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I’d never really seen the Milky Way until I saw it on a perfectly clear and moonless July night from a spot just below the Arc Dome in central Nevada. It spills a swath of patchy luminosity that seems to split the sky in half; a barred spiral galaxy, seen edge-on, and from within. One hundred billion intensely glowing stars, like sand grain jewels, each separated by miles. The photo above (taken by Steve Jurvetson last weekend from the Black Rock Desert in Nevada) reminded me of that experience.

The dark sky applet shows that the interior of Nevada (away from Las Vegas!) contains many of the least light-polluted areas of the United States.

Under a totally dark sky, you can distinctly see the star clouds in the foreground of the galactic center. It’s eerie to think that the 3-million solar mass black hole lurking in the center of the galaxy is just to the right of the bright luminosity of Baade’s Window near the boundary between Sagittarius and Scorpius.

The photo also shows Jupiter within a few degrees of Antares — a nice illustration of the fact that Jupiter appears slightly brighter than the brightest stars.

Newton used this similarity in apparent brightness to get the first real estimate of the staggering distances to the stars. He assumed that the stars are similar in absolute brightness to the sun, and he assumed that Jupiter (whose distance and angular size were known to him) is a perfect reflector of sunlight. This method underestimates the distance to Sirius by more than a factor of five, but it does a fairly reasonable job for Alpha Centauri.

I’m still really jazzed that the systemic users detected Gl 581 c prior to its discovery announcement.

A dramatic ESO press release “Artist’s impression” of the Gl 581 system is all over the web today. It shows a planet that appears quite dry, clearly drawing on a model of in-situ formation from silicates and iron. In all likelihood, however, the planet migrated from beyond the snowline in Gl 581’s protostellar disk. It likely contains at least an Earth’s mass worth of water, and the view from space would show the upper layers of a deep and stormy atmosphere. Jonathan Langton is running hydrodynamical simulations to try to get a sense of what the weather is like on this world, and we’re hoping to have an animation up very shortly. (See this brief description of yesterday’s splash image).

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One of my pet peeves is that it’s possible to produce far more accurate and photo-realistic press release images of extrasolar planets than is usually done. Artist’s impressions generally veer toward being luxuriously long on depicting what we don’t know and rudely short when it comes to presenting what we do know.

At the JPL Cassini/Huygens website, there is a trove of photos taken by the orbiter showing Saturn and its moons from different vantages and illumination conditions. The photos below were taken from a location near the ring plane, and show Rhea and Enceladus. The two pictures were taken one minute apart as Enceladus (314 miles in diameter) is occulted by the larger Rhea (949 miles across) as seen from the spacecraft.

This sequence of photos makes the most of the kinds of information that we do know about extrasolar planets, namely the system geometry, the relative sizes, the orbital dynamics, and the illumination. Note how the night side of Enceladus is eerily lit by the unseen Saturn. These particular photos, furthermore, are effortlessly discrete with respect to what we don’t know about extrasolar planets, namely the geological details of the surfaces. In the absence of concrete information, the surface is perhaps better left either to the mind’s eye or to the moment when we get the real image. In Cassini’s glorious up-close view, Enceladus was revealed to be far more bizarre and interesting than anyone had imagined:

The lighting in the Gl 581 press release image is pretty weird. We’re looking straight at the parent star, and yet planet “c” is seen in quarter phase, illuminated by a source of white light placed to the right of the scene. The star, however, is thought to be single.

The dynamic range of illumination in the scene is way off as well. If we’re looking straight at a star, then the field of view is completely flooded, saturated with light, and replete with lens flares. Planets are always lost in the glare if you’re looking straight at a star. Since any view of a star is seen through an optical system, I think it should be possible to achieve a better sense of optical dynamical range by correctly applying lens flares. Over the next year, we’ll be looking into this in much more depth.

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This website has an interesting discussion of how to correctly render the colors of stars. Dynamic range aside, and assuming that the star is a 3000K blackbody radiator (which isn’t quite right, but is a reasonably good approximation) the color should be a lighter shade of orange. As drawn, the color is more appropriate to the night-side glow of a hot Jupiter.

What about the perspective in the scene? At first glance, it looks like Gl 581 “b” might have been drawn a little too large. Using the information in table 1 of the Udry et al. preprint, and adopting a 1.7 Earth-diameter size for “c”, a Neptune-size for “b”, and 0.3 solar diameters for Gl 581 itself, we can draw the orbits and sizes of the planets to scale and almost have it fit correctly in an image that fits on the blog. (You may want to make your browser window wider):

In reality, because of pixelation, the tiny dots showing the planets are a bit larger than they should be. Ellipses are circles seen from an angle, so by applying a 1-dimensional re-scale with Adobe Illustrator, we can view the system to scale from a long distance away:

When I’m looking at the ESO press release image on my computer screen, the planet measures 7.5 cm across, and is located 45 cm from my eye. It subtends an angle of 9.5 degrees at the vantage from which its being viewed. The point of view is thus located 11 planetary radii above the surface of the planet, and drawn to scale, the geometry in the image looks like this:

As viewed from the skies of planet “c”, planet “b” subtends an angle of 36 arc minutes, and remarkably, would appear just slightly larger than the Moon appears from Earth. The parent star, on the other hand would subtend 2.3 degrees of the sky, which is about ~4.6 times larger than the Sun appears in our sky. (Given that Gl 581 “c” is in a habitable orbit, and given that the star is a red dwarf, it’s absolutely necessary to have the star fill more of the sky.) With this information, we can draw the correct angular sizes of the star and the planet “b” as seen from the vantage of the drawing. The planet “b” should be somewhat smaller than drawn, and the star should be somewhat larger. On the balance, however, the angular sizes aren’t that far away from being correct.

