by Paul Gilster | Aug 18, 2009 | Astrobiology and SETI |
Chalk up another win for the ‘life is ubiquitous’ school of thought. We now know that when the Stardust spacecraft passed through the gas and dust surrounding comet Wild 2 back in 2004, it captured samples that include glycine. Living things use glycine to make proteins, which made the preliminary detection of this amino acid a significant event, though one that had to be carefully analyzed. After all, terrestrial contamination could have accounted for the glycine gathered up by Stardust.
Image: The comet Wild 2 as imaged by the Stardust spacecraft. Credit: NASA/JPL.
Ensuing work, however, has ruled out the contamination scenario. The space-gathered samples show significantly more Carbon 13 than glycine from Earth, an isotopic marker that identifies the material as originating in the comet. That gets us back to a welcome thought, that life is common in the universe. Carl Pilcher (NASA Astrobiology Institute) has this to say:
“The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the universe may be common rather than rare.”
We’ve never found an amino acid on a comet before, but we’re now gaining evidence that points to the delivery of critical ingredients to the early Earth by comet and meteor impacts. Donald Brownlee (University of Washington) calls the discovery “a remarkable triumph that highlights the advancing capabilities of laboratory studies of primitive extraterrestrial materials.” More in this NASA news release. The paper is slated to appear in Meteoritics and Planetary Science.
by Paul Gilster | Aug 17, 2009 | Asteroid and Comet Deflection |
A recent report from the National Academy of Sciences points out that NASA has been tasked to locate 90 percent of the most deadly objects that could conceivably strike our planet. Yet only about a third of this assignment has been completed, and the money has yet to be found to complete the job. The agency calculates it needs about $800 million between now and 2020 to make the needed inventory, while $300 million would allow it to find most objects larger than 300 meters across.
The problem is that even the smaller sum is not available, and this AP story quotes space policy expert John Logsdon (George Washington University) as saying the money may never come through, calling the program “a bit of a lame duck.” In other words, there is not yet enough pressure on Congress to produce the needed funds. Meanwhile, asteroid detection remains a low priority for other governments as well, making this a problem we’re choosing to ignore in the absence of recent reminders of its potential.
Asteroid Numbers and Risk
The absence, at least, of recent reminders on Earth — we just saw what happened on Jupiter, with its admittedly larger gravitational well. The comet strike on that giant world reminds us of NASA’s current estimate that there are 20,000 objects — comets and asteroids — that are potential threats to our own world, each larger than 140 meters in diameter. We know the position of about a third of these. The AP story cites Lindley Johnson, manager of NASA’s Near Earth Object Program:
At the moment, NASA has identified about five near-Earth objects that pose better than a 1-in-a-million risk of hitting Earth and being big enough to cause serious damage, Johnson said. That number changes from time to time, as new asteroids are added and old ones are removed as information is gathered on their orbits.
The space rocks astronomers are keeping a closest eye on are a 430-foot (130-meter) diameter object that has a 1-in-3,000 chance of hitting Earth in 2048 and a much-talked about asteroid, Apophis, which is twice that size and has a one-in-43,000 chance of hitting in 2036, 2037 or 2069.
A New Asteroid Population Near Earth?
Meanwhile, an interesting paper by Takashi Ito (National Astronomical Observatory, Tokyo) and Renu Malhotra (University of Arizona) looks at the asymmetric distribution of craters on the lunar surface, questioning whether what we now know about near-Earth asteroids can account for what we see there. Various possibilities exist, including tidal forces breaking asteroids apart to create more numerous craters than we would expect, but there is also the possibility that there is an undetected population of objects co-orbiting with the Earth that has yet to be detected.
To study the issue, the authors simulated the orbital evolution of a large number of test particles representing near-Earth asteroids, working with one population made up of currently known NEAs, and one created as a synthetic group outside the known NEA orbital distribution. What emerged is interesting when weighed against lunar observations:
The NEA-like particles that we used in our numerical integrations, particularly the population A that does not include the particles with large random orbital velocity, have low relative velocity with respect to the Earth-Moon system. In other words, these particles are the “slowest” (relative to Earth) among all the known small body populations in the solar system.
Can the particles accurately model what we see on the Moon? The paper continues:
The fact that even these slow particles do not fully reproduce the asymmetric distribution of impacts as large as what is seen in the lunar crater record, suggests that there may exist a presently-unobserved population of small objects near the Earth’s orbit that have even lower average relative velocity than the currently known near-Earth asteroids do.
