Finding three planets around a single star is newsworthy in itself, but when the planets are Neptune-class things get more interesting. And when one of these worlds is found to be in the star's habitable zone, Centauri Dreams definitely drops everything for a closer look. Not only that, but the system around HD 69830, a Sun-like star some 41 light years away, is also the home of an asteroid belt, making the comparison with our Solar System that much closer. Here's what we know, as reported in a paper in the May 18 Nature: The orbital periods of the three planets are 8.67, 31.6 and 197 days, with that outer world located near the inner edge of the zone where liquid water could exist. In terms of mass, this planet is not Earth-like; in fact, the measurements show the new planets to be between 10 and 18 times the mass of Earth. So what we're probably detecting in the habitable zone is a planet with a rocky/icy core surrounded by a dense atmosphere. We know nothing, of course, about...

Yesterday's post on UMBRAS and occulter technology focuses attention on the characteristics of light, some of them counter-intuitive but well demonstrated. And since we've also been talking recently about the nearby star Epsilon Eridani, I've chosen an image of that star to illustrate some of the problems with planetary detections. What you see below is via Massimo Marengo (Harvard-Smithsonian Center for Astrophysics), who has done such outstanding recent work on untangling the riddle of Epsilon Eridani's debris disk. This is a false color image with red, yellow, green and blue representing different infrared wavelengths. I ran the same image last summer, when Marengo posted it on his own weblog (he had used it to illustrate his team's work in a presentation at the American Astronomical Society meeting in San Diego). Image: A false-color infrared image of Epsilon Eridani. Credit: Massimo Marengo (CfA). What I want to single out here are the artifacts in the image. The red/orange...

In Centauri Dreams' imagination, the name Epsilon Eridani is magic. Like many of us, my earliest speculations about life on other worlds always came back to the nearby, Sun-like stars like Tau Ceti, Epsilon Eridani and Centauri A and B. Frank Drake used the first two as his targets for Project Ozma in 1960, an effort that continues to inspire SETI work today. And Epsilon Eridani is joined by Vega, Fomalhaut and Beta Pictoris as the first stars found by the Infrared Astronomical Satellite (IRAS) to have a cool debris disk somewhat analogous to our own Kuiper Belt. The fact that this K2 star is likely to be orbited by the closest exoplanet to our Sun is also exciting. Its planet seems to be slightly larger than Jupiter, with estimates ranging from 0.8 to 1.6 Jupiter masses, and an eccentric orbit varying from 5.3 to 1.3 AU (here again we see how important it is to establish the effect of gas giants on terrestrial worlds in the habitable zone). At 10.5 light years from us, Epsilon...

I remember wondering, while still getting acclimated to the odd existence of 'hot Jupiters' in those amazing first years of exoplanet discovery, what the view from a terrestrial world in one of those systems might be like. After all, a Jupiter-sized mass in close solar orbit must make for some unusual visual effects. Do terrestrial worlds exist around these stars? For that matter, what are the constraints on terrestrial planet formation in systems where gas giants orbit farther out, well past the habitable zone? These questions are occasioned by the work of Sean Raymond (University of Colorado), whose paper on the subject will soon run in the Astrophysical Journal Letters. Raymond looks at how the presence of gas giants would affect the late stages of terrestrial world formation and presents the results of his simulations on same. Bear this in mind: gas giants, it is now thought, must form within the first few million years of the early protoplanetary disk. Whereas terrestrial worlds...

How planets form is not an issue that will be settled any time soon, but two models have emerged that continue to energize research. We saw yesterday in a review of Alan Boss' new paper that gravitational instability is one way to create a gas giant. But I spent most of yesterday's post talking about UV radiation and its effects on the atmospheres of planets around M stars, a key part of Boss' explanation of so-called 'super-Earths' in these environments. So let's back up and talk about gravitational instability itself. As early as 1997, the astrophysicist had proposed that planet-sized clumps could form relatively quickly due to instabilities in the disk of dust and gas surrounding a young star. Boss believed these clumps could be massive enough to form a gas envelope, but the model was hard to use in any predictive sense and demanded more intensive computer simulations than were then available. Later work by Thomas Quinn, however, bears Boss out. Quinn (University of Washington)...

What we know about the planets circling M-class dwarf stars is changing rapidly. Recent microlensing surveys have revealed the existence of two 'super-Earths' -- rocky worlds 5.5 and 13 times as massive as the Earth -- around distant red dwarfs. Microlensing has also produced two gas giants around such stars. And radial velocity surveys have found systems like Gl 876, an M-class star orbited by an outer pair of gas giants and an inner super-Earth. Other radial velocity catches are Gl 436 and Gl 581, each accompanied by a super-Earth in a short-period orbit. A curious fact emerging from these studies is that the frequency of gas giants around M dwarfs seems to be lower than around F, G and K-type stars. In a new paper, Alan Boss (Carnegie Institute of Washington) discusses the formation of these planetary types, arguing that disk instability rather than core accretion may be the cause of their formation. An additional, and in his view critical, factor: the loss of planetary gas...

That oh so interesting planetary system around 51 Peg continues to fascinate exoplanet hunters. After all, this was the first planetary discovery around a main sequence star, and the formidable dimensions of the radial velocity dataset accrued before and since the discovery may lead to other planets in the same system. For the latest on 51 Peg, check the online proceedings of last August's colloquium, in which eighty astronomers provide their thoughts on 'hot Jupiters' and discuss recent observational facts about this intriguing system. Centauri Dreams is always delighted to see conference proceedings made available online. The only catch here is that the format is PDF -- yes, it's a standard of sorts, but there has to be a better way...