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It’s hard to get a more profound sense of physical remoteness and isolation in the United States than to drive east from Walker Lake, Nevada as the Sun sinks below the western horizon. It’s like Mars.

On a transcontinental flight last month, I had a window seat away from the wing. The sky was clear over Nevada, and the sun angle was low. It was an ideal situation for high-resolution imaging of a habitable terrestrial planet. The airplane view provides an interesting link between the experience of driving across the landscape and examining the satellite photos. The area just east of Walker Lake imparts an impression of a planet that’s very different from the global idea of the “pale blue dot.” The lake itself is salty, alkaline.

Source: Google Maps

The satellite and aerial photographs show that Walker Lake seems to be an evaporating remnant of what was once a much larger body of water.

Four billion years ago, Gusev crater on Mars probably looked very similar, with a sour central lake receeding with bathtub-ring clockwork.

Image: NASA

On Mars, there are only a few spots where a high-level of zoom will reveal artificial features:

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On Earth, in the region to the East of Walker Lake, there’s very little that can’t be ascribed to natural processes. This smooth black curve seems to be a wave cut bench of the vanished shoreline:

This feature, however, would be more challenging for a planetary geologist to explain. It’s obviously younger than the channels that it cuts across. Perhaps it’s fresh material that welled up from a crack in the Earth’s crust? There are volcanos dotted across the Basin and Range province.

Just south of the region shown in the splash image for this post, there are some extremely strange landforms…

And as is often the case in planetary exploration, when one wants to see even more detail,

Image: NASA New Horizons Spacecraft (false color by oklo).

One day, one hour, and nine minutes ago, the New Horizons spacecraft sailed flawlessly through its closest approach to Jupiter. A day later, Jupiter still looms large in New Horizon’s field of view, with an angular size more than five times greater than the size of the full moon in our sky.

Jupiter, during its 4.5 billion year history, has been visited by at least seven other probes. Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Ulysses, Galileo, and Cassini have all successfully made the journey. This latest encounter was buried beneath the news of a 500-point drop in the Dow Jones Industrial Average. The flyby, in fact, hasn’t even made it onto the Astronomy Picture of the Day!

A decade ago, many of the metal atoms in the New Horizons spacecraft were still buried in the Earth’s crust. A bit more than a year ago, the assembled spacecraft was flown, in a sealed pressurized container, to Cape Canaveral for launch. All through the past several weeks, it’s been taking pictures of the Jovian system. Most of the data will be radioed back to Earth over the coming months. The image above was taken on Monday, and shows a Von Karman vortex sheet trailing away from the Little Red Spot, currently the second-largest storm in the Solar System.

In a sense, the Jupiter encounter was mostly utilitarian. It boosted the spacecraft’s heliocentric velocity (at the expense of Jupiter’s orbital energy) and cut down the travel time to Pluto.

The next scheduled mission to Jupiter is Juno, the Jupiter Polar Orbiter, which is scheduled to arrive at the Jovian system in 2016.

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Saturn reached opposition yesterday, marking the moment in our yearly orbit when the Earth draws closest to the massive ringed giant. At midnight, Saturn is currently the only planet visible in the sky. It’s an odd feeling to stare at the bright unresolved spot of light that encompasses the planet, the rings, and the moons into a tiny golden point, and to know that Cassini, our robot emissary, is actually out there, almost a billion miles away, taking photograph after photograph, and radioing them back to a mere mouse click away.

Schematic image of the solar system on 2/11/2007 created at Solar System Live.

Saturn and its rings are good reflectors of light, but nevertheless, in the vicinity of the planet, the glare is far from overwhelming. The ambient light levels are only a bit more than 1% that of a bright summer day on Earth. It would be easy to stare at the crisply defined terminator marking the day-night boundary on the planet and the arcs of black shadow cast by the rings. On the Cassini website, there are many views that show the planet as it would appear to human eyes.

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Cassini also has the ability to photograph in the infrared. The following false-color photograph shows visible and infrared images of the planet superimposed. In the infrared, Saturn glows with interior heat — still welling up from the planet’s formation — that illuminates the night from within.

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The picture above is not a bad approximation of what a younger more massive planet would look like to the naked eye. 2M1207 b, for example, which seems to have a mass about five times that of Jupiter, is in a 1700-year orbit around a young 25-Jupiter mass brown dwarf. At 1250 Kelvin, 2M1207b is still warm enough to be self-luminous in the visible region of the spectrum. It is also slightly illuminated by the light of its companion (whose ~2500K surface is intrinsically 100 times more luminous.) Methane absorption and Rayleigh scattering of incident light in 2M1207 b’s atmosphere likely give the weak crescent a bluish-green hue.

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