We end up with the possible existence of a population of slow objects orbiting the Sun close to the Earth-Moon system. Based on their simulations, the authors estimate the population of ‘slow NEAs’ as roughly fifty percent more than the fraction of known slow NEAs. The definition of ‘slow NEA’ is a near-Earth asteroid with a potential lunar impact velocity of less than 11 kilometers per second. These are objects, in other words, that are nearly co-orbiting with the Earth.
Observing Programs Not Optional
Do such objects exist? The authors note the need for more complete observational surveys of near-Earth asteroids to test their prediction. And they’re careful to run through the alternatives, which include (in addition to asteroid fragmentation) the possibility that future mapping missions will give us a better dataset regarding the crater asymmetry.
Whatever we learn about lunar cratering, this paper yanks our chain yet again — we need to learn much more about the objects that could do our planet harm. We’re finally approaching the technological stage when we could actually do something about a predicted impact. What remains to be seen is whether we’ll firm up our observing programs in time to put that technology to work in the event we spot something truly dangerous.
The paper is Ito and Malhotra, “Asymmetric impacts of near-Earth asteroids on the Moon,” submitted to Astronomy & Astrophysics (preprint).
by Paul Gilster | Aug 14, 2009 | Asteroid and Comet Deflection |
A note the other day from astrodynamics wizard Edward Belbruno (Princeton University) has put me in mind of the ongoing study of the L4 and L5 points being conducted by the STEREO mission. STEREO is a two-spacecraft observatory designed to study solar activity, but in September and October the craft will be making their closest approach to the two gravitational wells at L4 and L5, and it’s possible we’ll discover a resident population of asteroids in the process. If so, we may be looking at material from the birthplace of a long-gone planet.
Call this hypothetical world, as Belbruno does, Theia. We looked at this secondary mission for STEREO last February, but as Belbruno passed along a link to the 2005 paper on the subject of Theia for which he was lead author, it’s time to revisit it. The paper is a lively piece of work, noting that current thinking is that our Moon was the result of a giant impactor, a planetary-sized object that hit the Earth and produced debris that eventually formed the satellite. Formation from the iron-poor debris from the impactor’s mantle helps us explain the Moon’s low density relative to the Earth and also why the Earth and the Moon have the same oxygen-isotope abundances.
That impactor — Theia — is quite interesting. From the paper:
[The theory] explains why Earth and the Moon have the same oxygen-isotope abundances — Earth and the giant impactor came from the same radius in the solar nebula. Meteorites originating from the parent bodies of Mars and Vesta, from different neighborhoods in the solar nebula, have different oxygen-isotope abundances. The impactor theory is able to explain the otherwise paradoxical similarity between the oxygen-isotope abundance in Earth combined with the difference in iron. This is perhaps its most persuasive point.
How does STEREO fit in? The spacecraft will be studying the Lagrange points because it is there that the gravitational pull of the Sun and the Earth balance out, as shown in the diagram below. Belbruno and co-author Richard Gott speculate in this paper that planetesimals could have been trapped in L4 and L5, leading to the accretion of a planet at one of these locations. They point to the Trojan asteroids at Jupiter’s L4 and L5 points and similar objects associated with Mars and Saturn.
Note in the diagram that L4 and L5 allow a unique balance of forces, as explained in the accompanying caption.
Image: In the above contour plot we see that L4 and L5 correspond to hilltops and L1, L2 and L3 correspond to saddles (i.e. points where the potential is curving up in one direction and down in the other). This suggests that satellites placed at the Lagrange points will have a tendency to wander off (try sitting a marble on top of a watermelon or on top of a real saddle and you get the idea). A detailed analysis confirms these expectations for L1, L2 and L3, but not for L4 and L5. When a satellite parked at L4 or L5 starts to roll off the hill it picks up speed. At this point the Coriolis force comes into play – the same force that causes hurricanes to spin up on the earth – and sends the satellite into a stable orbit around the Lagrange point. Credit: NASA/Neil J. Cornish (WMAP).
But although both L4 and L5 are stable, gravitational perturbations from other planetesimals could eventually cause Theia, our Mars-sized impactor, to escape. The paper examines the mechanisms by which this could happen in considerable detail, concluding with this scenario:
Debris remains at L4 (as the Trojan asteroids prove). From this debris a giant impactor starts to grow like Earth through accretion as described above. As the forming giant impactor reaches a sufficient mass (~0.1mE), it gradually moves away from L4 through gravitational encounters with other remaining planetesimals and it randomly walks in peculiar velocity. It gradually moves farther and farther from L4 approximately on Earth’s orbit in a horseshoe orbit at 1 AU, until it acquires a peculiar velocity of approximately 180 m s-1. The giant impactor then undergoes breakout motion in which it performs a number of cycles about the Sun, repeatedly passing near Earth. In a time span roughly on the order of 100 years, it collides with Earth on a near-parabolic orbit.
