by Paul Gilster | Apr 6, 2007 | Exoplanetary Science |
A Neptune-class planet has been discovered around the nearby red dwarf GJ 674, and it’s an intriguing one. Using the HARPS spectrograph on the European Southern Observatory’s 3.6 meter telescope at La Silla (Chile), the discovery team determined that the new planet was 0.039 AU from its parent star, yielding a temperature of some 450 degrees K. With a minimum mass estimate of about 11 times the mass of Earth, it completes an orbit every 4.69 days. Whether GJ 674 b is largely gaseous or rocky is unknown, although further observations of its orbital eccentricity may yield clues.
We’re not down to Earth-mass planets yet, but this is an interesting find. This is the second-closest known planetary system (after Epsilon Eridani). GJ 674 is less than 15 light years away and it’s one of the brightest M dwarfs in our field of view. That makes the transit situation interesting, as Greg Laughlin noted in this systemic post:
At first glance, such an effort might appear to be hampered by the fact that the star is young enough to show significant photometric variability in synch with its 35-day rotation period. A central transit, however, would have a duration of only ~80 minutes — much shorter than starspot-induced variations — and would generate a clearly detectable dip of at least ~0.5% photometric depth.
Such a transit would also help us nail the composition of the planet, no small benefit. But whether or not the Transitsearch team finds a transit here, the broader story emerging from our study of these small red stars continues to develop. Consider this: Two red dwarfs are known to host giant planets — GJ 876 and GJ 849 — and both are relatively metal rich in the M dwarf scheme of things. M dwarfs with planets appear to be slightly more metal-rich than those without, although the authors of this paper are reluctant to push the data too hard given the relatively small sample.
But we can draw other conclusions with a little more force. Laughlin and his collaborators at the University of California (Santa Cruz) have been arguing for some time that according to the ‘core accretion’ model of planet formation, we should find Neptune-mass planets around M dwarfs but not many Jupiter-class worlds. The discovery team for GJ 674 points out that none of the 300-plus M dwarfs examined for planets using radial-velocity techniques has yielded a hot Jupiter, whereas GJ 674b is the fourth hot Neptune found.
Significant? Here’s what Xavier Bonfils (Observatório Astronómico de Lisboa) and collaborators have to say in their paper on this work:
Though that cannot be established quantitatively yet, these surveys are likely to be almost complete for hot Jupiters, which are easily detected. Hot Neptune detection, on the other hand, is definitely highly incomplete. Setting aside this incompleteness for now, simple binomial statistics shows that the probability of finding no and 4 detections in 300 draws of the same function is only 3%. There is a thus 97% probability that hot Neptunes are more frequent than hot Jupiter around M dwarfs. Accounting for this detection bias in more realistic simulations…obviously increases the significance of the difference. Planet statistics around M dwarfs therefore favor the theoretical models which, at short periods, predict more Neptune-mass planets than Jupiter-mass planets.
So we’re learning more about how the mass of a star relates to the planets that may form around it. Nor is it insignificant that microlensing surveys that have detected four planets around M dwarfs have revealed two that are apparently below a tenth of Jupiter’s mass, further strengthening the argument that Neptune-mass worlds are likely companions to such stars.
And bear one other thing in mind: M dwarfs are small enough that a given planetary mass exerts a correspondingly greater ‘wobble’ effect on them than on larger stars. “As a result,” says the Bonfils paper, “the detection of an Earth-like planet in the closer habitable zone of an M dwarf is actually within reach of today’s best spectrographs.” But let’s go get that transit first.
The paper is Bonfils et al., “The HARPS search for southern extra-solar planets. X. A m sin i = 11 Mearth planet around the nearby spotted M dwarf GJ 674,” submitted to Astronomy & Astrophysics, abstract available.
by Paul Gilster | Apr 5, 2007 | Culture and Society |
Could a cloud of two-foot wide sunshades 60,000 miles long save the Earth from a global warming emergency? Roger Angel (Steward Observatory, University of Arizona) has been studying the idea of making the spacecraft out of micron-thick glass weighing one gram per sunshade. That’s the weight of a butterfly for each unit, but we’re talking about trillions of them out at the L1 Lagrangian point, an almost fixed zone in relation to Earth whose mild orbital instability can be overcome by onboard intelligence. Total sunshade mass: 20 million tons.
This article in the Arizona Daily Wildcat has more on the improbable concept and what Angel is doing today:
One of the big problems for the project is getting the total mass of all the sunshades into space…so Angel came up with using electromagnetic force to propel the spacecraft up a two-kilometer launch tube.
The launch tube would have a series of electrical coils that propel the rocket until it accelerates to escape velocity, about 25,000 mph, the speed needed to escape Earth’s gravity.
Hmmm… A two-kilometer tall launch tube would be an architectural marvel but prohibitive in cost, and the electromagnetic liftoff would likely shred the sunshades on the way up. With NASA funding terminated, Angel may not have time to work out these problems; he’s now looking for ways to keep us from having to use sunshades in the first place by tackling global warming here on Earth. If the shades are ever launched, though, don’t expect immediate results. One to two centuries seems to be a realistic figure for dropping carbon dioxide levels and lowering the heat.
