by Paul Gilster | Mar 8, 2006 | Exoplanetary Science
Centauri Dreams has written before about Grover Swartzlander (University of Arizona), who is developing new ways to screen out the light of a star to make it possible for astronomers to study the planets around it. At the heart of Swartzlander’s effort is something called an optical vortex mask, which is said to be ‘a thin, tiny, transparent glass chip that is etched with a series of steps in a pattern similar to a spiral staircase.’
And here’s how this chip does its job: incoming light slows down more in the thicker parts of the chip than in the thinner ones, with the result that some waves of light eventually becomes 180 degrees out of phase with others. Reaching the ‘eye’ of the vortex, the waves that are 180 degrees out of phase with one another cancel each other out, so that a dark central core remains. Swartzlander says the effect is like light following the threads of a bolt; the distance between adjacent threads is crucial to the outcome.
So could this technology be used to see a planet that is ten billion times fainter than its parent star? A coronograph using such a mask might pull it off. A standard coronograph, which uses an opaque disk to block the star’s light, wouldn’t suffice because of diffraction around the disk, which as we recently saw, is a problem that has also been under intense investigation by Webster Cash at the University of Colorado, Boulder, not to mention the Jet Propulsion Laboratory’s own efforts.
Swartzlander has tested his ideas at Steward Observatory’s 60-inch Mount Lemmon telescope in Arizona, and thinks that he too can cancel out that diffraction problem. “Any small amount of diffracted light from the star is still going to overwhelm the signal from the planet,” Swartzlander explained. “But if the spiral of the vortex mask coincides exactly with the center of the star, the mask creates a black hole where there is no scattered light, and you’d see any planet off to the side.”
Centauri Dreams‘ take: The Mount Lemmon work was limited by the fact that the telescope there is not equipped with adaptive optics that could zero out atmospheric turbulence. Swartzlander is now applying for grants to improve the quality of the mask, and working on numerical simulations to make it more effective. All of which is promising in a variety of areas — there is talk of using the technology in microscopy as well — but in terms of planet hunting, the occulter spacecraft Webster Cash is proposing to NASA this spring uses technologies that seem to be much further along and therefore likely to pay off sooner for the purposes Terrestrial Planet Finder was originally designed to accomplish. Cash’s consortium — Ball Aerospace, Goddard Space Flight Center, Northrop Grumman and Princeton University — is ready to design a practical spacecraft around its ideas.
by Paul Gilster | Mar 7, 2006 | Exoplanetary Science
An interesting specialty in exoplanetary science is the simulation of planetary orbits. It’s intriguing, for example, to place a hypothetical terrestrial planet into a system with known giant planets to see what happens. After all, we know that many exoplanetary systems contain potentially stable orbits for such planets; in fact, one-fourth of known systems can support a planet in their habitable zone. And while we don’t know yet whether such worlds exist, we can draw useful conclusions from modeling their orbits if they are there.
Such are the premises of a new paper by Sean Raymond (University of Washington, Seattle) and Rory Barnes (Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder). The two simulate terrestrial planets in four systems: 55 Cancri, HD 38529, HD 37124, and HD 74156. And here is a key issue: most planets we’ve found so far are ‘hot Jupiters,’ in tight, close orbits around their primary. For a terrestrial planet to co-exist with such worlds, it must either form quickly and survive the inward migration of the gas giant (which presumably formed farther out in the protoplanetary disk) or it must form from material remaining after the passage of the gas giant as it moves inward. This would imply an early migration of the gas giant toward its star.
The first scenario is dubious in the extreme, with only one to four percent of terrestrial planets remaining in orbits within the habitable zone. But if the gas giant forms quickly enough, a new generation of planetesimals may form after it does. Raymond and Barnes work through such scenarios and make comparisons with earlier simulations, placing moon to Mars-sized planetary ’embryos’ between the giant planets and letting them evolve over time.
The result: Planets of up to 0.6 Earth masses, some with substantial water content, form relatively easy around 55 Cancri, whereas HD 38529 may support an asteroid belt but no terrestrial worlds. And no terrestrial planets form at all around HD 37124 and HD 74156. About 55 Cancri, the authors have this to say:
Our simulations of 55 Cancri suggest that a potentially habitable planet could form in situ. Such a planet would have a small enough eccentricity to remain in the habitable zone throughout its orbit, substantial mass and water content. However, as shown in Paper II, a Saturn-mass planet could exist on a stable orbit in the habitable zone of 55 Cnc. Such a planet may preclude the existence of habitable planet, although there remains the possibility of a habitable satellite of the giant planet.
