Unexplained sighting in the night sky. Credit for base image: Nayu Kim

Ever seen something in the sky that was unusual, and which you couldn't explain? I've received calls from some of you and have done my best to suggest explanations. Many of you have thanked me for my second-hand appraisals (second-hand, because I wasn't present to see what you saw). A few have rejected the "mundane" possibilities I offer, insisting what they saw wasn't what I proposed at all.

So this post is a condensed version of, "My Guide To Identifying Unexplained and Unidentified Apparitions in the Night Sky"–just a peek at my process of armchair evaluation of unexplained sightings.

Disclaimer: While I do believe that life is probably common in the universe, and that if one planet (ours) could develop intelligent life that in turn developed a technological, space-faring civilization, so could others. But in my experience as an astronomical observer of the world around me and the sky above, I have never seen anything for which my only possible explanation is a flying saucer. My process is to look for the simplest, most natural or human-related explanations first when possible.

The easiest ones are planets—in particular, Venus and Jupiter. Since planets move around in the sky, they regularly appear in different locations at different times in spots people didn't see them before. And, being so bright at times, these two often get questioned: is it a plane? Is it the International Space Station? Has a star gone supernova?

Fortunately, we know where the planets are at all times, so when I get a call asking about the brilliant white light shining in the west just after sunset, for example, it's easy to finger the culprit.

This happens with some bright stars on occasion, like Sirius. In fact, when I was a teenager, I had the personal experience of walking outside one night, looking up and seeing a brilliant, flickering prismatic apparition that I swore I'd never seen before. It took me some time to figure out that it was merely the brightest star in the night sky and it was supposed to look that way! I always think back to this experience when listening to your descriptions of the fantastic and strange things you've seen in the sky.

By the way, stars twinkle, planets don't (much). That's another way to tell them apart, other than consulting an app on your smart phone.

What about things that move–that is, with speed and direction different from normal "diurnal" motion (motion caused by Earth's rotation)? When you observe something moving, relative to the background stars or horizon, there are generally three (mundane) things it is likely to be: an aircraft, a spacecraft, or a meteor/meteorite.

Spacecraft (let's start with artificial satellites, the International Space Station, and in times of yore the Space Shuttle) can appear to move like a plane, but with the defining feature that they are always a single point of white light. Depending on how far from Earth they orbit they will move at different paces (just like aircraft at different altitudes), but since they are at least 150-200 miles above Earth's surface, they're too small to be seen as anything more than a point of light. And as the light they shine is actually reflected sunlight, they will be white. Some of them may flash, or pulse, as reflective surfaces like solar panels turn in the sunlight. Also, because they are in orbit in a ballistic trajectory, you won't be seeing them change direction.

An aircraft—or more correctly, at night, an aircraft's wing and fuselage lights—can appear as more than a single point of light, and these lights can bear color. Typical aircraft (commercial and private alike) have a lighting configuration in common: green for starboard, red for port, and blinking white at wingtips, tailtop, and tailtip. And if they're heading directly at you with their landing lights on, they may appear to flare up and barely move at all.

The shape the wing and fuselage-lights form (what kind of triangle or diamond they make) depends on the style of aircraft (where the wingtips are relative to the tail, etc.), but I'd say green, red, and flashing white are a dead giveaway for an airplane (or a flying saucer trying to look like one.)

Military aircraft can look unusual, depending on what they are and what kind of maneuvers they're on. They can even fly without lights on at all. In Flagstaff, Arizona, I once saw a simple triangle of steady, white lights fly over and have always assumed it was a stealth fighter on night maneuvers.

Meteors and meteorites (bits of interplanetary metal and rock that burn up in our atmosphere, or that are large enough to hit the ground before burning up completely, respectively) also have their hallmark behaviors and appearances.

Fainter ones will appear white, while brighter ones can show some color—blue, green, orange, depending on their composition and how hot they get. Most leave smooth, straight streaks, but some can exhibit "flame-like" raggedness, like long luminous gashes in the night. Some can even explode. But all of them move very fast, lasting only a couple seconds or typically less, yet crossing a good portion of sky in the process. You'll only see an airplane moving that fast if it passes thirty feet over your head.

If none of this helps you sort out what you've seen aloft, try this chart. And remember, if you're not sure what it is, it doesn't hurt to smile and make no aggressive moves….

The "Ice Giant" Neptune. Credit: NASA/Voyager 2

Would you believe we discovered the planet Neptune only one year ago? Weird; I seem to have heard about this planet all my life—it was even my favorite planet at one point, back in childhood. What's this paradox?

Well, not a paradox—just semantics. It has been one year since Neptune's discovery. One Neptunian year. It takes Neptune, the most distant known (official) planet, about 165 years to orbit the Sun one time.

