by Paul Gilster | Aug 24, 2011 | Outer Solar System |
2007 OR10 is an innocuous enough designation (discoverer Mike Brown calls it ‘an official license plate number’ based on date of discovery), but ‘Snow White’ isn’t. The dwarf planet that acquired the latter monniker from Caltech astronomer and KBO-hunter Brown seemed to deserve its name because at the time, Brown thought it was an icy chunk that had broken off from the dwarf planet Haumea. Ice in the outer system is almost always white, and that’s what you would expect on a world called ‘Snow White.’ But recent spectral analysis has revealed that while ‘Snow White’ is indeed covered in water ice, it’s not white at all. In fact, it is one of the reddest objects in the Solar System, about half the size of Pluto in its orbit at system’s edge.
What to make of this? It turns out that another dwarf planet fits the same characteristics, in being both red and covered with water ice. Although a bit smaller than Snow White, Quaoar is thought to have had an atmosphere and to have once been covered with ice-spewing volcanoes. As this Caltech news release points out, being smaller than the big dwarf planets Eris and Pluto, Quaoar could not hold on to volatile compounds like methane, carbon monoxide or nitrogen for long time frames. All that’s left at this point in our system’s history is methane, which over time and exposure to radiation would have been turned into reddish hydrocarbon chains.
Image: Caltech’s Mike Brown. Credit: California Institute of Technology.
So Quaoar, and possibly Snow White, are covered with irradiated methane, accounting for their hue. “You get to see this nice picture of what once was an active little world with water volcanoes and an atmosphere, and it’s now just frozen, dead, with an atmosphere that’s slowly slipping away,” adds Brown. It’s a view being pieced together with an instrument called the Folded-port Infrared Echellette (FIRE), used with the 6.5-meter Magellan Baade Telescope in Chile. The instrument’s spectral analysis revealing water ice told Brown just what he needed to know.:
“That combination—red and water—says to me, ‘methane,'” Brown explains. “We’re basically looking at the last gasp of Snow White. For four and a half billion years, Snow White has been sitting out there, slowly losing its atmosphere, and now there’s just a little bit left.”
Brown also talks about Snow White in a series of posts on Mike Brown’s Planets, from which this:
I love this spectrum of Snow White, since it tells a long complex history of a little icy world all in one glance. Snow White formed 4.5 billion years ago in the chaos that was the outer solar system. It had an evaporating atmosphere and a surface that was slowly gunking up from all of the frosts sitting in the sunlight on the surface. It would have been a cold, dark, uneventful place, until suddenly water burst out from the interior and began its slow slush flow on the surface before quickly freezing up. The volcanic period probably didn’t last long, and most of the atmosphere didn’t last much longer. The nitrogen went first, then the carbon monoxide. And finally, today, when we look at Snow White we see the very last gasps of a dying atmosphere covering a once dynamic but now dead and frozen world.
The methane finding will have to be confirmed as part of the larger study of volatile loss and retention on these distant objects. Volatiles have also been discovered on the surfaces of Eris, Makemake and Sedna. In fact, a model for volatile retention that has successfully explained the situation on these worlds seems to hold for every large KBO except Haumea, which is the parent body of a family of collision-born objects, and thus has a history more varied than most of its neighbors. From the paper on the spectral analysis, a clear view of where to go next:
While the size of 2007 OR10 has yet to be measured, the simple assumption that it has an identical albedo to Quaoar – the object whose spectrum its spectrum most resembles – places 2007 OR10 into a regime where it would be expected to retain trace amounts of methane on its surface. Such an object would be expected to have red optical coloration from methane irradiation, which both Quaoar and 2007 OR10 do have. In addition, such an object should have detectable signatures of methane if observed at sufficient signal-to-noise. Such methane signatures have been detected on Quaoar, but require higher signal-to-noise to positively identify on 2007 OR10. While additional measurements of the size and spectrum of 2007 OR10 are clearly required, we conclude that volatile retention models (Schaller & Brown 2007b) appear to continue to flawlessly predict both the presence and absence of volatiles on all objects in the Kuiper belt which have been observed to date.
