by Paul Gilster | Jul 19, 2006 | Exoplanetary Science |
Hard to believe that it’s been over ten years now since the discovery of 51 Peg B, the first exoplanet found around a main sequence star. So much has happened since, including over 160 exoplanet candidates identified within 200 parsecs, with most of these discovered through Doppler search methods examining radial velocities (although the nearby planet TrES-1 was found via transit methods). The last published list of exoplanets appeared in 2002, which is why the just published “Catalog of Nearby Exoplanets” is so welcome.
Appearing in The Astrophysical Journal, the catalog confines itself to the 200 parsec limit for good reasons. Yes, we have located exoplanets far beyond it, including some in the direction of galactic center found by the OGLE survey and several found through microlensing projects elsewhere. But within 200 pc, high resolution imaging and stellar spectroscopy allows satisfactory follow-up work, and we’re also working with stars whose parallax has been studied through the Hipparcos satellite. And as Paul Butler, Geoff Marcy and colleagues note in the paper, nearby host stars are bright enough to allow rich investigation by smaller telescopes, “…permitting careful assessment of velocity jitter, starspots, and possible transits.”
Why produce a catalog in a time of such rapid discovery? From the paper:
Without question, the catalog presented here will become out of date before it is printed. However, this catalog offers many attributes of unique value. First, it contains updated orbital parameters for 90 exoplanets, computed anew from our large database of Doppler measurements of over 1300 stars from the Lick, Anglo-Australian, and Keck Observatories obtained during the past 18, 7, and 8 yr, respectively (Butler et al. 2003; Marcy et al. 2005a). These new orbital parameters significantly supersede the previously quoted orbital parameters in most cases.
The updating is significant because the longer we observe a star using Doppler techniques, the more refined our datasets become. This allows further development of our models of various star systems and may help to reveal new planets within some of them. Refined estimates of stellar mass do their part in sharpening these calculations as well. Astrophysicist Greg Laughlin calls this a blockbuster paper and is using its data to re-examine the star that started it all, 51 Peg, hoping to tease a 51 Peg c out of the radial measurements.
There are five new exoplanets included in the catalog: HD 11964b, HD 66428b, HD 99109b, HD 107148b, and HD 164922b. Data for these detections was collected at the Keck Observatory. We wind up with 172 known exoplanets in the catalog, with this interesting concluding remark: “…the mass distribution increases sharply toward lower masses (roughly as the inverse of the minimum planetary mass) and toward larger orbital distance. Since these regions are where current surveys are most incomplete, this implies that many more low-mass and long-period planets await discovery as Doppler surveys cover a longer time baseline and become more precise.”
Which, of course, is what we are all waiting for. The scientists promise a forthcoming work that will discuss exoplanet candidates that fit this description. The paper is Butler, Wright, Marcy et al., “Catalog of Nearby Exoplanets,” The Astrophysical Journal 646 (20 July 2006), pp. 505-522, with abstract available here. This is an important work, and it seems worth mentioning that self-archiving on researchers’ home pages, a growing trend, would be a helpful way to get around publisher firewalls to make it available to a broader audience.
by Paul Gilster | Jul 18, 2006 | Exoplanetary Science |
We’ve recently discussed Greg Laughlin and Jeremy Wertheimer’s work on the possible role of Proxima Centauri in destabilizing the Centauri A and B debris disk and bringing volatiles to the inner system. Our deepening knowledge of the Centauri system is one of the most energizing aspects of the exoplanet hunt, for its proximity inexorably makes Alpha Centauri of high astrobiological interest.
And no one has done more significant work on planet formation around binary stars than Elisa Quintana and Jack Lissauer (NASA Ames). The two have examined the possibilities of terrestrial worlds around Centauri A and B and are continuing with the study of other binary scenarios. Now they have extended their analysis to binary systems whose stars are much closer to each other than Centauri A and B.
Their new paper is significant for planet hunters because more than half of all main sequence stars are in binary or multiple systems, whereas our basic models for planet formation have been based on single stars. That conventional model relies on the accretion process inside a disk of dust and gas that surrounds the newborn star. We’re now finding disks of such materials around binaries, with the mass of the disks comparable to those found around single stars, giving credence to the notion that planet formation in binary systems is not unusual.
Quintana and Lissauer’s paper reports on their simulations of the late stages of terrestrial planet formation in stars with separations between 0.05 AU and 0.4 AU, assuming a total mass of 1 solar mass for the two stars. The duo worked with fourteen different short-period binary combinations, varying the initial orbits of the stars and their eccentricities and factoring in the effects of gas giant planets on the model of Jupiter and Saturn.