A key contribution of this work is an illuminating discussion of the so-called ‘packed planetary systems’ (PPS) hypothesis, which suggests that all planetary systems have as many planets as they can support without becoming unstable. In other words, if you can determine that a stable region exists somewhere in a planetary system, then a planet ought to be there. This paper, which is the third in a series on predicting planets, helps the authors to conclude that the PPS idea is robust. From the paper:
It remains to be seen whether the predicted planets exist. We do not predict that stable regions will contain so much mass as to be borderline unstable. Rather, we suggest that any region in a planetary system which can support a massive planet will contain a planet. Future observations of these well-studied planetary systems will test the credence of the PPS hypothesis.
The paper is “Predicting Planets in Known Extra-Solar Planetary Systems III: Forming Terrestrial Planets,” available in abstract and full-text form at the arXiv site.
by Paul Gilster | Mar 7, 2006 | Interstellar Medium, Outer Solar System |
A number of readers have been asking about results from the Stardust mission, particularly as pertains to any interstellar materials returned by the craft. We’ll know a good deal more on March 13, when NASA holds a news conference at 3 PM EST (1800 GMT). The briefing will be available both on the Web and on NASA TV, with participation by, among others, principal investigator Donald Brownlee and JPL’s Peter Tsou. My understanding is that the team will largely confine its report to cometary samples, but these too may yield surprises.
by Paul Gilster | Mar 6, 2006 | Missions |
As reported by Larry Kellogg, the recent attempt to pick up a signal from Pioneer 10 may have come up short, although the team working on the project is processing the data in the attempt to pin down anything that isn’t noise. Says Kellogg:
Distance makes a difference… The 8 watt transmitter wouldn’t be putting out a full 8 watts (a night light of power) and the signals seen at Earth were buried down in the noise you get from just pointing an antenna out into space. It is like looking for a needle in the front lawn down in the grass.
One spike looks a lot like the one next to it.
If you see something you think is your needle you can narrow the band pass filter and magnify what you are looking at. You don’t see anything next to it though and so you have to look back and fourth and hope you recognize your needle.
We haven’t had any signals from Pioneer since 23 January 2003; the last telemetry data were received on 27 April 2002. With no real-time detection of the spacecraft’s carrier signal, the suspicion grows that we have lost Pioneer 10 forever (although the ongoing data analysis could still prove this wrong). If so, we can thank the probe for its extraordinary journey, and say thanks as well to the team that sent it on its way. A comprehensive book on the Pioneer program is Mark Wolverton’s The Depths of Space (Washington DC: Joseph Henry Press, 2004). The Pioneer 10 and 11 home page is here.
by Paul Gilster | Mar 6, 2006 | Research Tools
It’s a pleasure to report that the proceedings volume for last June’s New Trends in Astrodynamics conference in Princeton has been published. You can find the contents here. Three papers tackled issues with interstellar implications:
Gregory L. Matloff, Travis Taylor, Conley Powell, and Tryshanda Moton, “Phobos/Deimos Sample Return via Solar Sail” Ann NY Acad Sci 2005 1065: 429-440. An examination of sail technologies for a practical mission within the Solar System.
Marc G. Millis, “Assessing Potential Propulsion Breakthroughs,” Ann NY Acad Sci 2005 1065: 441-461. A summary of the methods, findings, and benefit predictions of breakthrough propulsion physics.
Paul A. Gilster, “The Interstellar Conundrum: A Survey of Concepts and Proposed Solutions,” Ann NY Acad Sci 2005 1065: 462-470. A look at the ingenious ways theorists have envisioned taking us to the stars with near-term technologies.
The Princeton event was a marvelous experience (my recollections are online), not just in the chance to talk to Ed Belbruno, whose work I have long admired, but also to have dinner with interstellar theorist Gregory Matloff and his wife, the artist C Bangs, as well as a long and enjoyable breakfast with the Matloffs and Claudio Maccone in Princeton’s Nassau Inn. Next August’s conference should feature a greatly expanded interstellar session, and I’ll have more details on participants as they become available.
