by Paul Gilster | May 4, 2005 | Exoplanetary Science
The European Space Agency’s DARWIN mission is all about detecting Earth-like planets, as well as analyzing their atmospheres and determining their ability to sustain life. With the mission currently in its ‘definition’ phase, where parameters are determined and designs weighed, Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) has made her Ph.D work in astrophysics available to interested scientists. Written for Karl Franzens University in Graz, Kaltenegger’s study presents her work with the DARWIN team at the European Space Agency. Here you’ll find an evaluation of alternative designs and mission targets; the section on defining the features of a habitable world alone is worth the download of this enormous PDF file.
Scheduled for launch some time after 2014, DARWIN is a space-based interferometer designed to operate by pooling the data from free-flying craft working in tandem. At the heart of Kaltenegger’s work is a discussion of target star selection for DARWIN. As matters currently stand, some 826 stars (including multiple systems) are in the running, a list that will be narrowed with further modeling. Some salient points:
At least 100 to 150 stars will be needed for observation since the frequency of extrasolar terrestrial planets remains unknown.
G-type stars like our Sun make natural targets, but other stellar types (M-class red dwarfs, for example) should not be excluded. “It is essential that we have a certain number of targets stars to derive conclusions for detections or non-detections. Even though the frequency of extra-solar terrestrial planets can only be guessed, we believe that a sample has to consist of about 100 to 150 stars to be able to derive conclusions,” Kaltenegger writes.
Target stars will be limited to those within 25 parsecs.
Binaries will be investigated in cases where the light from a second star does not overwhelm the detectors and mask any planetary signal; this will vary depending on the different stellar types of the stars observed.
Because most stars near the Sun are variable stars, the question of how much variability is acceptable while still retaining conditions for Earth-like planets to form must be investigated further. “We should be ready for surprises on what reactions exist to stabilize habitable conditions on planets, including reactions we might not be aware of now…”
Young stars should be secondary targets because of the time needed for terrestrial worlds to form. But studying different stages in planetary evolution is part of DARWIN’s objective, and some young stars will remain in the target sample.
Missions like DARWIN will add new depth to our understanding of exoplanetary systems. As Kaltenegger notes, the sensitivity of our current detection methods introduces an enormous bias in the direction of high mass companions close to the parent star; these are the most likely objects to be discovered by noting the apparent ‘wobble’ in a star’s motion as it is affected by such worlds.
Image: Darwin’s six telescopes look at light from space and analyse the atmospheres of Earth-like planets. Copyright: ESA 2002. Illustration by Medialab.
Can terrestrial planets form in such systems, or would those giant, close-in planets create instabilities fatal to their development? We won’t know the answers until DARWIN and Terrestrial Planet Finder provide us a big enough sample to draw some conclusions, and that makes the choice of target stars a critical matter.
“Search for Extra-Terrestrial planets: The DARWIN mission – Target Stars and Array Architectures” is available here, or via Kaltenegger’s page at the Center for Astrophysics site. Also see ESA’s home page for Darwin.
by Paul Gilster | May 3, 2005 | Astrobiology and SETI |
The idea that rocks may travel between planets is now widely accepted. But can rocks or other planetary ejecta wander between solar systems? A new paper examines this hypothesis, concluding that rocky materials and even life-bearing meteorites may make their way from one planetary system to the next. But here’s the catch: this transfer is only likely between stars in young stellar groups and clusters, where the distances and relative velocities between stars are low. The authors — the University of Michigan’s Fred C. Adams and Princeton’s David N. Spergel — note that most stars occur in binary systems, making the chance of such transfer that much higher.
The operative term is ‘lithopanspermia,’ the notion that life travels between worlds aboard meteorites. It is a variation on the older panspermia theory, which argued that life arrived directly from space. The concept dates back as far as Anaxagoras (5th Century B.C.) and was championed by Lord Kelvin, who declared in 1871, “…we must regard it as probable in the highest degree that there are countless seed-bearing meteoric stones moving about through space. If at the present instance no life existed upon this earth, one such stone falling upon it might, by what we blindly call natural causes, lead to its becoming covered with vegetation.”
Panspermia came to the public’s attention through the work of Swedish chemist Svante Arrhenius, whose Worlds in the Making: The Evolution of the Universe (New York: Harper & Row, 1908) talked of spores moving between planets by the pressure of starlight. It was given a powerful boost by the discovery of meteorites that are almost certainly Martian in origin.
Adams and Spergel calculate that although the odds of any given rock being captured by another solar system are low, the sheer number of ejected rocks is likely to be high enough that all solar systems share some materials with other systems in their birth cluster. From the paper:
This paper shows that young star clusters provide an efficient means of transferring rocky material from solar system to solar system. If any solar system in the birth aggregate supports life, then many other solar systems in the cluster can capture life bearing rocks. Only a fraction of these systems will feed biologically active rocks onto the surfaces of terrestrial planets, however, so the odds of successful lithopanspermia are low: In the limit of low speed ejecta, only a few systems per cluster are expected to be biologically seeded through this mechanism, although the efficiency is reasonably high… If the origin of life is relatively common and if life bearing rocks can be ejected at low speeds, then dynamical interactions in stellar birth clusters would provide an effective mechanism for spreading life.
