by Paul Gilster | Aug 25, 2006 | Exoplanetary Science |
Hunting for exoplanets isn’t a matter of peering into telescopes and seeing faint specks of light. It’s all about combing through data — reams and reams of data thankfully digitized — for the telltale signatures of planets. And it’s fascinating to reflect that in many cases the signatures we seek are in our possession in the form of already gathered radial velocity data. We must continue to re-examine our growing stellar libraries, which in the case of radial velocities only get richer over time as planetary influences become more pronounced.
And so we come to Mu Arae, a G-type dwarf star much like our Sun and catalogued as HD 160691. A new study by Krzysztof Gozdziewski, Andrzej Maciejewski, and Cezary Migaszewski re-examines this already intriguing planetary system to discover yet another planet, the fourth to be found there. These radial velocity measurements were made by the Anglo-Australian Planet Search project and build on the earlier detections of three worlds, two being Jupiter-style companions with orbits projected at 630 days and 2500 days respectively. A third planet has been characterized as a ‘hot Neptune’ in a 9-day orbit.
The new analysis finds an interesting fourth planet of about 0.5 Jupiter masses in a 307-day orbit. That fourth world makes Mu Arae the second known four-planet system, the other being 55 Cancri, and in both cases the planetary orbits are nearly circular, an architecture not so different from our own Solar System. Given longer data runs we thus begin to see solar systems in greater detail and can apply our ever more precise computing tools toward resolving their structure. Key to all this, of course, is making the software used to highlight these planetary nuances better and better.
Teasing planets out of the datasets takes time, as becomes clear in a second study, this one from the Geneva exoplanet search team, that delves into the Mu Arae system. From the paper:
“More than 25% of the known extrasolar planets populate systems with at least two planets. In most of these systems, the various planets were discovered sequentially, starting with the planet inducing the highest radial-velocity amplitude at short or intermediate period. The following planets unveiled themselves when, after a while, the residuals became high compared to the expected measurement precision and showed some structure as a function of time.”
Mu Arae turns out to be a classic case in point. Using data from the HARPS (High Accuracy Radial Velocity Planet Searcher) instrument at La Silla to update older observations, the Geneva team is now able to characterize the entire Mu Arae system in greater detail. The team confirms the presence of the Neptune-class planet orbiting every 9.64 days, and also the previously known gas giant now pegged as orbiting every 643 days. The other Jovian class world is given an orbital period value of 4206 days (this one is still in need of work), while the newly discovered planet orbits every 310.6 days.
The complete dataset involved in this study covers 8 years of observations and 171 nightly-averaged measurements whose precision varied depending on the instrument used. As the paper comments: “With increasing number of planets, and thus free fit parameters, the amount of possible solutions increases drastically, and the only way to constrain the orbits is to acquire many new and precise data points.”
Which is what the exoplanet hunt is all about, throwing dramatic light on our continuing analysis of radial velocity data and projects like systemic, an undertaking that has gathered all available radial velocity data for known planet-bearing stars. The current Sky & Telescope has an article on systemic under an apt title: “Virtual Planet Sleuths,” while the collaboration’s creator, Greg Laughlin (UC-Santa Cruz) discusses planet formation in the current issue of American Scientist.
by Paul Gilster | Aug 24, 2006 | Culture and Society, Outer Solar System |
So now we know what a planet is. As confirmed by the passage of a revised resolution at the International Astronomical Union’s general assembly today in Prague, a planet meets the following criteria:
It must be in orbit around a star
It must possess sufficient mass to allow it to assume a round shape; i.e., it assumes hydrostatic equilibrium
It is large enough that it has cleared the orbit through which it moves
The third item, of course, is the interesting part, for it rules out Ceres, about which there had been some controversy. I mean, it was one thing to consider 2003 UB313 as a planet, but to delve into the middle of the Solar System and define a new planet in medias res seemed a stretch too far for some people (though not for me). Pluto is also ruled out because it moves for part of its orbit inside the orbit of Neptune; Charon likewise is left without planetary designation.
What does happen to Pluto is that it becomes a ‘dwarf planet,’ a new kind of object that also includes Ceres and 2003 UB313, and of course we will be finding numerous other dwarf planets as we continue to refine our observations of the Kuiper Belt and surrounding space (as many as a dozen candidate dwarves are already on the IAU watchlist — click here for a list of known dwarf planets).
So there you have it, eight planets that can be considered ‘classical’ and a new category that subsumes all the rest. Which for the traditionalist in me is OK, while the logician says that the third criteria involving clearing out a planetary orbit is likewise sensible, and we can forget about the Pluto/Charon barycenter. Sigh.
There is precedent for planetary demotion, incidentally. After its discovery in 1801, Ceres was generally thought to be a planet, but by the early 1850s so many of what we now call ‘asteroids’ were being discovered that planetary status for all was ruled out. It shouldn’t be long before the IAU ruling becomes accepted and similarly relegated to a historical footnote, but the controversy was intense while it lasted.