ADDENDUM: A better way to put that last sentence would be: “One to two centuries seems to be a realistic figure for lowering the heat to compensate for greenhouse effects.” See Paul Dietz’ comment below.
And note this: Dr. Angel developed this concept as a potential solution to a planetary emergency, not as a final fix. He’s quoted on this in a Steward Observatory news release: “The sunshade is no substitute [for] developing renewable energy, the only permanent solution. A similar massive level of technological innovation and financial investment could ensure that. But if the planet gets into an abrupt climate crisis that can only be fixed by cooling, it would be good to be ready with some shading solutions that have been worked out.”
by Paul Gilster | Apr 4, 2007 | Culture and Society |
Back in the 1980’s, I was active as a shortwave listener. I was, in radio jargon, an SWL and not a ham, meaning I only listened and didn’t transmit. It was great fun to tune in distant stations, and the more challenging the better, which is why the Falkland Islands were always high on the list (I never received their station), and Tristan da Cunha was the ultimate catch (all but impossible here on the US east coast).
It wasn’t long before I drifted into utility DXing, listening for non-broadcast stations in remote places, everything from low-frequency aviation beacons to ship-to-shore communications, and I got a kick out of monitoring radiotelephone traffic from places like Little America (Antarctica) back to the States. Finally my interests converged and I started thinking about the ultimate DX — receiving a signal from the stars.
SETI efforts were in their early days then, but I began to wonder whether an amateur receiving rig could hope to snag some kind of extraterrestrial utility beacon. I joined SARA, the Society of Amateur Radio Astronomers, but finally realized that my talents lay in writing, not wiring, and that I didn’t have the skills to put together the equipment I needed. It’s a pleasure, though, to see that SARA is still active and that the SETI bug has now become more broadly established within the organization.
Now affiliated with the SETI League, SARA will be holding its annual technical conference at a storied place, the National Radio Astronomy Observatory in Green Bank, WV. This is where Frank Drake first turned human receivers on specific stars, choosing Epsilon Eridani and Tau Ceti as his targets and more or less inventing the modern discipline of SETI (interesting earlier ideas stretching back considerably farther in time also figure in to SETI’s lineage, about which more some other time).
The conference, to be held July 1-3, covers everything from gamma ray burst detection to astro-chemistry with a fine array of speakers you can see here along with abstracts of their talks. The SETI League itself is an attempt to privatize SETI work, reminding us of the contributions of amateurs as well as interested professionals in carrying on the search. And that reminds me of something Freeman Dyson said in a recent interview about the role of amateurs and the scientific hierarchy.
I like to remind young scientists of examples in the recent past when people without paper qualifications made great contributions. Two of my favorites are: Milton Humason, who drove mules carrying material up the mountain trail to build the Mount Wilson Observatory, and then when the observatory was built got a job as a janitor, and ended up as a staff astronomer second-in-command to Hubble. Bernhardt Schmidt, the inventor of the Schmidt telescope which revolutionized optical astronomy, who worked independently as a lens-grinder and beat the big optical companies at their own game. I tell young people that the new technologies of computing, telecommunication, optical detection and microchemistry actually empower the amateur to do things that only professionals could do before.
Dyson himself is an example, a man who simply became too busy to find time to get the standard degree (he had joined the Cornell faculty in 1951 as a physics professor without a PhD), and whose contributions have kept him similarly engaged ever since. In a 2005 commencement address at the University of Michigan, Dyson said he had “…fought all my life against the PhD system and everything it stands for.” While he is hardly an amateur, this remarkable scientist reminds us of the range of technologies that open up research to people wherever they stand in terms of formal credentials.
Sometimes I chuckle at the folly of my own preconceptions. I had thought until about ten years ago that I had missed out on the great era of amateur radio, assuming it to have occurred back in the 1920’s and 30’s, when people built their own equipment in their basements and television had yet to invade the home. But we’re in a golden age right now, in radio and much else, and looking at the resources available with a touch of my keyboard sometimes makes my head spin. It’s great to see the continuing efforts of the good people at SARA and the SETI League as they push the state of the art with their own work.
by Paul Gilster | Apr 3, 2007 | Asteroid and Comet Deflection |
The potential threat from near-Earth asteroids can sometimes seem purely theoretical, an academic exercise in how orbits are calculated and refined. But when we start quantifying possible damage from an asteroid strike, the issue becomes a little more vivid. Modeling potential impact points all over the planet, a University of Southampton (UK) team has worked out some stark numbers. The University’s Nick Bailey presented the results at the recent Planetary Defense Conference in Washington.
The researchers put a software package called NEOimpactor to work on asteroids under one kilometer in diameter and assumed an impact speed of 20 kilometers per second. Obviously, larger objects are out there and the impact velocity is arbitary, but asteroids in this size range seem to hit the Earth every 10,000 years, frequent enough that the next one that does hit will probably fit this description. Says Bailey:
‘The consequences for human populations and infrastructure as a result of an impact are enormous. Nearly one hundred years ago a remote region near the Tunguska River witnessed the largest asteroid impact event in living memory when a relatively small object (approximately 50 metres in diameter) exploded in mid-air. While it only flattened unpopulated forest, had it exploded over London it could have devastated everything within the M25.’