Assuming that confirmation proceeds as planned, Snow White and Quaoar will stand apart from the vast majority of KBOs as being large enough to hold on to volatile compounds, a trait that helps us to analyze their subsequent history. The paper is Brown et al., “The surface composition of large Kuiper Belt Object 2007 OR10,” accepted by Astrophysical Journal Letters (preprint).
by Paul Gilster | Aug 23, 2011 | Exoplanetary Science, Uncategorized |
Ever more refined radial velocity searches for exoplanets are reaching into the domain of lower and lower mass targets. It’s natural enough that we’re most interested in planets of Earth mass and even smaller, but as a new paper on the work of the European Southern Observatory’s HARPS instrument reminds us, one of the great values of this work is that we’re getting a broad view of how exoplanets form and evolve in their systems, no matter what their size. Characterizing not just planets but entire systems is becoming a profitable investigation.
But small worlds continue to fascinate us, particularly in the hopes of finding possible abodes for life. HARPS’ involvement in the hunt now includes an intense campaign to monitor ten stars that are relatively near our Sun, all of them slowly rotating and quiet solar-type stars. Mounted on ESO’s 3.6-meter instrument at La Silla Observatory in Chile, HARPS (High Accuracy Radial Velocity Planet Searcher) has produced more than 100 exoplanet candidates in its first eight years of operation, including not just Neptune-mass planets but super-Earths and intriguing systems like Gliese 581, with two possibly rocky planets near the habitable zone.
Moreover, from the system-wide point of view, the system around HD 10180 includes seven low-mass planets including the 1.5 Earth mass HD 10180 b. So when HARPS talks, we listen, and I want to quote this from the paper at the outset (internal references omitted for brevity):
… a recent investigation of the HARPS high-precision sample has shown that about 1/3 of all sample stars exhibit RV variations indicating the presence of super-Earths or ice giants… Indeed, planet formation models… show that only a small fraction (of the order of 10%) of all existing embryos will be able to grow and become giant planets. Hence, we expect that the majority of solar-type stars will be surrounded by low-mass planets.
Good news for small planets! If this is the case, we would expect that even a small sample like the current ten solar-type stars now under intense investigation by HARPS will turn up several Earth-like planets (i.e., rocky worlds in the inner system), and the new paper does not let us down. Three of the host stars involved in this program have already produced detections; these are HD 20794, HD 85512 and HD 192310. There are no giant planets here but study of the three stars has thus far yielded six low-mass planets, including three super-Earths around HD 20794 (82 Eridani), with semi-major axes of the planetary orbits measured as 0.12AU, 0.20AU and 0.35AU. The semi-major axis measures the radius of an orbit taken at the orbit’s two most distant points.
No habitable zone planets here, though, with even the furthermost planet reaching likely equilibrium temperatures of 388 K, which works out to about 115 degrees Celsius. Remember that equilibrium temperature is not the same thing as temperature at the surface. The equilibrium temperature of the Earth without an atmosphere is 255 K ( -18 degrees Celsius), but adding in the various effects of our atmosphere we come to an average of 288 K (15 degrees Celsius), so it’s clear how careful we have to be with these numbers, given how little we know about the planets in question. The surface temperature of a planet with a dense atmosphere will depend upon our atmospheric models.
That issue applies to the system around HD 85512 as well, which is described as the most stable of the stars in the HARPS sample. This star is found to have a possible super-Earth in an interesting orbit indeed, with a semi-major axis of 0.26 AU and a computed equilibrium temperature of 298 K, one that could place this potentially rocky world within the inner edge of the habitable zone. As my friend Ronald Botterweg reminds me in one of the comments to an earlier post, this equilibrium temperature is not far from that of southern France about now, but again, that has to be adjusted for atmospheric effects (for a paper analyzing different atmospheric models for this planet, see Kaltenegger et al., linked to at the end of this post).
