by Paul Gilster | Dec 14, 2006 | Exoplanetary Science |
If you’re thinking about detecting Earth-like planets around other stars, here’s an item that may set the pulse racing a bit faster. Michael Endl, who is an expert at the planet hunt around red dwarf stars (he’s searched for planets around 100 of them already), notes that the diminutive objects are prime targets for exoplanet hunters. And listen to this: “For the red dwarfs with the lowest masses, like Proxima Centauri, we are sensitive to planets down to two Earth masses using the standard radial velocity technique.”
Endl works at the University of Texas, out of which a study led by graduate student Jacob Bean has focused on planet formation around red dwarfs — we looked at this work not long ago. Few gas giants have been detected around red dwarfs. The study examined the dwarfs known to have planets: Gliese 876, Gliese 436, and Gliese 581. Of the three, Gliese 876 is perhaps the most intriguing, as it’s known to have two Jupiter mass planets and a likely third, lower-mass world orbiting around it.
Bean wanted to find out whether dwarfs like these known planet-bearers show high metallicity values (elements heavier than hydrogen and helium). Using computer modeling coupled with telescope observations, he was able to determine that the three dwarfs under study actually have significantly fewer metals than the 200 or so Sun-like stars known to harbor planets. Thus the possibility that the low number of high-mass planets found in our red dwarf surveys thus far may have something to do with low metallicity values in the stars selected for study.
And that makes sense, because a proto-stellar cloud with higher metallicity would be more likely to ‘grow’ a planet. Here’s co-author Fritz Benedict (University of Texas) on the matter:
“Just as rain drops need a speck of dust in the air around which to form, the formation of planets is thought to be assisted by a similar successful first step. More dust in the protoplanetary disk might increase the chances for planet formation.”
But it’s clear that these findings — on just three planetary systems — are the tip of the iceberg, so much work on red dwarf metallicity lies ahead. It’s significant because we need to know why there are so few gas giants around these stars, or whether there are gas giants in wider orbits that have simply not been discovered yet. We also need to know how best to target our planet searches, and if metallicity values for red dwarfs can be established in that regard, we can focus in on the most likely candidates.
I come back to Endl’s comment that radial velocity techniques are sensitive down to planets of just two Earth masses for some red dwarf studies. Given enough time to gather the RV data, we should begin to learn how often such worlds form around these stars. And Bean’s work on metallicity may help us estimate how often they’re joined by Jupiter-class objects in wider orbits. That’s exciting in its own right, but it also works toward filling in much broader patterns of planet formation that will help us characterize solar systems around a variety of stellar types.
The paper is Bean et al., “Metallicities of M Dwarf Planet Hosts from Spectral Synthesis,” Astrophysical Journal Letters 653 (December 10, 2006), L65-L68. And as referenced in our earlier article on this work, it’s also available at the arXiv site.
by Paul Gilster | Dec 13, 2006 | Outer Solar System |
If you’ve seen the Sierra Nevadas, you know what Bob Brown is talking about when he likens the mountain range found on Titan to those beautiful peaks in the western United States. Brown (University of Arizona) is team leader of the Cassini visual and infrared mapping spectrometer. Cassini was able to resolve features down to 400 meters (1300 feet) on its October 25 flyby. And suddenly we have a mountain range, dunes, and something resembling a volcanic flow under Titan’s inscrutable atmosphere.
Fascinatingly, at the top of the ridges are deposits of a white material that may be ethane snow or some other form of organic substance. Here’s Cassini scientist Larry Soderblom (USGS) on the organics:
“There seem to be layers and layers of various coats of organic ‘paint’ on top of each other on these mountain tops, almost like a painter laying the background on a canvas. Some of this organic gunk falls out of the atmosphere as rain, dust, or smog onto the valley floors and mountain tops, which are coated with dark spots that appear to be brushed, washed, scoured and moved around the surface.”
The Titan news is getting wide media coverage, so I’ll simply point you to this link to JPL’s news release on the subject. It’s interesting to me how quickly we get used to human operations in truly exotic places. After we’ve seen spectacular images like the backlit rings of Saturn, we tend to go about our daily routine forgetting that the vehicle is still out there and working. And then Cassini bobs back into the news with yet another spectacular image or paradigm-shifting dataset and we’re reminded just what an outstanding job our robotic explorers continue to perform.
A friend asked today how it was possible that we could be discovering entire mountain ranges for the first time on a world that had undergone so many surveys by Cassini. For the answer, examine the image below. These are composites made from radar, visual and infrared mapping spectrometer data. Take a close look and ponder what you’re seeing.
Image: This set of composite images was constructed from the best Cassini radar data and visual and infrared mapping spectrometer data obtained from all the Titan flybys up to the most recent flyby on Oct 25 (T20). The globe to the upper right is centered on 0 degrees longitude, and each of the other globes is labeled as to which longitude appears at the center of the disk. The two rightmost images in the bottom row are of the north and south poles of Titan, respectively. Credit: NASA/JPL/University of Arizona.
What stands out in the image is the sheer extent of what, if it were on Earth, we would call terra incognita. We see some detail from radar surveys, to be sure, and then the much more highly defined visual and infrared mapping spectrometer swathes. But you can see how little of the satellite’s surface is covered by the latter. In contrast to places like Triton, Titan seems more and more familiar, but in reality we’re only beginning to plumb its secrets. Ah for a Mars-style rover on that distant surface!
by Paul Gilster | Dec 12, 2006 | Deep Sky Astronomy & Telescopes |
What got me interested in Pismis 24-1 was simply the image. It’s one of those spectacular displays we’ve come to expect from Hubble, obtained using the telescope’s Advanced Camera for Surveys. Pismis 24-1 is part of the open cluster Pismis 24, some 8000 light years from Earth in the nebula NGC 6357 in Sagittarius. The cluster is filled with massive stars, but what seizes the attention is the juxtaposition of the cluster itself (the brightest stars in the image) and the gorgeous tapestries of the nebula in which it is embedded.