But let’s move on to other things, which tomorrow includes a new planet around Mu Arae, a system that is getting more crowded (and better characterized) all the time, and the methods used to find it.
Update: A telling comment from Hal Weaver at Johns Hopkins’ Applied Physics Laboratory. apropos of dynamically cleared orbits:
“Regarding the resolution itself, I’m with Andy Cheng in concluding that the situation is still somewhat muddled. What exactly is meant by a planet ‘clearing its neighborhood?’ Since many ‘plutinos’ … (including Pluto) …cross Neptune’s orbit, I’d say Neptune’s neighborhood still needs some clearing! … It just seems a bit risky to me to base a definition on a theoretical construct (‘dynamically cleared regions’) that’s only approximate at best and may change significantly as our understanding of planet formation evolves over time.
“I further note that there have been particularly large swings in the theories of outer solar system dynamical evolution during the past decade. What was ‘conventional wisdom’ five years ago has been replaced with the latest fad, and I don’t expect that situation to change any time soon.”
by Paul Gilster | Aug 23, 2006 | Tau Zero Foundation |
by Marc Millis
Centauri Dreams is pleased to report again on the status of the Tau Zero Foundation. Founded by Marc Millis, former head of NASA’s Breakthrough Propulsion Physics program, the Foundation’s goal is to support credible research into interstellar flight, with a realistic understanding that incremental progress toward this goal can only be made through persistent, long-term effort. Here Millis describes the current state of affairs, and discusses the necessary next steps for the young Foundation.
For those awaiting the debut of the Tau Zero Foundation, I thought I would take this time to let you know how it will be implemented. The first stage, setting up the basic operation and a pool of expert practitioners, is already happening. From the combined work of our practitioners we will debut a public website that explains the status of this work and the next practical steps to be taken toward interstellar flight. At that same time will be ready to accept general memberships.
Once the public website has become established and member donations grow, the means to invite research proposals over the Internet will be developed. Any solicitations, however, will be contingent upon the third stage – of having obtained sufficient philanthropic donations to support a suite of research tasks.
Meanwhile, the network of Foundation practitioners will continue its own work toward practical interstellar flight and through its progress and collaborations, the content on the public website will become more refined and up-to-date. One goal is to explain, in general public terms, the why, how, who and when of real interstellar flight.
Another goal is to identify, for students and other budding researchers, the next-step unknowns in need of solution. It is hoped that, by providing such information from reliable, visionary sources, students will have the tools to begin legitimate inquires on their own. As things progress, we would be pleased to hear from students how we could better serve their learning needs.
Your patience is appreciated. This will be a long term endeavor for the greater good of humanity.
For the time being, the Tau Zero Foundation will not be able to review any submissions. Even when the website is online, there will be strict submission requirements. If, after the public website has debuted, you find any errors or missing relevant information, please bring that to our attention. Such information will have to be traceable to work already published in the peer-reviewed literature. Other websites, conference papers, or personal theories are not suitable submissions.
Background information and previous postings about the Tau Zero Foundation are available on Centauri Dreams. We look forward to embarking on this enterprise and will keep you informed on the Foundation’s progress every step of the way.
by Paul Gilster | Aug 22, 2006 | Deep Sky Astronomy & Telescopes, Exotic Physics |
Gravitational lensing, discussed here recently as the motive for the FOCAL mission to the Sun’s gravity lens, is suddenly back in the news. This time it’s being used to make measurements of dark matter of a startlingly precise kind, measurements that in some quarters are being hailed as the first solid evidence that dark matter exists. Views of two merging galaxy clusters at optical and x-ray wavelengths are involved, with gravitational lensing being used to examine their mass.
The clusters under investigation seem prime candidates for this kind of work. Douglas Clowe (University of Arizona), who led the study, explains its significance:
“Prior to this observation, all of our cosmological models were based on an assumption that we couldn’t prove: that gravity behaves the same way on the cosmic scale as on Earth. The clusters we’ve looked at in these images are a billion times larger than the largest scales at which we can measure gravity at present, which are on the scale of our solar system.”
And that’s the key: if we make the assumption that gravity does indeed work the same way in the distant cosmos as it does locally, then the movement of galaxies leads to the inevitable conclusion that visible matter is only part of the universe, that there must be, in fact, five times more dark matter than the matter we can see. If there were not, galaxies would quickly fly apart from the gases within the clusters.
In the clusters under study, this gas is in the form of a plasma of hydrogen and helium created in a 10 million mile per hour collision. The collision produces drag on the gas, which shows up here in the form of a bullet-shaped cloud that is being shaped by the event. Usefully, the hot gas is affected by drag, but dark matter is not. The team can analyze the patterns of distorted light — the gravitational lensing caused by the gravity of the clusters being studied as it distorts the light from background galaxies — to extrapolate the mass of the clusters themselves and find out where most of that mass is located.
