by Paul Gilster | Jun 23, 2010 | Exoplanetary Science |
HD 209458b is perhaps the most persistently studied exoplanet we have, a transiting ‘hot Jupiter’ that has already revealed a slew of its secrets, including the detection of carbon dioxide, water vapor and methane. I confess that it sometimes seems like black magic to me that we are able to ferret out the signature of organic compounds on worlds we cannot even see. But the transit method is fruitful, and when scientists examine the light of the star during a planetary transit, the tiny portion of that light filtering through the planet’s atmosphere can be analyzed.
In the case of HD 209458b, we’re talking about a three hour transit, one that occurs every 3.5 days as this ‘hot Jupiter’ makes its rounds. Now we learn that the carbon dioxide detected here can also be studied in terms of its velocity. The result: We have indications of a vast storm, a wind flow that’s moving at speeds that defy the imagination. Ignas Snellen (Leiden Observatory, The Netherlands) led the team that performed this work:
“HD 209458b is definitely not a place for the faint-hearted. By studying the poisonous carbon monoxide gas with great accuracy we found evidence for a super wind, blowing at a speed of 5,000 to 10,000 km per hour.”
The storm itself is not surprising, but the fact that we are able to detect it is something of a coup. The planet in question is about 60 percent as massive as Jupiter, orbiting a Sun-like star 150 light years from Earth in the direction of Pegasus. Orbiting at 0.047 AU, the world is tidally locked, with surface temperatures thought to reach about 1000 degrees Celsius on the star-side, while the other remains much cooler. The temperature differential is what kicks up the enormous winds, now measured using the ESO Very Large Telescope and the CRIRES spectrograph, which produced spectra sharp enough “…to determine the position of the carbon monoxide lines at a precision of 1 part in 100,000,” according to team member Remco de Kok. From the paper on this work:
Since with transmission spectroscopy we probe the atmospheric region near the planet’s terminator, the blue-shift indicates a velocity-flow from the day side to the night side at pressures in the range 0.01-0.1 mbar as probed by these observations. Such winds may be driven by the large incident heat flux from the star on the dayside. Indeed three-dimensional circulation models indicate that at low pressure (<10 mbar) air should flow from the substellar point towards the antistellar point both along the equator and the poles.
But HD 209458b has yielded even more, as the astronomers were able to measure the velocity of the exoplanet as it orbits its star, using the information to refine estimates of the mass of both star (1.00 plus or minus 0.22 Sun masses) and planet (0.64 plus or minus 0.09 Jupiter masses). Further measurements showed how much carbon dioxide is present in the planet’s atmosphere. HD 209458b turns out to be about as carbon-rich as Jupiter and Saturn, leading to speculation that it was formed in the same way.
The paper is Snellen et al., “The orbital motion, absolute mass, and high-altitude winds of exoplanet HD 209458b,” Nature 465 (24 June 2010), pp. 1049-1051.
by Paul Gilster | Jun 22, 2010 | Outer Solar System |
Suppose for a moment that you have some novel ideas about astrobiology on Io. The idea seems extreme, but there are scientists who argue for the notion, as we’ll see in a moment. In any case, if you wanted to observe Io, how would you go about it? The best solution is a spacecraft, as it was when Voyager 2 sped through the Jupiter system and discovered what tidal effects can do to a small moon. The Galileo probe, despite the failure of its high-gain antenna, was able to send up-close data about both Europa and Io, confirming that the latter’s volcanic activity was 100 times greater than what we experience on Earth. And then there was Cassini’s lovely view.
Image: Gliding past Jupiter at the turn of the millennium, the Cassini spacecraft captured this awe inspiring view of active Io with the largest gas giant as a backdrop, Credit: Cassini Imaging Team, Cassini Project, NASA.