Pismis 24-1, the brightest star in the cluster, was originally thought to weigh up to 300 solar masses, making it twice the assumed upper mass limit for individual stars. But as the image below shows, it’s at least a binary, and ground based observations suggest that it may even be a triple system, with the third star too tightly bound to be resolved. If so, it’s a whopper of one, each of the three stars averaging about 70 solar masses. Ahead for these behemoths is a short lifetime of a few million years and a violent end as a supernova before the inevitable collapse into a black hole.
How unusual is this configuration? According to a Hubble news release, there are 18,000 solar-mass stars for every star with 65 solar masses or more. But the numbers are deceptive, since stars like our Sun live 3,000 times longer than their giant counterparts. So the galaxy today holds millions of solar-mass stars for each of its vast cousins. This work, conducted by Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía) should be useful as we firm up our knowledge of the conditions leading to supernova explosions, those titanic events that seed the universe with heavy elements.
Image credits: Credit: NASA, ESA, and J. Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain).
by Paul Gilster | Dec 11, 2006 | Exoplanetary Science |
You would think Alpha Centauri would be a prime hunting ground for extrasolar planets simply because of its proximity. But the problem for direct imaging is the sheer brightness of Centauri A and B, creating a halo of diffuse light around the pair. Getting through the glare isn’t easy, but a search based on twin techniques — adaptive optics and CCD imaging — covering a wide-field around the Centauri system has just been completed. Results on the CCD work, using European Southern Observatory equipment, have now been made available and they’ve come up short on planetary detections.
As reported by Pierre Kervella (Observatoire de Paris-Meudon) and Frederic Thévenin (Observatoire de la Côte d’Azur), the team found no co-moving companion objects between 100 and 300 AU. And that’s useful information, because it puts some constraints on possible planets around these stars. From the paper:
Within the explored area, this negative result sets an upper mass limit of 15-30 M J to the possible companions orbiting α Cen B or the pair, for separations of 50-300 AU. When combined with existing radial velocity searches…and our adaptive optics results…this mostly excludes the presence of a 20-30 M J companion within 300 AU.
First of all, note what this is not telling us. We can draw no conclusions about possible terrestrial-sized worlds orbiting within 3-4 AU of either Centauri A or B, for the equipment is not sensitive enough to detect planets that small. Thus the scenario that continues to fire the imagination of many of us — habitable planets around one or both Centauri stars — is still viable. We’ve simply learned that we can rule out massive super-Jupiters in wide orbits.
And that gives us further insight into the Alpha Centauri system itself, for some recent work has indicated that the mass of Centauri B could be higher than what earlier models have suggested. Specifically, radial velocity studies have come up with mass estimates that differ by 28 Jupiter masses (plus or minus 9) from the results of long-baseline interferometry. If the missing mass is in the form of an unseen companion, we can now exclude at least one planetary configuration that might have accounted for it.
The paper is Kervella and Thévenin, “Deep imaging survey of the environment of α Centauri,” accepted as a research note by Astronomy & Astrophysics and available as a preprint online. The team’s earlier work using adaptive optics (which feeds directly into the present paper) is Kervella et al., “Deep imaging survey of the environment of α Centauri: I. Adaptive optics imaging of α Cen B with VLT-NACO,” available here. Centauri Dreams‘ earlier story on the latter is also available.
by Paul Gilster | Dec 9, 2006 | Sail Concepts |
I don’t want to move past Gregory and James Benford’s interesting sail ideas without pausing to examine another paper that ran in the preceding issue of JBIS. It’s a look at what we might do in the near-term with solar sails, written by Gregory Matloff (CUNY), Travis Taylor (BAE Systems) and collaborators. And it focuses on what inspired Centauri Dreams (the book) in the first place, the question of where we stand right now in terms of deep space propulsion.
In other words, never mind the politics or the economics. If we had to launch a mission soon (obviously with a robotic rather than a human payload), how far could we go and how fast? Matloff and Taylor lay out the near-term possibilities for reaching the heliopause (roughly 200 AU) and the Sun’s inner gravity focus (550 AU) using both sails and other propulsion options. The reference sail used here is a 100 meter disc massing some 157 kilograms, with structure and payload adding up to 100 kg for a total mass of 257 kg.
Let me just quote the paper’s conclusion:
A number of options exist for near-term interstellar exploration using robotic solar-photon sailcraft including sail unfurlment at the perihelion of a parabolic solar orbit, sail unfurlment at the perihelion of an elliptical solar orbit, and Jupiter-gravity-assist after sail unfurlment from an elliptical solar orbit. Although all of these techniques can propel a 257 kg sailcraft with an areal mass thickness of 0.0082 kg/m2 to the heliopause (200 AU) within a human working lifetime, [and] to the Sun’s inner gravitational focus at 550 AU from the Sun in a human lifetime, they are all slower than equivalent missions launched using higher technology sailcraft.
Note what Matloff and Taylor are saying about mission times — this defines the human reach at present using a mix of solar sail and solar-electric technologies. Reaching 200 AU takes about a human working lifetime (i.e., some 40 years), while the gravity focus takes a full lifespan, averaged here at 80 years. A critical factor is the areal mass thickness of the sail. The authors draw on Italian theorist Giovanni Vulpetti, who developed calculations based on areal mass thicknesses that were considerably thinner (0.001-0.002 kg/m2). As you might expect, such vehicles are capable of higher cruise velocities.
The paper is Matloff et al., “Near-Term Interstellar Sailing,” Journal of the British Interplanetary Society Vol. 59 No. 2 (February 2006), pp. 59-62.