Thus STEREO’s ongoing task will be to look for evidence of asteroids at L4 and L5, ancient planetesimals that never became part of the impactor. As this NASA news release from last April notes, telescopic searches from Earth have turned up no such objects, but have only been able to rule out kilometer-sized asteroids. STEREO will be looking at much closer range. As the above diagram shows, the L4 and L5 Lagrange points are actually large regions of space (some fifty million kilometers wide, according to this NASA news release) so the examination will be lengthy.
Public participation is encouraged and you can get involved in the L4/L5 campaign here. Meanwhile, the paper in question is Belbruno and Gott, “Where Did the Moon Come From?” Astronomical Journal Vol. 129 (March, 2005), pp. 1724-1745, available online. More on asteroid detection and its attendant problems on Monday.
by Paul Gilster | Aug 13, 2009 | Exoplanetary Science |
Why some planets are the size they are remains something of a mystery. I’m looking at the discovery paper for a planet called WASP-17b, which is said to be twice Jupiter’s size but only half its mass. That raises questions about the mechanisms at work, for you can’t explain the bloated nature of this world with the models of planetary evolution we’re now working with without factoring in massive tidal effects.
In one sense, WASP-17b is completely anomalous. In addition to its size, it orbits its star in retrograde fashion, opposite the direction of the star’s spin. But in other important respects, the new planet joins the ranks of other bloated worlds like HD209458b (the first such world to be discovered), and a flock of other huge planets that includes TrES-4, WASP-12b, XO-3b and HAT-P-1b. TrES-4 shows a density about fifteen percent that of Jupiter, with a radius 1.78 times larger than Jupiter’s.
Image: Orbiting close to its parent star, WASP-17b may look something like this, a huge world bloated by the tidal forces produced by its highly unusual orbit. Credit: NASA/Hubble.
But WASP-17b now seems to take the prize in terms of size. Its primary, about 1000 light years away, is an F6 star in Scorpio, which it transits every 3.7 days. So bloated is this object that Coel Hellier (Keele University, UK) says it is “…only as dense as expanded polystyrene, seventy times less dense than the planet we’re standing on.” That makes it the least dense planet known, and tidal effects seem to be the only way to account for the fact. From the paper (internal references deleted for brevity):
It has been proposed… that tidal dissipation associated with the circularisation of an eccentric orbit is able to substantially inflate the radius of a short-orbit, giant planet. If a planet is in a close (a < 0.2 AU), highly eccentric (e > 0.2) orbit then planetary tidal dissipation will be significant and will shorten and circularise the orbit. Orbital energy is deposited within the planet interior, leading to an inflated planet radius. This process is accelerated by higher atmospheric opacities: as the planet better retains heat, shrinking of the radius is retarded, and a larger radius causes greater tidal dissipation.
This planet’s tidal effects — compression and stretching — must be remarkably powerful. The evidence that WASP-17b is in a retrograde orbit is strong, and the authors note that the angular momenta of star and protoplanetary disk (and thus the planets that emerge from that disk) all derive from the same parent molecular cloud. By all rights, this planet should be orbiting in the same direction as its star.
The theory, then, is that WASP 17-b started out in a prograde orbit and migrated to its current separation of 0.05 AU, being flung in the process into its unusual orbital arrangement. A disruptive brush with another planet or star is about the only thing that could explain the result. The authors note that radial velocity measurements could look for planets further out in this system that might have been involved in this early event, but it is possible that such worlds were ejected from the system in the process. WASP-17b may be the only giant planet remaining.
The paper is Anderson et al., “WASP-17b: an ultra-low density planet in a probable retrograde orbit,” submitted to the Astrophysical Journal and available as a preprint.
by Paul Gilster | Aug 12, 2009 | Astrobiology and SETI |
When it comes to exoplanet speculations, we’re still in the era when data are few and dominated by selection effect, which is why we began by finding so many ‘hot Jupiters’ — such planets seem made to order for relatively short-term radial velocity detections. It’s a golden age for speculation, with the promise of new instrumentation and a boatload of information from missions like Kepler and CoRoT to be delivered within a few years. What an extraordinary time to be doing exoplanetary science.
The big questions can’t be answered yet, but it shouldn’t be long before we have an inkling about what kind of stars are most likely to produce terrestrial planets. And maybe a qualification is in order. M-dwarfs are so common in our galaxy — some estimates run to seventy percent of all stars and up — that finding habitable worlds around them would hugely increase the possible venues for life. But is there any way we could call planets around M-dwarfs ‘Earth-like?’ Maybe in terms of temperature in a specific habitable zone on the surface, but little else applies.