But the Voyagers are now close to leaving the Solar System, while Galileo was sent to its destruction in the clouds of Jupiter back in 2003. Cassini is orbiting Saturn. Today, you’d have to rely on getting instrument time on the Hubble space telescope to follow-up your Io ideas. But we’ve just had word of a new adaptive optics technology that does Hubble one better. In fact, tests at the Large Binocular Telescope in Arizona have shown that the system — First Light Adaptive Optics, or FLAO — delivers image quality more than three times sharper than Hubble while using only one of the LBT’s two 8.4-meter mirrors. Add in the second mirror and this ground-based system is expected to achieve image sharpness fully ten times that of Hubble.
I share the excitement of Simone Esposito, the leader of a team from Italy’s Arcetri Observatory of the Istituto Nazionale di Astrofisica (INAF), which developed the system in collaboration with Steward Observatory (University of Arizona). Says Esposito:
“The results on the first night were so extraordinary that we thought it might be a fluke, but every night since then the adaptive optics have continued to exceed all expectations. These results were achieved using only one of LBT’s mirrors. Imagine the potential when we have adaptive optics on both of LBT’s giant eyes.”
Indeed. Using a secondary mirror that is pliable enough to be manipulated by actuators pushing on 672 magnets on its back, the technology seems to be a breakthrough in adaptive optics, taking us close to the point where we can see through the atmosphere as clearly as if it did not exist. You can see the results below:
Image: A double star as observed with the LBT in standard mode (left), and with the adaptive correction activated (right). Because of atmospheric blurring, the fainter companion of the star cannot be identified in the images taken in standard mode, while it is easily visible when the adaptive module is activated. A third faint star also becomes visible in the upper right part of the frame, thanks to the increased sensitivity of the telescope in adaptive mode. Credit: INAF/University of Arizona.
Life’s Chances on Io
The revolution in ground-based astronomy that adaptive optics continues to foment is nothing short of breathtaking. Meanwhile, let’s back up a bit to that earlier comment about astrobiology and Io. Dirk Schulze-Makuch (Washington State University) made the case for astrobiology on the seemingly hostile world in a paper last year in the Journal of Cosmology. Io’s plasma particle interactions with Jupiter, its lack of a substantial atmosphere and its extreme temperature gradients all argue against life there. Nor do we see impact craters, indicating a malleable surface that is being constantly reformed.
But there is this to be said about the place: It formed in a part of the Solar System where water ice is plentiful and geothermal heat could have made the origin of life possible. We can imagine a scenario where water was lost on the surface and life went deep underground, where both water and carbon dioxide may still be plentiful. Schulze-Makuch’s view:
Geothermal activity and reduced sulfur compounds could still provide microbial life with sufficient energy sources. Particularly, hydrogen sulfide is probably a common compound in Io’s subsurface…. Volcanic activity is prevalent on Io and lava tubes resulting from that activity could present a favorable habitable environment. Microbial growth is common in lava tubes on Earth, independent of location and climate, from ice-volcano interactions in Iceland to hot sand-floored lava tubes in Saudi Arabia. Lava tubes also are the most plausible cave environment for life on Mars… and caves in general are a great model for potential subsurface ecosystems.
Underground microbial life on Io would, then, be protected from low temperatures and shielded from radiation, in an environment with both trapped moisture and nutrients like sulfide and hydrogen sulfide. Schulze-Makuch speculates that sulfur could play a large role here as a potential building block of life, noting that there is no evidence to this point for any organic molecules on Io and little hint of carbon of any kind. But it’s also worth keeping in mind that any organic molecules would be extremely difficult to find in Io’s atmosphere — they wouldn’t last long given the radiation environment at the surface. Energy, of course, is plentiful, and the author studies the possibility of chemical and magnetic energy’s role in astrobiology.
I’m not putting my money on Io as a home for life, and even Schulze-Makuch notes that the possibility has to be considered a long-shot. When we have exploratory robotic systems in Jupiter space again, Europa is obviously a much more likely candidate, and so, for that matter, is Ganymede. But a habitable niche in Io’s subsurface can’t be ruled out, even if radiation levels make shielding a robotic probe of the planet a dicey proposition. Until such a probe becomes possible, though, a clear view of Io with adaptive optics may be our best way to observe it.