In fact, let me go ahead and quote from the Kaltenegger paper, which calls HD 85512 b “…with Gl 581 d, the best candidate for habitability known to date.”:
We focus our analysis on HD 85512 b. We show the influence of the measurement uncertainties on its location in the Habitable Zone as well as its potential habitability. We find that HD 85512 b could be potentially habitable if the planet exhibits more than 50% cloud coverage. A planetary albedo of 0.48 +/- 0.05 for a circular orbit, and an albedo of 0.52 for e=0.11 is needed to keep the equilibrium temperature below 270K and the planet potentially habitable.
And this:
If clouds were increasing the albedo of HD 85512 b, its surface could remain cool enough to allow for liquid water if present. HD 85512 b is a planet on the edge of habitability.
But back to the original HARPS paper. HD 192310 has been under investigation for several seasons following the earlier discovery of a Neptune-mass planet there. HARPS confirms that earlier discovery and adds another possibly Neptune-class world, the two semi-major axes being 0.32 AU and 1.18 AU. According to the paper, we’re again bracketing the habitable zone, with equilibrium temperatures on the order of 355 K and 185 K — possibly at the very inner and outer edges of the habitable zone, respectively.
So far, then, three of the ten stars observed in this program have yielded low-mass planets. From the paper:
Although statistics is poor over only ten targets, it is interesting to note that this 30% value was already announced by Lovis et al. (2009) who based their analysis on the larger (< 200 stars) HARPS high-precision program. Theoretical works by Mordasini et al. (2009) actually forecasted that the frequency of small Neptunes and super-Earths on short and intermediated orbits would be considerably higher than that of Saturns and Jupiters. The recent amazing discoveries made by the KEPLER satellite using the transit technique further strengthen this fact. Borucki et al. (2011) report that the probability of finding low-mass planets is considerably higher than for Jupiter or Saturn-mass planets. Furthermore, when summing up the frequency of finding a planet of any mass, they end up with a probability of about 30%, again in perfect agreement with the results of Lovis et al. (2009).
All good news for finding Earth-class worlds as we push the radial velocity method into this mass range. It’s interesting, too, to look at what this paper has to say about Alpha Centauri, Centauri B being one of the ten targets on the HARPS list for the study. As the work continues, the researchers have to contend with the bright magnitude of the Centauri stars, which “may result in poorer RV precision due to incomplete light scrambling across the spectrograph’s entrance slit.” Another major issue: Alpha Centauri B is a member of a triple star system, which means the radial velocity analysis must include a complete and precise orbital model. All of this is tricky but a thorough reading of the paper yields the conviction that HARPS is up to the task.
Tau Ceti is also a member of the list — this is one of the two stars from the original Project Ozma that Frank Drake made famous back in 1960 (the other being Epsilon Eridani). Tau Ceti as yet shows no planetary signatures, and again I’m going to turn to Centauri Dreams regular Ronald Botterweg, who has been in the thick of our ongoing exoplanet discussions for many years. Ronald analyzed the metallicity of the ten stars in the HARPS sample and found that eight of them have lower metallicity than the Sun (seven, in fact, have considerably lower metallicity than Sol). Which leads Ronald to quote a recent Greg Laughlin post on systemic:
“First, among host stars with masses similar to the Sun that harbor giant planets, there’s a strong preference for metal-rich stars. This is the classic planet-stellar metallicity effect. Second, among low-mass stars, there’s a dearth of giant planet candidates. This is the known giant planet-stellar mass effect.”
Interesting stuff, and I’m pleased at the way readers here have been digging into these papers, which not only alerts me to new work but points to issues I might otherwise have missed. Solar-type stars of low metallicity are places where we find few giant planets, the latter seeming to favor high-metallicity stars of solar size and larger. Meanwhile, the relatively high metallicity content of Centauri B, which might lead us to expect a gas giant, is presumably offset by its position as a close binary. We’ll now wait with great interest to see how the HARPS work continues on the vital and fascinating question of smaller worlds in the Alpha Centauri system. With two other teams also on the case, I suspect we won’t have to wait too much longer before we learn something definitive about the planetary situation around our nearest neighbor.