M-dwarfs vs. the Early Sun
An M-dwarf planet in the habitable zone is, around many such stars, going to be subject to the kind of solar flare activity that could either prevent life from gaining a foothold altogether, or else serve as an evolutionary stimulus. Either way, conditions like this don’t seem Earth-like, even if the early Earth was subjected to its own barrage of harsh ultraviolet radiation before life forms could produce enough oxygen to yield an ozone layer. Rocco Mancinelli (SETI Institute) talked about this at the recent IAU Symposium on Solar and Stellar Variability — Impact on Earth and Planets. Here he discusses the importance of UV:
“We also see ultraviolet radiation as a kind of selection mechanism. All three domains of life that exist today have common ultraviolet protection strategies such as a DNA repair mechanism and sheltering in water or in rocks. Those that did not were likely wiped out early on.”
Clearly, intense ultraviolet can’t be considered prohibitive to life. But again, that’s from Earth’s history around a G-class star. In addition to their flare activity, M-dwarf planets are likely to be tidally locked, producing weather patterns that will keep meteorologists up nights and probably reducing habitable zones to specific areas on the Sun side. Life may well be possible, but even if Kepler turns up M-dwarf rocky worlds in large numbers, we’ll be talking about conditions that are only marginally like Earth, though obviously with an astrobiological fascination all their own. What we may one day find living on such worlds should be exotic creatures indeed.
The Case for K-class Stars
Recently we’ve been kicking around the subject of K-class stars in the comments to various posts here, and with K stars we really can start talking about planets much more like our own. Here I fall back to the IAU meeting, where Jean-Mathias Grießmeier (ASTRON, The Netherlands) looked at the role of magnetic fields in determining how likely life is to develop. Such fields provide a shield against incoming charged particles from stellar mass ejections as well as pervasive solar winds. They also offer protection against high-energy cosmic rays.
And here’s the quote from Grießmeier that resonates with me. He’s looking at the kind of stars we might expect to find life around, and concludes that our Sun probably wouldn’t top a list of such stars as compiled by the average extraterrestrial astronomer:
“The Sun does not seem like the perfect star for a system where life might arise. Although it is hard to argue with the Sun’s ‘success’ as it so far is the only star known to host a planet with life, our studies indicate that the ideal stars to support planets suitable for life for tens of billions of years may be a smaller slower burning ‘orange dwarf’ with a longer lifetime than the Sun ? about 20-40 billion years. These stars, also called K stars, are stable stars with a habitable zone that remains in the same place for tens of billions of years. They are 10 times more numerous than the Sun, and may provide the best potential habitat for life in the long run.”
K stars — now we’re talking! A stable habitable zone that offers a long period for life’s development, and a population that far outnumbers G-class stars like the Sun. It’s nice to speculate about the closest such star, the K-dwarf Alpha Centauri B, but of course we still have to resolve the question of planetary formation in binary systems like this one. We should have some answers fairly quickly, what with two ongoing attempts to find planets in the Alpha Centauri system, and may well know about Centauri planets before we start getting hard returns from Kepler.
Describing a Life-Bearing Planet
What does Grießmeier lean to when it comes to planets that would make good astrobiological candidates? Planets more massive than the Earth by two or three times, where higher gravity can make it easier to retain the atmosphere, and a larger liquid iron core offers robust magnetic field protection. The clincher here is the slower cooling of such a planet, allowing it to keep that magnetic protection for longer periods.
Meanwhile, Manfred Cuntz (University of Texas, Arlington) told the IAU meeting about his own team’s work on ultraviolet radiation and its effect on DNA. This is also quite interesting:
“The most significant damage associated with ultraviolet light occurs from UV-C, which is produced in enormous quantities in the photosphere of hotter F-type stars and further out, in the chromospheres, of cooler orange K-type and red M-type stars. Our Sun is an intermediate, yellow G-type star. The ultraviolet and cosmic ray environment around a star may very well have ‘chosen’ what type of life could arise around it.”
So many life factors, and so many stars to study! More in this IAU news release, which also looks at Edward Guinan’s work at Villanova, where he’s been studying stars that are analogues to the Sun at various stages of their life cycles. Among the findings: The Sun rotated ten times faster four billion years ago than it does now, thus producing a far stronger magnetic field. Our young Sol emitted X-rays and ultraviolet radiation several hundred times stronger than the Sun does today.