The paper is Schulze-Makuch, “Io: Is Life Possible Between Fire and Ice?” Journal of Cosmology Vol. 5 (2010), pp. 912-919 (available online).
by Paul Gilster | Jun 21, 2010 | Astrobiology and SETI |
Are planets common around brown dwarfs? We aren’t yet in a position to say, but the question is intriguing because some models suggest that the number of brown dwarfs is comparable to the number of low-mass main sequence stars. That would mean the objects — ‘failed’ stars whose masses are below the limit needed to sustain stable hydrogen fusion — could be as plentiful as the M-dwarfs that far outnumber any other type of star in the galaxy. If planets form around brown dwarfs, then we have to add them to our list of possible abodes for life.
Evidence for Brown Dwarf Planet Formation
But first, to the planet question. We can find suggestive analogs to planet formation around brown dwarfs in nearby space. The star Gl 876, some fifteen light years away, is not a brown dwarf, but this M-dwarf is only 1.24 percent as luminous as the Sun, with most of its energy being released at infrared wavelengths. We now know that at least three planets, two of them gas giants similar to Jupiter, orbit the star. Among brown dwarfs themselves, we have cases like 2M1207b, MOA-2007-BLG-192Lb and 2MASS J044144. In fact, the planet orbiting the second of these brown dwarfs is one of the smallest exoplanets known at 3.3 Earth masses.
As Andrey Andreeschchev and John Scalo (University of Texas) noted in a 2002 paper (thanks to Centauri Dreams regular ‘andy’ for the tip), we can extrapolate from what we find in our own Solar System to lower-mass stars, with simulations indicating that terrestrial-mass planets can form around low-mass objects like these as long as sufficient disk material is available. The authors study whether or not such planets can be habitable, noting this key fact about brown dwarf evolution: The brown dwarf is continually fading as it releases gravitational potential energy. As the object fades, its habitable zone moves past any worlds in it.
Time, Tides and Habitability
Is there time, then, for life to form on such a planet? When andy sent the pointer to this paper, he added an intriguing comment of his own:
It’d be interesting to come up with some scenarios for evolution on such a planet whose star decreases in luminosity as it ages (as opposed to more conventional stars that brighten as they age) – perhaps life might begin in the cloud layers of an initially Venus-like planet, moving to the surface as the atmosphere cools and the oceans rain out of the atmosphere, and finally moving to a more Europa-like state with the oceans frozen under an ice layer.
Now that’s a chewy science fiction scenario for the writers who frequent these pages to work on. Andreeshchev and Scalo note that a brown dwarf planet will be within the tidal lock radius, meaning the planet will always present one side to its star even when the brown dwarf is young, but we do have some studies showing that atmospheres can remain viable in such settings, so this may not rule out life. A bigger question is just how long the habitable zone will remain habitable and how, as andy notes, life might adapt. Clearly, evolutionary time-scales on a brown dwarf planet could be much different from those on Earth, but the paper notes that a habitability duration of less than 0.1 billion years would present real issues about the viability of complex life.
I can’t get Andreeshchev and Scalo’s diagram reproduced well enough to display well here, but they study the duration of residence in the evolving habitable zone as a function of the planet’s distance from the brown dwarf, assuming a circular orbit. They find that much depends on how we set limits on the habitable zone, but in general habitability durations of a billion years are possible for planets within 2-3 Roche radii for brown dwarfs above 0.03 solar masses. The Roche limit defines how close a planet can be to its host star before being torn apart by tidal forces. A habitable zone duration of up to 4 billion years is possible only close to the Roche limit, but could theoretically occur for brown dwarfs as small as 0.04 solar masses.
In fact, if you push these numbers to their upper limits, you can work out a habitable zone that has a duration of up to 10 billion years for a brown dwarf with a mass of 0.07 solar masses, as long as you’re willing to skirt the Roche limit about as close as possible. The authors are working, by the way, with a habitable zone definition that involves liquid water at the surface, the classic formulation of habitable zone rather than more recent extensions of the idea.
Temperature and Intelligence
This is a short but fascinating paper, and here’s something that catches the eye:
…if development of intelligence is partially driven by cooling episodes, as suggested by Schwartzman & Middendorf (2000), then on BD planets cognitive evolution may be expected to contain a stronger continuous component than on Earth.
