by Paul Gilster | Feb 21, 2007 | Exoplanetary Science |
At just over 150 light years from Earth in the constellation Taurus, the Hyades is the nearest open star cluster to Earth. We’ve been scouting the terrain in clusters recently, looking at globular clusters like 47 Tucanae and open clusters like M37, both of which are under intense scrutiny. But the first exoplanet to be identified definitively in either kind of cluster seems to be Epsilon Tauri b in the Hyades [but see below re a 2.5 Jupiter mass planet in the system comprising pulsar PSR B1620?26 and its white dwarf companion — that one is in the globular cluster M4].
It’s an interesting world, a gas giant that’s a little less than 2 AU out with an orbital period of 1.63 years. This is the first planet discovered around a red giant, its star the most massive of all planet hosts known. [My mistake: several planets evidently orbit red giants — see comments below, and check here for another example]. That leads to intriguing speculation: Should we expect planets around other red giants, and do our current planet formation theories work in this environment?
The latter point bears watching. Greg Laughlin (UC-Santa Cruz) argues that in the case of Epsilon Tauri b, the cluster’s harsh ultraviolet radiation should have disrupted the protplanetary nebula.
The UV radiation environment in the original Hyades cluster was fierce. The protostellar disks of the individual Hyads were likely photoevaporated before the growing planetary cores were able to reach the runaway gas accretion phase that gives rise to Jupiter-mass planets (see our paper on this topic). When we get the full inventory of planets in the Hyades, I think we’ll find plenty of Neptunes and terrestrial planets, but almost nothing in the Jovian range. Indeed, work by Bill Cochran and the Texas RV group has demonstrated that the Hyades are generally deficient in massive planets.
So how do we explain Epsilon Tauri b? Laughlin thinks this may be an example of a planet forming via the process known as gravitational instability, which can produce massive planets and is little affected by nearby ultraviolet radiation. Gravitational instability is a model in which instabilities within the protostellar disk can cause gas giants to coalesce. The rival core accretion model sees such planets growing from small cores of rock and ice that acquire new mass through collisions, eventually growing large enough for their gravity to draw in nearby gas.
Laughlin is saying that the likely dispersal of the protostellar disks in the Hyades stars (thanks to UV) makes core accretion less likely in the case of Epsilon Tauri b (there simply wasn’t time). Whereas if gravitational instability produces a planet for every few hundred stars formed, as Laughlin believes, then there is no reason not to expect such a world in an open cluster like the Hyades. That’s a win for gravitational instability, though Laughlin still sees core accretion as the dominant model, writing elsewhere that “…the weight of observational and theoretical evidence seems to be shifting against the gravitational instability hypothesis.”
The paper is Sato et al., “A Planetary Companion to the Hyades Giant Epsilon Tauri,” accepted by The Astrophysical Journal but not yet available at the arXiv preprint site.
by Paul Gilster | Feb 20, 2007 | Culture and Society, Missions |
Strange Paths offers a robust essay on the topic of interstellar arks, one that considers our future among the stars without warp drives or other breakthroughs that get us past the speed of light barrier. Star Trek and its ilk offer familiar, short-term travel analogous to our own relatively brief journeys in the Solar System. The real thing may be different:
The way toward stars becomes however quite unfamiliar if we consider that such Triumph of Physics could possibly not happen, and that the famous constant of Einstein c, the speed of light (3E8 m/s), represents an horizon speed which is impossible to exceed and which is even extraordinarily difficult to approach, so that we would begin to see outer space like it is seen by astronomers: a vastness compared to which that of terrestrial oceans is nothing.
The author looks at two alternatives, the first being a relativistic rocket able to take advantage of time dilation at velocities close to light speed so that the crew experiences a much shortened interstellar journey. Such a craft (the author assumes a vehicle driven by antimatter) is well beyond our technology, but it offers up the kind of mind-bending travel Carl Sagan was among the first to describe. Says Strange Paths, “Thus, in just 12 years of proper time (but 113,243 years for the stationary observer), which is a long time but still bearable in a comfortable spaceship, one could traverse the whole Galaxy, whose diameter is 100 000 light-years.” The relevant mathematical arguments are supplied.
But what energizes the essay is the long-haul ark, a vessel driven by a what the author calls thermonucleoelectric propulsion, channeling fusion plasma through a magnetic conduit for thrust. The beauty of the ark is that it is a self-contained world, housing its own society and capable of maintaining itself in stellar systems unlike our own. Thus the author’s choice of Epsilon Eridani as a target, it being relatively close to the Sun at 10.5 light years and, as a K2 star, not terribly dissimilar from ours.
