinite future but on equipment we’ve currently got in Saturn space. And the Enceladus picture remains fascinating because of the possibility that some microbial systems on Earth that operate far beneath the surface may offer examples of how life could evolve on a cold and distant moon of Saturn.
We’ve already found a dozen icy particle jets coming out of Enceladus’ south polar regions, all pumping material into a plume that extends for thousands of kilometers. A 2005 Cassini flyby revealed, among other things, water vapor, methane and simple organic compounds, even as other Cassini instrumentation showed the moon’s south polar region to be anomalously warm. If there is liquid water under the south polar region, could life have evolved there? If so, the paper raises the possibility that methane may be a biomarker. For that matter, could life have come there from elsewhere? The paper argues both are possible:
The two categories of theories for the origin of life on Enceladus have different implications for how long habitable conditions must persist there for life to be probable. For the panspermia theories, life arrives at Enceladus intact ready to reproduce. In this case no origination time is required and life can utilize a pre-existing habitable environment instantly. For the organic soup or chemosynthetic theories for the origin of life within liquid water aquifers in Enceladus, the best we can conclude is that the origination time could be as low as 10 Myr or as along a 500 Myr. As discussed above, there are no direct geophysical estimates of the persistence of the jet activity on Enceladus. However, estimates for the timescale to freeze an ocean on Enceladus are ~30 Myr… which may be consistent with the timescale for the origin of life. Thus, there is no reason to conclude that any liquid water on Enceladus is too young to have been a site for the origin of life.
All of which is consistent with the possibility of living systems. Christopher McKay (NASA Ames) and colleagues run through three Earth ecosystems found deep below the surface that do not depend on oxygen or organic material produced by photosynthesis, relating these to plausible energy sources under Enceladan ice. An exciting possibility is that biological materials might be carried out into the plume streaming from Enceladus, available for future sample return missions. But Cassini itself, in its extended mission, may be able to use another flyby to determine whether the methane and other hydrocarbons found in the plume are consistent with biological origin by looking at the ratio of non-methane hydrocarbons to methane. The biological signature should be distinctive.
Fascinating stuff, as is the issue of whether any detected life would be biochemically related to life on Earth (via, perhaps, Solar System-wide panspermia) or completely different, evidence of a ‘second genesis.’ As the paper notes, both finds would be of huge interest, but the second would offer prospects both more exciting and more daunting in terms of detection, with life perhaps using a different set of basic molecules. The paper is McKay et al., “The possible origin and persistence of life on Enceladus and detection of biomarkers in the plume,” in press at Astrobiology.
by Paul Gilster | Jul 15, 2008 | Exoplanetary Science |
We’ve been paying a lot of attention to Centauri A and B in the past two years, but what about Proxima Centauri? After all, this is the closest star to our Sun, a fifth of a light year out from the two major Centauri stars, and free of the close binary problem. You would think this small red dwarf would rank higher on our list of astrobiologically interesting places, but until recently, that red dwarf status has been an encumbrance. It has been only within the last eleven years that the presumed tidal locking of planets in the habitable zone of such stars has been found not to be a necessary deterrent to the formation of a stable climate.
Today, M dwarf interest grows. There’s at least the chance of a workable ecosystem around such a star, assuming flare activity (common to these stars) might act more as an evolutionary stimulus than a deterrent to life. Moreover, the long lifetimes granted to M dwarfs mean that stable environments could exist for many billions — perhaps hundreds of billions — of years. This is why we’ve seen a recent florescence of M dwarf studies, with a keen interest in their astrobiological prospects, and why Proxima Centauri remains an interesting target. And although it hasn’t gotten the press of its larger siblings, Proxima has generated studies that are closing in on characterizing its system.
Image: Alpha Centauri, with components Centauri A and B doctored for clarity. The arrow marks Proxima Centauri. Credit: European Southern Observatory.
We can already say this about Proxima planets: If they exist, they are no larger than 0.8 Jupiter masses in the range of orbital periods ranging from one to 600 days. That’s from radial velocity studies published in the late 1990s. This work is now complemented by seven years of high precision radial velocity data gathered with the UVES spectrograph at the European Southern Observatory. Michael Endl (McDonald Observatory) and Martin Kürster (Max-Planck-Institut für Astronomie) address the question of what kind of planets we can exclude from the habitable zone of Proxima Centauri based upon these data.
Proxima’s habitable zone, remember, is in close because this is a small star — the authors assume 0.12 solar masses, a reasonable estimate if on the high side, for reasons they explain in their paper. The habitable zone then becomes 0.022 to 0.054 AU, which corresponds to an orbital period ranging from 3.6 to 13.8 days. And the UVES data make it clear that no planet of Neptune mass or larger exists out to a distance of 1 AU.
For periods of less than 100 days, no super-Earths are detected larger than about 8.5 Earth masses. And for the actual habitable zone of Proxima Centauri we can rule out planets larger than 2-3 Earth masses in circular orbits. Needless to say, this doesn’t rule out planets of Earth mass or smaller in this zone.
The sensitivity of these studies is only improving:
With the results from this paper we demonstrate that the discovery of m sin i ? 1 M? [one Earth mass] is within our grasp. Since sensitivity is a function of RV [radial velocity] precision, number of measurements and sampling, adding more points to the existing data string in a pseudo-random fashion, will allow us to improve the detection sensitivity over time.
Do note the above qualifier: We can rule out Proxima planets of 2-3 Earth masses in circular orbits. This is one of several limitations on the study:
Limits for planets on eccentric orbits are typically slightly higher… Planets with masses above our mass threshold for circular orbits can still exist around Proxima Cen on eccentric orbits. We also considered only the case of a single planet. The RV signals of a multi-planet system with several low-mass bodies, [are] likely to be more difficult to detect by a pure periodogram analysis (depending on their period spacing and mass ratios) and require signi?cantly larger data sets. Simulations to determine the mass limits for multiple planets [are] beyond the scope of this paper.
Thus Proxima Centauri remains a prime target, if one for which we still have no discovered planets. We are moving ever closer to the ability to detect Earth mass planets inside the habitable zone around this star, a capability we’ll hope to refine still more in coming months. After all, a Mars-size planet in the right place could provider an abode for life even if beneath our current threshold for detection. The paper is Endl and Kürster, “Toward detection of terrestrial planets in the habitable zone of our closest neighbor: Proxima Centauri,” accepted for publication in Astronomy & Astrophysics (available online).