Indeed, while a 100 meter asteroid could cause relatively localized damage across several countries, doubling the object to 200 meters causes tsunamis on a global scale, assuming an oceanic hit. In terms of casualties, the study sees China, Indonesia, India, Japan and the US as the most vulnerable, though obviously a direct hit on any heavily populated area would be catastrophic.
Economically speaking, where the infrastructure is tells much of the tale. Put dense development along the coastlines of economically prosperous areas and you open yourself to the threat of tsunamis and earthquakes emmanating from a wide variety of impact areas. Sweden’s long coastline thus places it in high danger economically, while an impact in the north Atlantic could send devastating tsunamis into both Europe and America. Severe economic effects would clearly result from a strike involving China or Japan.
Image: The areas of maximum infrastructure vulnerability following an asteroid impact. Credit: Nick Bailey/University of Southampton.
Although we’re currently engaged through projects like the Spaceguard survey in cataloguing NEOs larger than one kilometer in diameter, the smaller objects represented in the Southampton study are largely undetected. The risk of being blindsided by such an object emphasizes our need to develop a space-based observation platform for tracking asteroids of this size, along with providing more accurate information about the movements of larger Earth crossers. Bailey again: “The threat of the Earth being hit by an asteroid is increasingly being accepted as the single greatest natural disaster hazard faced by humanity.”
by Paul Gilster | Apr 2, 2007 | Sail Concepts |
A Finnish team has introduced a new wrinkle on the solar sail idea. Or more specifically, on the general principles of the magnetic sail, which would tap the propulsive power of the solar wind to push a ‘sail’ created as a field around the spacecraft itself. The so-called ‘electric sail’ would use fifty to one hundred 20-kilometer long charged tethers, their voltage maintained by a solar-powered electron gun aboard the vehicle. We’re talking about tethers made of wires that are thinner than a human hair, thin enough that each can be wound into a small reel.
But unwind the tethers and you get interesting results. The electric field of each wire now extends tens of meters into the solar wind flow. A single tether yields the equivalent effective area of a sail roughly a square kilometer in size. You can see the promise of deploying multiple tethers to reach high velocities. What’s more, this sail allows the spacecraft to ‘tack’ towards the Sun as well as sailing outward from it.
All told, the electric sail points toward faster transport within the Solar System. The team, whose work was published in late March, has been running supercomputer simulations to verify the concept. Their numbers are encouraging: taking an average figure for the solar wind (which can range from 300 to 800 km/s), their sail can generate speeds up to 100 km/s, which would get a probe to Pluto in less than four years and into the nearby interstellar medium in fifteen.
Image: In this phase the wires have been deployed and the electron gun has been started. The blue lines symbolise the electron beam of the gun. The spinup propulsion arms and associated fuel tank have been jettisoned to save mass. The solar wind acts on the wires, bending them slightly. The electric field around the wires is depicted by a dashed red line. Credit: P. Janhunen/Kumpula Space Centre.
Although we lose the solar wind as we move beyond the outer planets, the Finnish design does have interstellar implications, at least by extension. From the paper:
In interstellar space the plasma ?ow is rather slow. Thus the electric sail cannot be used for acceleration, but it can instead be used for braking the spacecraft. There are some concepts such as the laser or microwave sail which are designed to “shoot” a small probe at ultrahigh speed towards e.g. a remote solar system. In these concepts the power source is at Earth so that the accelerated probe needs no propulsive energy source. Stopping the probe at the remote target is very dif?cult, however, if one has to rely on power beamed from the starting point. The electric sail might then provide a feasible stopping mechanism for such mission concepts. In other words, one would shoot a probe to another solar system at ultrahigh speed using a massive and powerful laser or microwave source installed in near-Earth space, brake the probe before the target by the electric sail action in the interstellar plasma and ?nally explore the extrasolar planetary system with the help of the electric sail and the stellar wind. A similar idea was proposed by Zubrin and Andrews (1990) for their magnetic version of the solar wind sail.
Note the difference between the electric sail (and magsail concepts like Robert Winglee’s M2P2) and the standard solar sail. For one thing, the electric sail relies on a stream of charged particles (the solar wind) to push it, while a solar sail taps the momentum of solar photons. For another, a solar sail is a physical structure that faces problems of deployment that only get magnified as you go to larger and larger sail designs. An electric sail’s deployment problem comes down primarily to tether extension, a much less demanding proposition. And that puts sails front and center as we look to ramp up travel times to the outer planets.
The paper, available for download, is Janhunen et al., “Simulation study of solar wind push on a charged wire: basis of solar wind electric sail propulsion,” Annales Geophysicae 25 (March 29, 2007), pp. 755–767. And here’s a backgrounder, including an interesting animation, from the Kumpula Space Centre in Helsinki.