I leave it to the science fiction writers to come up with depictions of the societies that may result. And I’ll end with the thought that if we do decide brown dwarf planets are not uncommon, and that complex life may find ways of evolving on such worlds, then nearby space may be littered with astrobiologically interesting destinations that are largely unknown to us. Or will be until infrared surveys like WISE tell us just how common brown dwarfs really are in our stellar neighborhood.
The paper is Andreeshchev and Scalo, “Habitability of Brown Dwarf Planets,” Bioastronomy 2002: Life Among the Stars. IAU Symposium, Vol. 213, 2004 (abstract).
by Paul Gilster | Jun 18, 2010 | Astrobiology and SETI |
Long-term thinking means planning for the consequences of things that are beyond our current capacity. What happens on the farside of the Moon is a case in point. Getting humans back to the Moon is going to happen sooner or later, and one day we will have bases there, as well as a human or robotic presence at the L4 and L5 Lagrangian points of the Earth-Moon system. That means an ever growing blanket of electromagnetic radiation from our various activities. At the same time, we want to protect the farside, which is ideal for future radio telescope or phased array detectors. What to do?
Italian physicist Claudio Maccone has brought this issue to Vienna, speaking before the United Nations Committee on the Peaceful Uses of Outer Space. Maccone is proposing a radio-quiet zone on the farside that will guarantee radio astronomy and SETI a defined area in which human radio interference is impossible. It’s an idea with a pedigree, going back to 1994, when the French radio astronomer Jean Heidmann first proposed a SETI observatory in the farside Saha Crater with a link to the nearside Mare Smythii plain and thence to Earth.
An IAA study committee grew out of this and a series of discussions since, with Heidmann’s death in 2000 being followed by Maccone’s taking over the project. As Maccone told the UN committee meeting, radio interference becomes a serious threat when we consider the uses to which the Lagrangian points could be put. The Earth/Moon system has five such points, as shown in the diagram. Potential space operations could take place at all of these except, let’s hope, L2, which as you can see in the diagram is located so that a base there would flood the farside with interference. The first principle is then clear: Leave the L2 point alone.
As for the others, the diagram makes it equally clear that inteference from L4 and L5 would reach large parts of the farside, but Maccone’s figures show that a protected area is still possible. He is proposing a Protected Antipode Circle, defined as a circular piece of land 1820 kilometers in diameter, centered around the antipode on the farside and spanning an angle of 30 degrees in longitude, in latitude and in all radial directions from the antipode. The rationale, as presented by Maccone in a 2008 paper:
(1) PAC is the only area of the Farside that will never be reached by the radiation emitted by future human space bases located at the L4 and L5 Lagrangian points of the Earth-Moon system (the geometric proof of this fact is trivial);
2) PAC is the most shielded area of the Farside, with an expected attenuation of man-made RFI ranging from 15 to 100 dB or higher;
3) PAC does not overlap with other areas of interest to human activity except for a minor common area with the Aitken Basin, the southern depression supposed to have been created 3.8 billion years ago during the ‘big wham’ between the Earth and the Moon.
Image: PAC, the Protected Antipode Circle, is the circular piece of land (1820 km in diameter along the Moon’s surface) that Maccone proposes to be reserved for scientific purposes only on the farside of the Moon. At the center of the PAC is the antipode of the earth (on the equator and at 180? in longitude). Credit: Claudio Maccone.
Where to locate our radio astronomy and SETI facilities within the vast region of the PAC? The problem is that this is a rugged region, but about 5? south along the 180? meridian (at 179 degrees east longitude, 5.5 degrees south latitude) we find the 80 kilometer Daedalus Crater. Daedalus is the most shielded crater of all from Earth-made radio pollution even as we extend our radio interference into space at the Lagrange points (and again, assuming we declare L2 off-limits).
Image: An oblique view of the Crater Daedalus on the Lunar Farside as seen from the Apollo 11 spacecraft in lunar orbit. The view looks southwest. Daedalus (formerly referred to as I.A.U. Crater No. 308) is located at 179? east longitude and 5.5? south latitude. Daedalus has a diameter of about 50 statute miles (? 80 km). Credit: NASA.