The scientists used the same initial disk parameters they used to simulate planet formation around the individual Centauri stars in their 2002 paper (see below), and compared their results to planet formation simulations using the Sun, Jupiter and Saturn that assume the same initial protoplanetary disk. The result: The planetary systems forming around binaries less than 0.2 AU apart turn out to be similar to those forming around single stars, while fewer planets form around systems with wider spacing. At 0.3 AU, the accreting disk is perturbed enough to make the formation of terrestrial worlds near 1 AU unlikely, although even in the wider spacings smaller terrestrial worlds (think Mercury) do emerge.
Centauri Dreams‘ take: We haven’t yet detected exoplanets orbiting both members of a main sequence binary system, but this is likely the result of our methods — Quintana and Lissauer note that short-period binaries are not included in radial velocity search programs because of their complicated spectra. Such planets are doubtless out there, and their parent stars may play an interesting role in planet hunting. From the paper:
An additional benefit of understanding the differences between planet formation around single stars and that around close binaries is that for eclipsing binaries, the contrast ratio between brightness of the stars and that of the planet(s) is reduced during the eclipse. For a total eclipse of identical stars, this reduction is a factor of two; as lower mass main sequence stars can be just slightly smaller but significantly less luminous, the detectability of the planet can be enhanced by more than a factor of two when the fainter star transits the brighter one. In an evolved close binary having undergone mass transfer, the fainter star can actually completely eclipse its much brighter companion, leading to an even larger improvement in planetary detectability.
The paper is Quintana and Lissauer, “Terrestrial Planet Formation Surrounding Close Binary Stars,” now accepted for publication in Icarus and available here. The scientists have completed a second set of simulations on wide binaries and are writing up the results. I’ll have a summary of those findings soon, and at some point in the near future, I want to return to the 2002 paper “Terrestrial Planet Formation in the Alpha Centauri System” (available here as a PDF) a key study that has changed our thinking on terrestrial worlds and habitability around the nearest stars.
by Paul Gilster | Jul 17, 2006 | Outer Solar System |
With New Horizons now six months out and closer to Jupiter than it is to the Sun, the creatively-acronymed Jupiter Encounter Science Team (JEST) has turned in its observation plan. New Horizons will pick up a gravity assist from the gas giant in February of 2007, on its way to the 2015 encounter with the Pluto system. That also means that from January to June of 2007, the spacecraft will make more than 500 observations on everything from the Galilean satellites to the Jovian magnetosphere, with rich results expected.
If you are tracking New Horizons, check out the mission locator page. Alan Stern’s regular reports are also vital; Stern is principal investigator for the mission and has, throughout planning and launch, used the Web effectively to keep the public informed about its progress. In the latest update, he reports on a rare occultation of a star by Pluto in June. Such events provide the opportunity for studying Pluto’s atmosphere, with the interesting result that conditions have changed since earlier occultations in the 1980s. Back then, Pluto was moving past perihelion and thus about as warm as it ever gets.
Is the atmosphere of this distant world ‘snowing out’, falling to the ground as flakes of frozen gases? Not yet, says Stern, though there is no certainty about what will happen by the time New Horizons arrives. You can read more in Stern’s report, including news of the encounter with asteroid 2002 JF56, which provided reassuring evidence that the spacecraft’s equipment can track and image moving targets. New Horizons passed 2002 JF56 at a relative speed of over 70,000 km/hr.
The equipment in question is the spacecraft’s moving target image motion compensation system, but continuing testing of other onboard systems goes well, including the upload of the latest version of the probe’s autonomy software. Flight software uploads are scheduled for August and September. New Horizons is now in axial spin mode to save fuel and will pass the distance of Ceres from the Sun in July.
Keep an eye on Ceres: NASA’s Dawn mission is still scheduled for orbital insertion around the distant object in 2015, the same year New Horizons reaches Pluto. If all goes well, our knowledge of the Solar System’s ‘dwarf planets’ should soon increase exponentially.
by Paul Gilster | Jul 15, 2006 | Culture and Society |
Stewart Brand is a leading proponent of long-term thinking, the sort of thing that builds cathedrals and, perhaps one day, starships. In this excerpt from his book The Clock of the Long Now (New York: Basic Books, 1999), Brand discusses science fiction and the various forms of futurism.
