Response to the August 12 post about Fermi's Paradox was heavy, an indication that the physicist's famous question -- Where are they? -- will not go away. A number of readers asked for background on my statement (drawn from Milan ?irkovi?'s paper) that the average age of Earth-like planets in the Milky Way is now thought to be 6.4 billion years, an indication that there should be planets that have had a two billion year head start on Earth in terms of evolving intelligent life. The number 6.4 billion is a broad estimate, to be sure, but it has been the subject of intense investigation by Charles H. Lineweaver and colleagues. Lineweaver's key paper "An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect" ran in Icarus Vol. 151, No. 2 (2001), pp. 307-313 (available here in PDF form), and was followed by a paper in collaboration with Yeshe Fenner and Brad Gibson titled "The Galactic Habitable Zone and the Age...

We now have 156 confirmed extrasolar planets orbiting 133 stars, making for 17 multiple planet systems. Keeping up with these fast-breaking discoveries is a challenge, but Julia Espresate at the Instituto de Astronomía (Ciudad Universitaria, Mexico) has produced two useful catalogs now available on the arXiv site. The first lists stellar data including spectral type, luminosity, rotation period, stellar metalicity, age and other factors for the 133 stars. The second provides a breakdown of data for the 156 extrasolar planets so far detected. Good information on extrasolar planets has been available on the Internet for a while, as witness the Extrasolar Planets Encyclopaedia. What has been lacking is a source that ties together the planetary information with data on the characteristics of the parent stars -- the latter tend to be widely scattered in the scientific literature. Thus the utility of Espresate's work, which also reveals how great are our gaps in knowledge of these systems....

A star in the constellation Serpens may be a close match for our own Sun. HD 143436 (also known as HIP 78399, from its listing in the Hipparcos survey) is an 8th magnitude star that's 140 light years from Earth and visible with binoculars. According to a recent article on the star by astronomer Ken Croswell, both its spectral type and its absolute magnitude are closely similar to Sol, and the star appears to be equally hot, with a temperature of 5768 Kelvin vs. the Sun's 5777 K. In terms of mass, HD 143436 is the Sun's twin. Finds like this are intriguing because they raise the possibility that similar stars have similar solar systems, within which may lurk a terrestrial world. No one knows whether this star has a planet like Earth around it -- or any planets at all, for that matter -- but we do know that the close stellar match calls for further work. Exactly how old HD 143436 is remains conjectural, with an uncertainty either way of 2.9 billion years. The star may be as old as the...

By now we all know what a 'hot Jupiter' is -- a gas giant orbiting breathtakingly close to its parent star. The radial velocity searches for extrasolar planets that have found so many new worlds are particularly sensitive to high-mass planets in close orbits, so it makes sense that the early list of discoveries would be populated mostly with hot Jupiters. It's intriguing (and typical of the entire field of extrasolar planet detection) that this is a category of planet few scientists expected to find, especially in such numbers. But look what has happened to the planet hunt. In 2000, Geoff Marcy and Paul Butler detected the first planet with a mass below that of Saturn. It orbits the star HD 46375, some 109 light years away in the constellation Monoceros. The duo also discovered a planet 70 percent of Saturn's mass orbiting the star 79 Ceti, 117 light years away in the constellation Cetus. In 2004, a team led by Portuguese researcher Nuno Santos discovered a planet 14 times the size...

So many of the planets discovered in the last ten years have been gas giants, circling their parent stars in extremely tight orbits. We assume there are rocky, terrestrial worlds out there in abundance, but until more advanced detection techniques are in place, how can we be sure? An important answer may be offered by BD +20 307, a Sun-like star some 300 light years from our Solar System. It's surrounded by a warm disk of silicate dust particles that shows all the signs of being formed from the collision of rocky bodies up to planet size. Located in the constellation Aries, the star has one more ace up its sleeve. Its dust -- found in greater profusion than has ever been observed around a Sun-like star this long after its formation -- exists at distances comparable to that of the Earth from the Sun. Finding such an infrared dust signature at Earth-like distances (i.e., 1 AU) has long been a goal of researchers. As revealed in the July 21 issue of the British science journal Nature,...