The paper is Pepe et al., “The HARPS search for Earth-like planets in the habitable zone: I — Very low-mass planets around HD20794, HD85512 and HD192310,” accepted by Astronomy & Astrophysics (preprint). See also Kaltenegger et al., “A Habitable Planet around HD 85512?” submitted to Astronomy & Astrophysics (preprint).
by Paul Gilster | Aug 22, 2011 | Outer Solar System |
New Horizons continues on its inexorable way to Pluto/Charon, now some 21 AU out, which places it between the orbits of Uranus and Neptune. The latest report from principal investigator Alan Stern tells us that the 2011 checkout of the spacecraft was completed on July 1, a two-month process that included a test of the REX radio occultation experiment, coordinating with the Deep Space Network as the Moon interrupted a radio signal from Earth. According to Stern, spacecraft tracking over May and June shows New Horizons on a ‘perfect course’ toward the distant world, one that will demand no course correction until, at the earliest, 2013.
I wanted to bring Stern’s report into play here because of the image below, which shows Pluto’s newly discovered moon P4 along with the other moons now known in the system. The fact that I hadn’t yet run it told me that it was time to do some catching up with this impressive mission.
Image: These two images, taken about a week apart by NASA’s Hubble Space Telescope, show four moons orbiting the distant, icy dwarf planet Pluto. The green circle in both snapshots marks the newly discovered moon, temporarily dubbed P4, found by Hubble in June. The new moon lies between the orbits of Nix and Hydra, two satellites discovered by Hubble in 2005. It completes an orbit around Pluto roughly every 31 days. Credit: NASA, ESA, and M. Showalter (SETI Institute)
P4 (the name is temporary) was found during a search for rings around Pluto. At an estimated diameter of 13 to 34 kilometers, it’s the smallest moon yet discovered in this system. Obviously, the more we learn about Pluto/Charon, the better the New Horizons team will be able to plan for its brief period of close up observations. And it’s likely we’ll find still more tiny moons in a system that is thought to have been formed by a collision between Pluto and another large body in the early Solar System. P4 was found with Hubble’s Wide Field Camera 3 on June 28 and later confirmed through subsequent imagery. Still no signs of any Plutonian rings, however.
Meanwhile, John Spencer, a member of the New Horizons mission science team, has posted an interesting look at the effort to find a Kuiper Belt object that New Horizons will study after the encounter with Pluto/Charon. We’ve talked before about the Ice Hunters project, where volunteers can help pursue the search using the power of networked computers at home. Hunter now reports on his trip to Hawaii, which provided the chance to work with the Subaru telescope on the summit of Mauna Kea, an experience would-be astronomers can only envy. Spencer talks about recapturing ‘the romance of the old way of connecting with the universe,’ something many working astronomers seldom do these days, and describes the trip to the top:
After a night and a day of acclimatization, adjusting our bodies to the thin air, we climbed into 4WD vehicles and made the half-hour drive to the summit as sunset approached. It is always an amazing transition from the relative domesticity of Hale Pohaku and its mamane trees to the vast, alien, apparently lifeless landscape of the summit and its giant telescopes. This was the first time at Subaru for some in our group, so Josh Williams, the telescope operator, gave us a quick tour of the darkened, cathedral-like space of the dome, almost filled by the huge bulk of the telescope with its 8-meter diameter mirror. We also made quick trip around the catwalk outside the dome, to admire the fabulous view, before returning to the warmth and comfort of the control room, where we were to spend the night.
The work involved calibration observations, warm-up tests using near-Earth asteoids, and finally the acquisition of the images that might lead to KBO finds. But toward the end of the observing period, a broken coolant hose ended operations (and kept Subaru down for another three weeks). Spencer says the team left with 70 percent of what they were there for, in any case, and his laptop hard disk returned from the journey with data that will appear soon on Ice Hunters.