The result: The dark matter, which passes through the cluster collision without interacting with normal matter, has moved ahead of and apart from the hot gas, which did interact. And the dark matter is far more massive than the ordinary matter in the gas cloud. “The bottom line is, there really is dark matter out there,” said Dennis Zaritsky (University of Arizona). “Now we just need to figure out what it is.”
Image: Hot gas detected by Chandra in X-rays is seen as two pink clumps in the image and contains most of the “normal,” or baryonic, matter in the two clusters. The bullet-shaped clump on the right is the hot gas from one cluster, which passed through the hot gas from the other larger cluster during the collision. An optical image from Magellan and the Hubble Space Telescope shows the galaxies in orange and white. The blue areas in this image show where astronomers find most of the mass in the clusters. The concentration of mass is determined using the effect of so-called gravitational lensing, where light from the distant objects is distorted by intervening matter. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving direct evidence that nearly all of the matter in the clusters is dark. Credit: Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
Centauri Dreams‘ take: Readers sometimes ask what obscure subjects like dark matter (and the even more mysterious ‘dark energy’) have to do with propulsion and travel between the stars. It’s clear that they have everything to do with the basic forces that control matter’s interactions and with the age and composition of the cosmos. But these are broader subjects than the specific issue of interstellar travel.
The answer is that the makeup of the universe tells us a great deal about our propulsion options, and when it comes to interstellar flight, propulsion is the name of the game. Dark matter forces us to understand mass, with all that implies about accelerating objects to appreciable fractions of the speed of light. Dark energy leads us to contemplate the repulsive forces that push against the fabric of space, leading to space drive concepts that may one day become reality. Understanding these fundamental components of the universe could provide the breakthrough we need to venture out into the galaxy.
The paper is Clowe, Bradac, Zaritsky et al., “A direct empirical proof of the existence of dark matter,” scheduled for publication in Astrophysical Journal Letters and available online.
Update 8/24/06: For a more detailed analysis of this research, see this Cosmic Variance post, and check out as well Anthony Kendall’s fine article here.
by Paul Gilster | Aug 21, 2006 | Culture and Society |
Centauri Dreams continues to admire the clarity of the draft IAU resolution on the definition of a planet. Although the criteria are easily understood, they also present teaching opportunities (imagine all those schoolchildren learning what a barycenter is, and why Pluto/Charon make a double planet thanks to the location of their center of gravity!). This sound definition also grows from the properties of the planets themselves and is based on the best current information on planetary formation.
Galileo Galilei had some thoughts on naming things that seem apropos, and what better source to consider when defining a planet? The Tuscan astronomer/mathematician (1564-1642) could have been speaking of the current controversy when he said, “Names and attributes must be accomodated to the essence of things, and not the essence to the names, for things come first and names afterward.” I submit that the draft resolution does a fine job accomodating the named thing to its essence — that essence being gravity — despite the occasional oddball result.
As Greg Laughlin notes, for example, the Moon will become a planet in roughly 3.5 billion years as the barycenter of the system moves above the surface of the Earth (by that time, it will be at a distance of 81.3 Earth radii). Which would create havoc for textbook writers of the far future as they adjust to yet another planet popping into definition, but Laughlin’s genial jeux d’esprit (picked up by CNN here) is a reminder that definitions themselves partake of the arbitrary and almost inevitably produce consequences that give us pause.
Image: Justus Susterman’s 1636 portrait of Galileo. What would the celebrated planetary observer have thought of our current controversy over defining a planet?
So while a Division for Planetary Sciences committee strongly supports the IAU resolution, other opinions are strongly in the other camp, as witness these two from Johns Hopkins, the first from Richard Conn Henry (Department of Physics and Astronomy):
“I think the notion that Pluto is a planet is absurd. When it was initially discovered, it was thought to be vastly more massive than it turned out to be. Its orbit is radically different from that of all the other planets. Down with Pluto, is what I say!”
And then there’s Andy Cheung, from JHU’s Applied Physics Laboratory:
“Yes, keeping Pluto as a planet is the correct decision. However, the new definition of planet does not work for me, because “hydrostatic equilibrium” is an idealization — it is approximately correct for planets like Earth but is not exact. There is still no criterion for deciding how far from hydrostatic equilibrium an object can be and still qualify as a planet. Much of the science of geophysics deals with the different ways, and the reasons why, planets are not quite in hydrostatic equilibrium. Also the suggested term ‘pluton’ is a bad idea, in my opinion.”
Well, he’s got a point about ‘pluton.’ But under the premise that no definition will satisfy everyone, and knowing that the existing definition seems to satisfy few, Centauri Dreams hopes that the IAU resolution will pass so that 1) we can move to a definition that is clear-cut and based on reasonable assumptions and 2) we can get past the naming controversy and move on to more significant topics. But I will say this: anything that can get the planets of our Solar System all over the front page of my local newspaper has to be congratulated for energizing public interest in space in ways even Huygens and Cassini couldn’t.