How unlikely would it be to find a 200-year old person? That's the comparison astronomer Lee Hartmann (Harvard-Smithsonian Center for Astrophysics) is using in talking about a dust disk around a pair of red dwarf stars. The disk looks conventional enough -- as examined by the Spitzer Space Telescope, its inner edge is about 65 million miles from the binary stars, and it seems to extend outward for 650 million miles. That kind of disk should lead, according to current theory, to planetary formation within a few million years. But the disk in question has been estimated to be 25 million years old, and it shows no evidence whatsoever of having created a planetary system. In fact, a dust disk that old shouldn't exist at all; most newborn stars show no dust disks after just a few million years. All that material has by that time gone into the making of full-sized planets. Image: Astronomers were surprised to discover a 25-million-year-old protoplanetary disk around a pair of red dwarf...

A planet with three suns in its sky staggers the imagination -- how can a stable orbit exist for such a world? In fact, the only references are science fictional, which is why Maciej Konacki, a senior postdoctoral scholar in planetary science at Caltech, refers to the first such planet to be found as a 'Tatooine' planet. The name is a nod to Luke Skywalker's home world in the first of the Star Wars movies. An earlier science fiction reference might be Stanton Coblentz' Under the Triple Suns (1955), one of the few novels to posit planets around star systems this complicated. But Konacki isn't writing science fiction. He found his Jupiter-sized planet around the triple star system HD 188753 using a new method of measurement that lets him examine stellar velocities even in close multiple-star systems. Such systems have generally been avoided by planet hunters because precision velocity measurements are much easier to make around single stars, and also because theory suggested that...

Big mirrors make all the difference in optical astronomy. A 100 meter telescope (compensating for atmospheric disturbances) could separate two points on the moon two meters apart. Compare that to the 95 meters the Hubble Space Telescope can resolve and you can see that there is a case for Earth-based optical searches for planets around other stars. But how do we build such gigantic mirrors? Today's most advanced designs -- 8 to 10-meter instruments -- are created around mirrors that were constructed from smaller mirror segments. They use computers to achieve the needed fine-tuning so that the mirrored parts act like a single surface. Stepping this technology up to the 100-meter level, as astronomers in Europe are now discussing, would open up the search for terrestrial worlds around hundreds of nearby stars. As examined earlier in these pages, a 100-meter telescope should be able to detect Earth-like planets around stars as far as 100 light years away, a sphere containing some 1000...

A planet circling the star HD 149026 is certainly not the most massive extrasolar world we've discovered. But it does take honors on one count: it boasts the largest solid core ever found. Detected by a consortium of American, Japanese and Chilean astronomers, the planet is roughly equal to Saturn in mass though significantly smaller in diameter. It is being studied not only by analyzing its gravitational effects on HD 149026, but also by virtue of the fact that it transits the face of the star, dimming the starlight and allowing much more extensive measurements of its size, mass and density. Located some 250 light years from Earth, the planet takes 2.87 days to circle its primary. Modeling its structure provides indications that the new planet's core is 70 times the mass of the Earth. And that gets us into interesting territory, for it has implications for our theories about how planets form. The so-called 'core accretion' theory of planet formation says that planets begin as small...

If you want to see planet building happening before your eyes, turn your attention to TW Hydrae. Located 180 light years from the Sun in the constellation Hydra (the water snake), TW Hydrae is ten million years old, a celestial infant, with a mass four-fifths that of the Sun. Now researchers have discovered that the protoplanetary disk surrounding it contains more than enough material to form at least one and probably more Jupiter sized planets. Behind the new study are David Wilner (Harvard-Smithsonian Center for Astrophysics) and colleagues, whose work was just published in the June 20, 2005, issue of The Astrophysical Journal Letters. The team has shown that a vast swarm of pebbles extending out a billion miles from the star is in the early stages of planet formation. The small objects, according to current models, should grow in size as they continue to collide and eventually form planets. "We're seeing planet building happening right before our eyes," said Wilner. "The...

An unseen planet is the cause of the unusual geometry of a dusty ring around the star Fomalhaut (HD 216956). So say University of California at Berkeley scientists after examining a detailed, visible light image from the Hubble Space Telescope's Advanced Camera for Surveys. What's striking about the image isn't the ring itself, but the fact that it seems to be offset, evidently the result of the hidden planet reshaping the ring with its gravitational pull. The suspected planet is thought to be orbiting far away from Fomalhaut, along the ring's inner edge, making it anywhere from 4.7 billion to 6.5 billion miles away from the star. What the Hubble observations show is that the ring's inner edge is much sharper than its outer one, a sign that the ring is being gravitationally shaped by the unseen body. Another key pointer: the ring's unusually narrow width, some 2.3 billion miles. It would be much more diffuse without an object keeping it confined. Image: The top view, taken by NASA's...