According to Kevin Kelly, ‘Isaac Asimov once said that science fiction was born when it became evident that our world was changing within our lifetimes, and therefore thinking abut the future became a matter of individual survival.’ The nanotechnology futurist Eric Drexler concurs: ‘I have found over the years that people familiar with the science fiction classics find it much easier to think about the future, coming technologies, political effects of those technologies, and so on.’ At Global Business Network (GBN), the scenario-planning business that employs me, we frequently send out science fiction books to the Network membership, and when we can get writers such as William Gibson, Bruce Sterling, David Brin, and Vernor Vinge to attend scenario workshops with client organizations, the quality of work deepens. Scenario planning involves exploring several widely variant futures of an organization’s world persuasively in depth. Skill in science fiction adds to the depth.
Still, the most important developments in the future, says Freeman Dyson, keep being missed by both the forecasters and the storytellers: ‘Economic forecasting misses the real future because it has too short a range; fiction misses the future because it has too little imagination.’
Too little imagination? Yes, for a structural reason almost never taken into account. At any time the several ‘probable’ things that might occur in the future are vastly outnumbered by the countless near-impossible eventualities, which are so many and individually so unlikely that it is not worth the effort of futurists or futurismists to examine and prepare for even a fraction of them. Yet one of those innumerable near-impossibilities is what is most likely to occur. Reality is thus statistically forced always to be extraordinary. Fiction is not allowed that freedom. Fiction has to be plausible; reality doesn’t.
Notice Brand’s use of the coinage ‘futurismists.’ Here’s the distinction he’s aiming at: ‘Futurists’ focus on the rigorous and objective analysis of possible futures. ‘Futurismists’ propound what is essentially a belief structure that is highly subjective in nature and often comes with an agenda. Brand quotes Paul Saffo on the macro-myopia of the futurismists: “We overexpect dramatic developments early, and underexpect them in the longer term.”
by Paul Gilster | Jul 14, 2006 | Astrobiology and SETI |
Centauri Dreams was amazed to realize that almost two years have passed since Christopher Rose and Gregory Wright posed a bold challenge to SETI researchers. In an article in the September 2 (2004) issue of Nature (a cover story, no less), the duo suggested that we are more likely to achieve extraterrestrial contact through artifacts — organic material embedded in an asteroid or comet, say — than through radio or optics. Larry Klaes, posting a link in a comment here on the Rose/Wright discussion, recently jogged my memory about this article, which deserves a renewed look.
Rose (Rutgers University) is a professor of electrical and computer engineering; his work with wireless technologies convinced him that “…it’s often MUCH (many many orders of magnitude) better from an energy use perspective (and perhaps from others like message persistence at the destination) to write a message down in some medium and LITERALLY toss it to the recipient than it is to radiate the message electromagnetically — assuming delay beyond light transit time can be tolerated.” He and physicist Wright (Antiope Associates) explored this notion in a piece that should remain required reading for anyone serious about the SETI hunt. They maintain current work on the idea on their Cosmic Communications page, from which the above quote is drawn.
The problem with artifacts, of course, is time. If we’re in a hurry, then we want to get the word out (or receive it) the fastest way possible, and with our current technology, that means working via radio or optics at the speed of light. But if we can extend our thinking to messages not so much for conversation as for archival purposes, recording the great works of a civilization (think of the Voyager golden discs), then imagine sending a highly compressed matter packet on a journey of millions of years. And imagine looking for such messages here.
From the original paper:
Any message presumably arrived after the Solar System became habitable (that is, after most of the protoplanetary debris had cleared), so whatever carried the message would be less eroded by impacts than an asteroid. Interplanetary radar could search for objects with anomalously smooth radar signatures. Alternatively, a message could have a retrore?ector attached to produce an anomalously large radar cross-section. Of course, an even simpler strategy is to use a powerful radio beam to illuminate these regions and see whether anything answers back. More active message types (ecological seeds or probes) are also conceivable, but are not necessary to exploit inscribed-matter ef?ciency.
Shades of Scottish astronomer Duncan Lunan, who once discussed a ‘phantom’ radio echo from the outer Solar System as a possible return signal from a space probe from Epsilon Bootes! I’m glad Larry noted Rose and Wright’s work in his recent comment, as it was apropos of another proposal to fling matter at a distant star, namely, a relativistic probe to Alpha Centauri whose effects upon the presumed debris disk there would announce its arrival. Who is to say that such objects have not come to our own Solar System, perhaps in far more complex form, with the obvious conclusion that we may be more likely to find evidence of extraterrestrials in nearby space than by trying to detect anomalous radio signals from other stars or looking for optical beacons.
Evidence for ET in a grain of sand? I find it a beautiful concept. The older SETI paradigms benefit when we bring such robust questioning to their premises. You can learn more about Christopher Rose and Gregory Wright on their respective Web sites. Centauri Dreams‘ original 2004 story on Rose and Wright’s work is here.