Image: The Subaru dome (left) is silhouetted by the Milky Way, as the telescope searches for KBOs. The search area is among the star clouds in the upper left of the image. (Credit: John Spencer).
Whether or not New Horizons gets to make that flyby of a distant Kuiper Belt object depends upon a number of things, among them NASA approval of an extended mission (it’s hard to see how this could be turned down given the rarity of our getting a spacecraft this far from the Sun), as well as the discovery of an appropriate KBO. The New Horizons team is looking for an object at least 50 kilometers across for a flyby and high resolution imagery, as well as spectroscopic investigations and study of possible moons or traces of an atmosphere. I can only echo Spencer’s invitation for those of you who haven’t yet done so to join the Ice Hunters search today. Remember, although we’ve found more than 1000 objects beyond Neptune’s orbit, it’s estimated that there may be half a million objects bigger than 30 kilometers across out there.
by Paul Gilster | Aug 19, 2011 | Culture and Society |
Why think seriously about mounting an effort to reach the stars? In yesterday’s New York Times, Dennis Overbye runs through some of the basic drivers:
- The discovery of a habitable planet around a nearby star would create intense interest in sending a probe or, depending on how technology develops, mounting an expedition
- The demands of human nature include a basic restlessness that has always impelled us to explore
- The danger of a future impact from an asteroid or other space debris will force us to think not only about how to mitigate the threat, but also about a ‘backup’ plan for humanity
The article is worth looking at for the gorgeous Adrian Mann illustration alone — it shows a future starship on a ‘shakeout’ cruise near Jupiter. Overbye then goes on to discuss the 100 Year Starship Study and its upcoming symposium, with plentiful references to Project Icarus and the Tau Zero Foundation. It’s good to see the press continuing to focus on the real goals of the 100 Year Starship Study, given that Jill Tarter (SETI Institute) is quoted in the article as saying that some of the proposals for talks she has seen have been a ‘mixed bag.’ And she adds “Maybe you have to be a little bit crazy to think about this seriously.”
Or maybe not. Overbye refers to possibilities that are well within the realm of known physics even while they challenge (monumentally) our current engineering skills. I was glad to see reference, for example, to the kind of enormous solar sails that, boosted with a close pass by the Sun and made of incredibly thin and reflective materials, could get us to the Alpha Centauri system in a millennium. And although he isn’t mentioned by name in the article, Robert Forward’s ideas on ‘lightsails’ or ‘photon sails’ that would be pushed by laser or microwaves make an appearance, as do ion drives. Forward envisioned cutting the travel time to decades.
All of this means that propulsion systems galore must be on the table if we are looking toward a starship launch that might not occur in this century. Overbye refers to Marc Millis’ term ‘incessant obsolescence’ in terms of how technology may change as we pursue these studies, but what Millis really means by the term is the possibility that a starship launched with the fastest technologies of its day might eventually be caught by a faster one launched much later, leading to real questions about how long to wait before launching anything. It’s interesting to note that Andrew Kennedy, who has written about what he calls the ‘wait equation,’ will be a speaker at the upcoming 100 Year Starship Study symposium to be held in Orlando at the end of September.
“The agenda ranges far beyond rocket technology to include such topics as legal, social and economic considerations of interstellar migration, philosophical and religious concerns, where to go and — perhaps most important — how to inspire the public to support this very expensive vision,” writes Overbye, who calls the study “perhaps the ultimate startup opportunity.” Indeed, and the multi-disciplinary approach demanded by the challenge of starflight may be one of its greatest attractions. It forces us to acknowledge that if we are seriously talking about sending humans on what could be generations-long journeys, our investigations have to range far beyond propulsion into fields like biology, environmental science, sociology and psychology.
David Neyland (director of tactical technology for DARPA, and the man behind the 100 Year Starship Study) likes to talk about the tools we have now vs. our theoretical knowledge to develop them further. Would Einstein and Marconi have been able to come up with communications devices like cellphones if asked to sketch out a way for people to stay in touch in 1910? They surely had the scientific knowledge but were not at a position to foresee the engineering. The challenge of starship thinking is even greater. We don’t know for sure that we’re asking the right questions, making the need for divergent voices at the symposium that much greater.