What about the L1 and L2 points of the Sun/Earth system? Here we have a problem, because there are already plans for placing satellites there (the original plan for NASA’s Space Inteferometry Mission was to place it at the Sun-Earth L2 point, for example). Maccone’s thoughts on this:
This radio pollution of the Moon Farside by scientific satellites located at the Lagrangian Points L1 and L2 of the Sun-Earth system is, unfortunately, unavoidable. We can only hope that telecom satellites will never be put there. As for the scientific satellites already there or on the way, the radio frequencies they use are well known and usually narrow band. This should help the Fourier transform of the future spectrum analyzers to be located on the Moon Farside to get rid of these transmissions completely.
Maccone’s hope is that the PAC will be approved by the United Nations, in pursuit of which he made his presentation in Vienna. Ultimately, declaring the area comprising the PAC an international land under the protection of the United Nations — or by direct agreement among the governments of the space-faring nations — is the only way we can hope to preserve the central farside from activities that would compromise its unique scientific value.
An urgent matter? You bet. It may seem like a remote future given our current problems, but the time will come when commercial and industrial interests will lead to more and more satellites in orbits much higher than geostationary, with consequent degradation to the farside’s ‘zone of quiet.’ Uncontrolled radio frequency interference is already a serious problem for Earth-based radio astronomy. The farside can be the ideal place for radio astronomy and SETI facilities, but only if we act to protect it.
For more on these issues and the mathematics of working out the best protected area location, see Maccone, “Protected antipode circle on the Farside of the Moon,” Acta Astronautica 63 (2008), pp. 110-118.
by Paul Gilster | Jun 17, 2010 | Missions |
When I was a kid, interstellar destinations were sharply defined. It seemed obvious that you didn’t even consider Alpha Centauri, because a double-star primary system surely wouldn’t allow stable planetary orbits. So you looked around for single stars. Moreover, these should be stars a lot like the Sun, so that when Frank Drake began SETI with Project Ozma, it made all the sense in the world to focus on Tau Ceti and Epsilon Eridani. Both were similar enough to our own star to suggest that they would have planets, and maybe one like ours.
For that matter, we had no idea in those distant days whether the Sun was a statistical fluke in having planets or simply a garden-variety star with a system that was all but inevitable. These days we keep finding interesting planets, but so far (other than perhaps in the Gliese 581 system) we haven’t found anything enough like the Earth to consider any nearby system an obvious target for an interstellar probe. All that may change, and swiftly, when we learn more about Alpha Centauri’s system, and if WISE finds a close brown dwarf. The latter two possibilities are going to be resolved within a few short years.
The Closest Probe Targets
The Project Icarus team — the interstellar probe study, not JAXA’s IKAROS sail — has been considering potential targets for their creation, just as the Project Daedalus crew did back in the ’70s (the glorious era of starship creation in the Mason’s Arms pub — there were giants in those days). Ian Crawford studies the matter in the Icarus blog, noting that a realistic maximum distance for an early probe is about 15 light years. This assumes a fusion starship design capable of 0.15c going after a target allowing a mission duration of no more than 100 years.
These are interesting numbers. As Crawford (University of London) notes, at present we know about 56 stars in 38 different stellar systems within this range. We have to be careful here, for not all stars within this volume of space have been discovered, and moreover, there are discrepancies, if slight, between the various catalogs of nearby stars. Crawford leans toward the RECONS (Research Consortium on Nearby Stars) catalog of one hundred nearest stars.
Now we find out just how tricky the target choice is. Let’s assume just for argument that the hunt for Alpha Centauri planets comes up dry. What’s the next possibility? It turns out that among the 56 closest stars, we have one spectral type A, which is Sirius, and one F (Procyon). Two G-class stars are available, Centauri A and Tau Ceti. Five K stars are possible, including Centauri B. But the overwhelming majority of nearby stars, 41 out of the 56, are M-class dwarfs. We round out the list with three white dwarfs and three probable brown dwarfs, although these numbers should increase with further WISE data.