Could a gas giant around Epsilon Eridani, even one in as eccentric an orbit as the one we now know to be there, provide satellites that could be used as a resource base for an interstellar ark? There seems to be no reason why not, though I think the author is overly pessimistic about the possibility of nearby terrestrial worlds, the Centauri stars now looking more and more promising in that regard, especially as we consider Proxima Centauri’s possible role in delivering volatiles to inner system planets around both Centauri A and B (if they do indeed exist).
But it will take another decade or so before we start to accumulate enough terrestrial-style worlds to draw even the most preliminary judgments about their frequency. Until then, speculation like this, richly detailed and backed by solid mathematical description, offers us a look at how humans can reach the stars with technology that could reasonably develop absent major breakthroughs in our understanding of physics.
On the societal level, the discussion of human psychology in such enclosed spaces is fascinating, and I would also offer up a paper by Edward Regis Jr. called “The Moral Status of Multigenerational Interstellar Exploration.” This one was in the Finney and Jones volume Interstellar Migration and the Human Experience (Berkeley: University of California Press, 1985). Regis considers the psychological and ethical questions interstellar arks present and finds no show-stoppers:
We conclude then that just as no rights of Earthlings are infringed by their not being included as passengers on a space ark leaving for the stars, no rights of star voyagers are infringed by their not living their lives on Earth. A multigenerational interstellar expedition is no more and no less morally permissible than the very existence of human life on our own planet.
Strange Paths has more to say about how life aboard an interstellar ark would be conducted, and how the ark can be constructed in such a way as to maximize livability in terms of environment and social interaction. And here’s something to ponder that may awaken old memories:
Compared to the Present, the spiritual horizon is identified to the horizon of social activity previously discussed. Arkonauts live a morally new situation, in lived and in long-term aims. They should develop an original mentality. The need of getting along would come first. The word trajectory reminds of “transient”. But such transient is a whole life and a whole civilization, with the result that the stellar goal will become almost secondary. The majority of individuals populating this travel will belong to a generation “not leaving, not arriving”. For this majority, the end of the way would just appear like a distant future. Certainly, in background the reach of targets will structure the community, but the stake making the happiness of the travellers will remain for the humankind its present.
The science fiction trope from way back is the interstellar ship whose inhabitants have forgotten their mission; indeed, they no longer recall that they are on a ship at all. Think Heinlein’s story ‘Universe’ (later part of Orphans in the Sky) or Aldiss’ Non-Stop (published here in the US as Starship). But we can hope for outcomes better than this. Alan Bond and Anthony Martin, of Project Daedalus fame, have written about so-called ‘worldships’ powered by nuclear pulse engines or solar sails, and Gregory Matloff has speculated on sail-driven craft on a 1000-year journey to the Centauri stars. These are voyages that may one day prove viable with manufacturing help from nanotechnology.
Strange Paths concludes this energizing essay with the thought that no matter how extravagant the propulsion and structural demands of an interstellar ark may be, the examination of its parameters now may somehow contribute to a future in which it happens. So it is with interstellar studies, which routinely posit technologies that are, at the least, next-generation, and extrapolate from them to show what could be done. We are in the lengthy but critical stage of base-building, creating the intellectual framework from which great things may one day grow.
by Paul Gilster | Feb 19, 2007 | Outer Solar System |
Perhaps ten to twenty kilometers under Europa’s global shell of ice there looks to be an ocean. That ice sheet is pretty thick for even our best drilling rigs but, says William B. McKinnon (Washington University, St. Louis), the deformities make a good case for its being relatively thin in comparison to the world it encircles. The smooth and largely uncratered surface implies that the ice has been active in recent geological time.
McKinnon made the case for a Europa mission at the American Geophysical Union meeting last December and continues to advocate close study of the Jovian moon, which seems to offer one of the most intriguing habitats for life’s development in our Solar System. The Galileo mission, due to its serious antenna problems, couldn’t get enough images to see active geysering, as we’ve found on Saturn’s moon Enceladus, but we do see what McKinnon calls “…lots of interesting ice tectonics, and surface eruptions with weird colors and spectral signatures whose compositional implications everyone just loves to argue about.”
All of which makes the case for a mission that can do close up work:
“To go into orbit around Europa with high-resolution cameras, spectral imagers and sophisticated, ice-penetrating radars of the sort mapping Mars right now, would allow us to really characterize that ocean and give us clues about the biogenic potential of the surface materials. We’d see to the bottom of the ice shell, I predict. It would be a fantastic proof of concept.”
Is a Europa mission feasible? Of course. Cassini has demonstrated what’s possible in such challenging environments. But NASA’s budgeteers are going to have trouble landing the $2 billion such a mission would cost, and there are the competing possibilities of Titan and Enceladus, not to mention the Moon program. But Ronald Greeley (Arizona State) couldn’t agree with McKinnon more on the importance of such a mission, and spoke about it yesterday at the American Association for the Advancement of Science meeting in San Francisco.
