Not long ago, Ramses Ramirez (Earth-Life Science Institute, Tokyo) described his latest work on habitable zones to Centauri Dreams readers. Our own Alex Tolley (University of California) now focuses on Dr. Ramirez' quest for 'a more comprehensive habitable zone,' examining classical notions of worlds that could support life, how they have changed over time, and how we can broaden current models. We can see ways, for example, to extend the range of habitable zones at both their outer and inner edges. A look at our assumptions and the dangers implicit in the term 'Earth-like' should give us caution as we interpret the new exoplanet detections coming soon through space- and ground-based instruments. by Alex Tolley The Plains of Tartarus - Bruce Pennington In 1993, before we had detected any exoplanets, James Kasting, Daniel Whitmire, and Ray Reynolds published a modeled estimate of the habitable zone in our solar system [1]. They stated: "A one-dimensional climate model is used to...

Who among us hasn't speculated about the ultimate fate of the Solar System? The thought of our Sun growing into a vast red giant has preoccupied writers and readers since the days when H. G. Wells so memorably captured a far future scene through the eyes of his Time Traveler. And what a scene that was: "I cannot convey the sense of abominable desolation that hung over the world. The red eastern sky, the northward blackness, the salt dead sea, the stony beach crawling with these foul, slow-stirring monsters, the uniform poisonous-looking green of the lichenous plants, the thin air that hurts one's lungs: all contributed to an appalling effect. I moved on a hundred years, and there was the same red sun—a little larger, a little duller—the same dying sea, the same chill air, and the same crowd of earthy crustacea creeping in and out among the green weed and the red rocks. And in the westward sky, I saw a curved pale line like a vast new moon." Wells was working on a...

We've recently looked at gas giant planet formation, and specifically the stages in which Jupiter seems to have formed -- this is the work of Thomas Kruijer (University of Münster) and colleagues as summarized in A Three Part Model for Jupiter's Formation. Whether or not the details of Kruijer and team's model are correct, it seems evident that gas giants must form quickly, based on current theories. These involve the formation of a large solid core, with gas accretion building up a thick atmosphere at a time when the disk around the parent star is still rich in materials. In this thinking, planets like the Earth come along much later than the gas giants that are the first to form. Get a few million years into the evolution of a stellar system and there should be evidence of a gas giant, if one is going to form, but terrestrial worlds can take up to 100 million years to emerge. This has captured the interest of Nader Haghighipour (University of Hawaii), whose work was presented at...

One of the memorable things about 1995 (and this was also the year of the first detection of an exoplanet around a main sequence star) was the release of the Galileo spacecraft's descent probe. Dive into that howling maelstrom, it would seem, and instant obliteration should follow. But the probe had been designed with a heavy duty heat shield to protect it during its journey. It kept transmitting after scorching its way into Jupiter's atmosphere at 47 kilometers per second, 30 km/sec faster than Voyager 1. The probe returned data for fully 58 minutes before its demise. Here's how two science fiction novelists handle a descent into the Jovian clouds: Slowly, the fine fretwork of the ammonia cirrus clouds above him became obscured by brown and salmon layers of intervening chemistry, the air stained a nicotine-coloured haze of complex carbon molecules. Soon it was warmer than a summer's day out there, and already the gondola was enduring more than ten atmospheres of pressure, the...

How close would a passing star have to come to produce drastic results on the outer Solar System? According to researchers at the Max Planck Institute for Radio Astronomy in Bonn, roughly 3 times Neptune's distance would be disruptive enough to explain what we see beyond that planet's orbit today. Led by Susanne Pfalzner, the scientists have been modeling close stellar flybys of other planetary systems for years, but have only recently turned their attention to the eccentricities of our own system, where conditions beyond Neptune pose questions. Image: Artist's concept of a stellar system in the making with a protoplanetary disk surrounding a young star. Credit: NASA JPL-Caltech. A look at the Solar System's formation highlights the problematic nature of the process. Out to the orbit of Neptune, planets, dwarf planets and asteroids orbit with only small differences in orbital inclination, indicative of the flatness of the original disk from which all these objects drew their birth...

Beta Pictoris b continues to instruct us in the ways of exoplanet finding. Consider: The young world was identified in 2008 through direct imaging via the Very Large Telescope at the European Southern Observatory site at Cerro Paranal (Chile). Actually seeing an exoplanet is no small feat. We are in this case talking about a bright A-class star some 63 light years away in the wash of whose light we can pick out a comparatively small planet. But it was also a young planet, putting out plenty of heat amidst the large debris disk, the first such disk ever imaged. The earliest detections of planets around main sequence stars have involved radial velocity, using Doppler methods that can tell us the rate at which the star moves toward and then away from the Earth as it is affected by the planet orbiting it. But radial velocity is a tough call at Beta Pictoris because these changes are tiny, and we are dealing with a star those fast rotation and stellar pulsations obscure the needed signal....

The first time I ran into the term 'water world,' it had a seductive quality. After all, we think of habitable zones in terms of water on the surface, and a world with an overabundance of water suggested a kind of celestial Polynesia, archipelagos surrounded by a planet-circling, azure sea. But we immediately run into problems when we think about planets with substantially more water than Earth. For one thing, we may have no land at all. Let's leave aside the icy moons of our Solar System that may well contain oceans beneath their surface and concentrate on exoplanets in the interesting size range of two to four times the size of Earth. We have to ask what would happen if a planet were completely covered with water, with no run-off of nutrients from exposed rock. Such an ocean could be starved of key elements like phosphorus. Or how about a planet with a high-pressure zone of ice effectively cutting off the global ocean from the rocky mantle? A world with enough water -- 50 times...

Although it appears in the same constellation as seen from Earth (Centaurus), Omega Centauri has nothing to do with the Alpha Centauri stars that so interest interstellar flight theorists. The brightest globular cluster visible in our skies, Omega Centauri is anything but close (16,000 light years out) and, containing several million stars, is the largest globular cluster in our galaxy. We may in fact be looking at the core of a dwarf galaxy once absorbed by the Milky Way. But although it's quite distant, Omega Centauri may be the source of the relatively nearby Kapteyn's Star, just 13 light years from the Sun. Where this gets intriguing is that Kapteyn's Star, (named after Dutch astronomer Jacobus Kapteyn) is known to have at least two planets, one of them considered the oldest known potentially habitable planet -- let's call it a 'temperate Super-Earth', as Guillem Anglada-Escudé and team have done -- at 11 billion years old. Steven Kane (UC-Riverside), working with graduate...

Speaking of getting really, really close to a star, as we were yesterday in our discussion of the Parker Solar Probe, I couldn’t help but turn to new computer models of the ‘ultrahot Jupiter’ WASP-121b. I still find it delightful that the earliest exoplanet detections involved a category of planet that few scientists had imagined existed. These days we routinely discuss gas giants blisteringly close to their hosts, and even manage to extract information about their atmospheres through transmission spectroscopy, but few people expected such planets when we began to discover them. In fact, Apollo 11’s Buzz Aldrin had a role to play in what may be considered to be the first prediction of the worlds we would start calling ‘hot Jupiters.’ Working with John Barnes on his novel Encounter with Tiber (Grand Central, 1996), Aldrin asked physicist Greg Matloff whether a hydrogen-helium atmosphere as found in a Jupiter-class world could survive in an inner stellar system. Here’s how Matloff...