But I like what Kelvin Long, founder of Project Icarus, has to say. “A lot of us are quite young. We grew up hearing about the Apollo program,” he said. “We want to be part of a significant journey. We personally think we may be doing something important, driving humanity out to the stars.”
If you’re intrigued by the ‘wait equation,’ see Kennedy, “Interstellar Travel: The Wait Calculation and the Incentive Trap of Progress,” Journal of the British Interplanetary Society Vol. 59, No. 7 (July, 2006), pp. 239-247.
by Paul Gilster | Aug 18, 2011 | Exoplanetary Science |
One of the problems with building a backlog of stories is that items occasionally get pushed farther back in the rotation than I had intended. Such is the case with an article in Astrobiology Magazine that talks about how much of a factor a large moon may be in making a planet habitable (thanks to Mark Wakely for passing the link along). It’s an interesting question because some have argued that without our own Moon, the tilt of the Earth’s axis, its ‘obliquity,’ would move over time from zero degrees to 85 degrees, a massive swing that would take the Sun from a position over the equator to one where it would shine almost directly over one of the poles.
The resulting climate changes would be severe, potentially affecting the development of life. The thinking is that just as the direction of the tilt of a planet varies with time — astronomers say that it ‘precesses’ — so does the orbital plane of the planet. The gravity of a large moon like ours affords a stabilizing effect by speeding up the Earth’s rotational precession and keeping it out of synch with the precession of the planet’s orbit. When rotational and orbital plane precession are synchronized, the obliquity begins to change chaotically. The Moon’s job, then, is to keep the two out of synch, minimizing the kind of fluctuations that would play havoc with life on the planet.
Image: The image shows Earth’s axial tilt (or obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left). Credit: Wikimedia Commons/Astrobiology Magazine.
Jason Barnes (University of Idaho) and colleagues are behind the latest work (presented at the most recent American Astronomical Society meeting) which suggests another interpretation, arguing that the effect of the Moon on the Earth’s obliquity has been overstated — the Moon is not in fact crucial for the development of life. We can hope this bears out, because estimates of how many terrestrial planets will have a substantial moon get as low as one percent. Most of these worlds, then, under previous thinking, would experience huge changes in their obliquity, pointing toward a ‘rare Earth’ conclusion.
But that thinking is under challenge. Recent work by Sebastian Elser (University of Zurich) argues that the chances of large moons for such planets are as high as 10 percent. And Barnes’ team contrasts the gravitational effects of the Moon with those of other planets orbiting the Sun. The conclusion: The Moon does provide some stability to our planet, but the pull of Jupiter and, to a lesser extent, other planets orbiting the Sun would tend to keep the Earth’s obliquity swings in check. In fact, Barnes has determined that the Earth’s obliquity without a moon would vary only ten to twenty degrees over half a billion years. That’s enough to cause major climate changes, but it would “…not preclude the development of large scale, intelligent life,” Barnes adds.
I was intrigued by the team’s findings about planets with retrograde motion in their orbits. With or without a moon, the obliquity variations of a planet in this configuration — spinning in the opposite direction from their star — should be smaller than those orbiting in the same direction as the star. Barnes believes that the initial rotation direction of a planet should be random, voicing his suspicion that “whatever smacks the planet last establishes its rotation rate.” If this is true, the odds on retrograde precession are 50/50, lengthening the odds for relatively modest obliquity.
So we get help from the spin of a planet where it is retrograde, and also the combined gravitational effects of other planets in the system that help to reduce the planet’s tilt. If Barnes and team are right, then worlds lacking a large moon are still very much in the running for the development of life, a stability that he reckons may account for 75 percent of the rocky planets in the habitable zone. We’re a long way from confirming that idea, but it’s refreshing to hear this assertion that a large moon may not be a sine qua non after all, given how little we know about exoplanetary moons and the likelihood of their emergence in the right size range.