Greeley targets Europa’s potential for life, noting that the moon holds more water than all the oceans of Earth. If we can find the right energy source, perhaps through gravitational and magnetic interactions with Jupiter, we have a case for interesting things happening under that ice.
Another Europa plus: the presence of organic minerals mixing with the moon’s surface ice. “Ultimately,” says Greeley, “we want to get down through that ice shell and into the ocean where any action is. So it matters whether the ice is 10 yards thick, or 10 miles or more. The data we have today will never answer that question.”
Image: The image on the left shows a region of Europa’s crust made up of blocks which are thought to have broken apart and “rafted” into new positions. In this false color image, reddish-brown areas represent non-ice material resulting from geologic activity. White areas are rays of material ejected during the formation of the 25-km diameter impact crater Pwyll (see global view). Icy plains are shown in blue tones to distinguish possibly coarse-grained ice (dark blue) from fine-grained ice (light blue). Credit: NASA.
To test the thickness of Europa’s ice, Greeley advocates a study of the shell’s tidal flexing as the moon orbits Jupiter. A thick ice layer, one that goes all the way down to the ocean floor, would produce small tidal flexing, but a thinner shell should demonstrate much more effect. A spacecraft flying a high-precision altimeter could make such a measurement.
And here’s an interesting twist. Putting the spacecraft into orbit around Ganymede rather than Europa might be the best solution, the radiation environment at Europa itself being quite harsh. Greeley believes the Europan tidal effect could be measured from Ganymede orbit and offer a mission design that doesn’t produce radiation damage to sensitive electronics within a few months (not to mention the interesting possibilities around Ganymede). All of which points to more and more workable Europa concepts while leaving the big one unanswered. Given NASA’s budgetary woes, how do we fund the Europa gambit?
by Paul Gilster | Feb 18, 2007 | Culture and Society |
Star Trek technology will be the subject of two upcoming shows on the History Channel, with at least one segment devoted to the interstellar warp drive and the possibility of making it real. The Tau Zero Foundation’s Marc Millis will make an appearance in the context of his work on advanced propulsion for NASA. Star Trek Tech is to air on February 18th, with Star Trek: Beyond The Final Frontier following on the 19th. Click here for Dr. Millis’ background statement on the Tau Zero Foundation. For more, have a look at Centauri Dreams‘ archive of Foundation coverage.
by Paul Gilster | Feb 17, 2007 | Deep Sky Astronomy & Telescopes |
The night sky has always been a kind of time machine, allowing us to look farther into the past the deeper we look into space. But the heavens are also a time machine in another sense — by looking carefully, we can find stellar systems in almost every stage of development. We recently saw an example in the Helix Nebula, an object that suggests what our Solar System may look like in five billion years, after the Sun has gone into its red giant phase and then collapsed into a white dwarf.
Now have a look at the Sun as it may have been five billion years in the other direction, back when it was coalescing out of its own primordial materials. The Pillars of Creation image taken by Hubble has become iconic, a majestic, breathtaking vista of a star-forming region in M16, the Eagle Nebula. Below, we see a Hubble image of the Pillars overlaid with Chandra X-ray Observatory data showing infant stars being born.
Note the bright x-ray sources, most of which are young stars. Much harder to see in the image is E42 (see the caption below for its location). It is an evaporating gas globule (EGG), one of 73 found in the Pillars of Creation region. Only four seem massive enough to form a star, and of these only E42 seems to have the mass of the Sun. All four of these proto-stars are thought to be the youngest ever identified by astronomers, according to Jeffrey Linsky (JILA, Boulder). And all are apparently too young to be emitting x-rays.
Image: The Chandra data show bright X-ray sources in this field, most of which are young stars. In this image, red, green, and blue represent low, medium, and high energy X-rays. The Chandra data have been overlaid on the Hubble Space Telescope image to show the context of these X-ray data. E42 is located in the left pillar on the right edge of a node jutting out to the right about two-thirds of the way down the pillar. Credit: X-ray: NASA/CXC/U.Colorado/Linsky et al.; Optical: NASA/ESA/STScI/ASU/J.Hester & P.Scowen.
The question to be asked now is whether E42 will ever grow up. 7000 light years away, the Pillars show evidence of a nearby supernova explosion, according to a French study. Says Linsky, “My guess is that the shock wave from the supernova may have been far enough away so that E42 and some of the other stars may have survived. But I guess we will have to wait another thousand years or so to get the answer.”
A supernova is likely the cause of the heavy elements in the gas that formed our Sun and Solar System. That would indicate that the Sun was born under conditions not so different from what we see in the Pillars of Creation, an environment rocked by shockwaves and permeated by ultraviolet radiation. Chemical abundances tell the tale and help us interpret the signs of an otherwise unrecoverable past.