The Case for ? Eridani
So far we know of planets around two of the 56 nearest stars, Epsilon Eridani (a K2 at 10.5 light years) and the M-dwarf GJ 674, which pushes our distance limit at 14.8 light years. So how about Epsilon Eridani? Crawford writes:
The planet orbiting epsilon Eri is a giant planet, with a mass about 1.5 times that of Jupiter. It has a highly eccentric orbit, which brings it as close to its star as 1.0 AU (i.e. the same distance as the Earth is from the Sun), to as distant as 5.8 AU (i.e. just beyond the orbit of Jupiter in our Solar System), with a period of 6.8 years. Although this would span the habitable zone (i.e. the range of distances from a star on which liquid water would be stable on a planetary surface given certain assumptions about atmospheric composition) for the Sun, this orbit lies wholly outside the likely habitable zone for a K2 star like epsilon Eri.
From an astrobiological perspective, then, Epsilon Eridani b doesn’t seem promising:
…being a gas giant, this planet itself it not a likely candidate for life, and its eccentric orbit wouldn’t help in this respect either (although it is possible that the planet may have astrobiologically interesting moons, perhaps similar to Jupiter’s moon Europa, which could in principle support sub-surface life).
Other planets in the Epsilon Eridani system? Maybe. An unconfirmed sub-Jupiter mass planet in a distant (40 AU) orbit may be there, and perhaps the system houses more Earth-like worlds. We’ll find out with further exoplanet investigation, and it should also be mentioned that Epsilon Eridani is circled by an interesting dust and debris disk. So we have to keep this young star on our list, even as we hold our thinking open about Alpha Centauri, and we have to realize how many other stars may soon be shown to have planets. Crawford again:
Clearly it would be of great interest if planets were discovered orbiting closer stars. Currently there have been no such planets discovered, but they are very likely to exist. Based on the detection rate to-date, and allowing for the known biases in the detection methods, it has been estimated that roughly 30% of main-sequence stars will have planets with masses less than 30 Earth masses. Thus, we might expect 16 or 17 of the nearest 56 stars to be accompanied by planets and, given the current lack of data on very low mass planets, it could easily be more. Although not targeted at any of the nearest stars, statistical results from the Kepler mission (which is looking for low-mass planets orbiting solar-type stars by the transit method…will greatly improve these estimates within the next few years.
Image: This artist’s diagram compares the Epsilon Eridani system to our own solar system. The two systems are structured similarly, and both host asteroids (brown), comets (blue) and planets (white dots). Epsilon Eridani is our closest known planetary system, located about 10 light-years away in the constellation Eridanus. Its central star is a younger, fainter version of our sun, and is about 800 million years old — about the same age of our solar system when life first took root on Earth. Observations from NASA’s Spitzer Space Telescope show that the system hosts two asteroid belts, in addition to previously identified candidate planets and an outer comet ring. Credit: NASA/JPL-Caltech.
Finding Out Where We’re Going
When I first started thinking about unmanned robotic flight to a nearby star, my unexamined assumption was that any such probe would deliver the first data we had about the presence of planets in that system. But that was twenty-five years ago, and now we’re in the exoplanet era. We’re building equipment of such sophistication that we can talk about detecting exoplanets from the ground, and future space-based missions will certainly do spectroscopic analysis looking for biosignatures on interesting nearby worlds. And all this will happen long before a true interstellar probe can be built, so we have plenty of time to work with. Unlike Daedalus (whose team chose Barnard’s Star because of a mistaken detection of planets there), the Icarus team knows enough to keep the parameters tight and wait for more information.
Crawford’s money is on Alpha Centauri in the end, not only because of its proximity but because it’s not a single target but three (assuming a sufficiently ingenious mission trajectory), each of a different stellar class. As for me, I’ll agree with Crawford most of the way, though I still hedge my bets by keeping an eye on WISE and the possibility of a brown dwarf within three light years. The beauty of all this, as I said above, is that we’ll have hard answers very soon.