Centauri Dreams‘ take: Note that young stellar systems offer the best opportunity for capturing biologically active materials from another system, because the planets are still forming and collisions between planetary debris would be common. In this setting, rocky materials may well contribute to the formation of planets which would thus have the potential for life more or less built into them. Also note the authors’ view that a key figure in their study — the fraction of captured material that falls onto the surface of habitable planets — is in need of additional calculations. Much work remains to be done, but the current conclusion is intriguing: “…optimistic circumstances allow a cluster, once biologically seeded, to transfer life to the majority of its solar systems through the process of lithopanspermia.”
Image: The Alpha-Monocerotid meteor outburst in 1995. Is it possible that life falls onto habitable worlds from such events? Credit: S. Molau and P. Jenniskens, NASA Ames Research Center.
Fred Hoyle and Chandra Wickramasinghe have studied the properties of biological material falling onto planetary surfaces; see their 1999 paper “The viability with respect to temperature of microoganisms incident on the Earth’s atmosphere,” Astrophys. Space Sci. 268, 45-50. The duo have argued for decades that complex organic materials and even primitive organisms might have arrived on Earth by way of comets or meteorites. Giving further credence to the hypothesis is the survival of viable bacteria on the Surveyor 3 moon lander, retrieved by the Apollo 12 astronauts after two and a half years on the Moon.
“Lithopanspermia in Star Forming Clusters” is now available on the arXiv site and has been accepted by Astrobiology for future publication.
by Paul Gilster | May 2, 2005 | Missions, Outer Solar System |
The so-called ‘Pioneer Effect’ continues to trigger study. Both Pioneer 10 and 11, as discussed in these pages back in November, have shown changes in their expected trajectories since they moved 20 AU beyond the Sun. In fact, since 1980 radio signals from the Pioneers have been slowly shifting to shorter wavelengths, which seems to imply a slight but interesting deceleration. This has led to at least one proposal for a mission to investigate the Pioneer effect.
Both Galileo and Ulysses data have been examined for evidence of a similar effect; while Galileo’s data were too limited for use, Ulysses did show a provocative, though extremely slight, change to its own acceleration (though at a much smaller distance from the Sun). Now a new paper notes the difficulties in measuring the Pioneer anomaly, and discusses a way of using asteroids and comets to measure gravitational effects in the outer Solar System. The paper is by computer scientists Gary Page and John Wallin (George Mason University), along with Jornada Observatory (NM)’s David Dixon.
According to the authors, three asteroids (5338, 8405, and 2001XA255) of the fifteen in a suitable orbital geometry have sufficient size and brightness to be observed for significant periods beyond 20 AU. The paper continues:
Thus, these asteroids should provide a mechanism for observing the gravitational field in the outer solar system and permit its use in investigating the Pioneer Effect and in a broader context, the mass distribution in the outer solar system. Additionally, many of the other candidate asteroids could be observed in the near future, when they are not in the Pioneer Effect region, in order that their orbits be tied down with observations when they are close (perhaps including high precision radar observations). This could be done in anticipation of continuing observations when they move further out and become subject to the Pioneer Effect.
Make no mistake, we may find that there are simple explanations to what is going on with the Pioneers. Anything from measurement errors to gas leaks aboard the aging spacecraft could prove to be the culprit — interplanetary dust, for example, has been suggested as a drag force, or perhaps the cause of a red shift in the spacecraft’s radio signals. But the suggestion that new physics may be involved is tantalizing. Moreover, the Pioneers are unusually useful platforms for such study. Spacecraft launched more recently engage in course corrections that could mask subtle effects like what the Pioneers are experiencing. How these tiny craft move as they are buffeted by the solar winds through their long exit from the heliosphere has much to tell us.
As the authors conclude: “The bottom line is that the Pioneer Effect seems well-founded and has not been convincingly explained in terms of known physics and engineering parameters of the spacecraft involved. Although spacecraft systematics remain the most likely explanation for the Pioneer Effect, its potential existence is of great interest for a variety of fundamental physical reasons.”
Centauri Dreams‘ take: This paper bears interest beyond the Pioneer Anomaly it sets out to investigate. Assuming an explanation for the anomaly is found within conventional physics, the authors’ study of using solar system bodies like the three named asteroids as ‘gravitational probes’ of the outer Solar System is ingenious, and may provide useful data on the Kuiper Belt. While comets are problematic for this purpose because of their frequent outgassing, as the authors point out, asteroids fit the bill nicely. Thus the authors’ conclusion: “There is nothing quite as useful as a big, unwieldy, dynamically dead chunk of rock for investigating small variations in Newton’s Laws.”
“Utilizing Minor Planets to Assess the Gravitational Field in the Outer Solar System” is available on the ArXiv site. Also be aware of Laurent Nottale, “The Pioneer anomalous acceleration: a measurement of the cosmological constant at the scale of the solar system,” found here.