With exquisite timing, the SETI Institute has announced the first of a series of workshops to study the habitability of planets around M-class red dwarfs. The issue became highly visible recently with the announcement of the rocky planet discovered around the red dwarf Gliese 876, some 15 light years from Earth. Although thought to be too hot for life as we know it, the new planet is a solid world orbiting a main sequence star, raising the question of genuinely terrestrial worlds around such stars. 'Main sequence' refers to stars that burn hydrogen in their cores, those that show up in a well-defined band on the famous Hertzsprung-Russell (HR) diagram, which plots the intrinsic brightness of stars against their surface temperatures (intrinsic brightness is the observed brightness of the star corrected for distance). Moving off the main sequence takes you into the domain of red giants, red and yellow supergiants, and white dwarfs. But way down on the lower right of the HR diagram, and...

Yesterday's post "On Red Dwarf Stars and the Hunt for Life" incorrectly stated the lifespan of an M-class red dwarf star as 100 times that of the Sun. The correct figure is ten times as long, making an age limit of perhaps 100 billion years for the average red dwarf. G-class stars like the Sun are expected to live about ten billion years. The red dwarf Gliese 876 is about 11 billion years old, more than twice the age of the Sun.

'Normal' is a tricky word when you're talking about extrasolar objects. As in 'normal star,' a phrase used during yesterday's news briefing about the new planet detected around Gliese 876, and in much of the press coverage since. The planet's low mass (about 5.9 times the mass of Earth) rules out the possibility that it is in any sense Jupiter-like, and the natural assumption is that this is a rocky world in a tight orbit around an M-class red dwarf.

Finding a planet that resembles the Earth is the ultimate goal of our exoplanetary explorations. It implies the possibility of life on a world not so different from our own, and encourages the speculation that Earth-like worlds are out there in huge numbers. We certainly haven't found such a place yet, but we're getting closer. Which is why Gliese 876, a red dwarf some 15 light years from our Solar System, made news today at a National Science Foundation briefing. No planet yet found -- and that includes roughly 155 extrasolar planets to date -- is as similar to Earth as the one found here. Gliese 876 is in the direction of the constellation Aquarius, and it is known to possess two larger gas-giant worlds as well as this much smaller neighbor. Not that conditions on the newfound world would be exactly habitable by our standards. The planet, some seven and a half times the mass of Earth, orbits its host star once every 1.94 days, and it's only two million miles from it (about...

We should have some interesting news about exoplanets tomorrow afternoon. That's when a media briefing will be given to reporters at the National Science Foundation in Arlington VA. The briefing is titled "Scientists Make New Discovery About Planets Outside Our Solar System," and although I have a hunch what this one is about, I'm not confident enough to run with it here. But it's intriguing that Jack Lissauer (NASA Ames), who is participating in the briefing, has done groundbreaking work on planetary formation in binary systems, as discussed in these pages back in December. Other participants in the briefing include exoplanetary pioneers Geoff Marcy (University of California, Berkeley) and Paul Butler (Carnegie Institution), as well as Eugenio Rivera from Lick Observatory. Rivera has previously worked with Lissauer on the 'resonant' orbits of two planets around the red dwarf Gliese 876, some 15 light years from Earth. Michael Turner, who heads NSF's Directorate of Mathematical and...

The first of the giant, close-in exoplanets (widely known as 'hot Jupiters') was discovered a decade ago around 51 Pegasi. Since then, we've found numerous other examples of such objects, challenging our theories of planetary formation and revising estimates of how common habitable worlds may be. Now a team of Canadian astronomers using the MOST (Microvariability & Oscillations of STars) space telescope has found yet another interesting anomaly: a planet that seems to force its star to rotate in synchrony with the planet's orbit. The planet is in orbit around the star tau Bootis. MOST has revealed that the star varies in light output in relation to the orbit of the planet around it. The explanation offered by the team studying tau Bootis b is that the planet has forced the outer envelope of the star to rotate so it always keeps the same face toward the planet. The effect is probably limited to the outer layers of gas in the star, in much the same way the Moon can cause a tidal bulge...