Doing the math backward, 2011 – 165 = 1846. Neptune was officially discovered on September 23rd of that year in a sort of virtual team effort-slash-international-rivalry extravaganza by at least three people. (To be nit-picky, it was actually July 12th when Neptune completed one Neptunian year since its discovery, since the Neptunian year is a couple of months short of exactly 165 Earth years….)

Usually, the discovery of a celestial object is credited to one individual: the first to spot it through a telescope. Sometimes there is battle over the credit because two people may have spotted the object around the same time, and documented proof of who found it first is needed to settle the debate, or lack thereof to let the controversy go on.

But Neptune was the first planet to be discovered mathematically, based on observed perturbations in the motion of Uranus (Uranus, incidentally, was the first planet to actually be discovered; it's difficult to say that any of the other planets were "discovered" in the conventional sense, as all of them—Mercury, Venus, Mars, Jupiter, and Saturn—are naked-eye objects, and so have been available to human eyesight since human eyesight was invented).

In Neptune's case, in France and in England, Alexis Bouvard and John Couch Adams, respectively, each deduced that Uranus' orbital perturbation was caused by the influence of another as yet undiscovered planet. Another Frenchman, Urbain Le Verrier, mathematically predicted the position of this unseen lurker, and German astronomer Johann Galle turned a telescope to the predicted position, finding Neptune less than a degree away. Eventually, credit for Neptune's discovery—or at least the process of discovery–was divvied up among the group.

Why was Neptune once my favorite planet in childhood? Well…because it's blue, of course! Just as good a reason as any, and blue was my favorite color. Like its slightly larger near-twin, Uranus, Neptune's upper atmosphere is made mostly of hydrogen and helium gas, but the presence of methane tints it blue. The thick atmospheric shell is believed to extend downward 10 to 20 percent the distance to the planet's center.

Deeper down is thought to exist a mantle containing significant amounts of volatile materials, like methane, ammonia, and water. This hot, dense, high-pressure fluid shell might be thought of as an "ocean" more than an atmosphere—although the distinction between gas and liquid is blurred by the extraordinary temperature and pressure of the material (taking an extreme example of what I mean, even the Sun's core, with a density a hundred times greater than liquid water on Earth, is considered a gas–or plasma–by virtue of its great temperature).

Nevertheless, sometimes Neptune, and Uranus, are referred to as "liquid giants"—or even "ice giants", though it's difficult for me to reconcile the term "ice" with the multi-thousand degree temperatures deep down inside these worlds….

Under all that stuff, Neptune's solid core, made of rock and nickel iron primarily, has a mass not much greater than planet Earth—as if, at the center of this giant piece of liquid confection, there exists a crunchy center, a shrouded planet Earth. What a gem! And maybe literally, as it is speculated that deep inside the planet's mantle, carbon atoms coming from methane may be squeezed by the pressure into diamond crystals, which "rain" down upon the rocky core. Neptune, planet of riches…if only….

At Chabot, we'll be having a Neptune's First Anniversary since Discovery celebration on that date, September 23rd. Hope you can join us!

Artist concept of an Earthlike Exoplanet. Credit: ESO

It's been 15 years since the first extrasolar planet (exoplanet: planet orbiting a star other than our Sun) was confirmed to exist. Since that time and until the launch of Kepler, subsequent detections have racked up the number by about one a month to well over 400. Planets, see? Worlds! What would we name them if we knew them more intimately, as we do Jupiter and Neptune?

What are they like, and how are we seeing them? Well—we don't exactly see them; we detect their presence by their affect on their parent stars: small wiggles and wobbles that their gravity and motion cause in their star, or tiny dips in star brightness when they pass in front of it ("transit").

As for what they're like, so far we've detected mostly very large planets—gas giants, like Jupiter, Saturn, Uranus, and Neptune—that are relatively close to their stars. "Hot Jupiters" they've been called, for obvious reasons. The bigger the planet, the greater its gravitational or eclipsing effects; the closer the planet to its star, the shorter its orbital period and so the more frequently we can detect their influences.

NASA's Kepler mission has a slightly different goal than finding Hot Jupiters. Using the "transit method" of looking for small drops in star brightness, Kepler is looking for Earth-sized planets orbiting their stars at Earth-like distances—in a nutshell, we're looking for environments similar to those of Earth, since Earth's environment is the one that we know supports life.

Since the drop in light caused by the transit of a small planet is much more difficult to detect, Kepler is located in space, outside of Earth's distorting atmosphere. And since Kepler is looking for transits that should occur only one time in many months, and which may last only hours, it must observe constantly over a long time, without "blinking".

So what has Kepler come up with so far? NASA recently released data from Kepler taken in the first 43 days of its science mission. How many candidates for exoplanethood, you ask? The report announced a total of 706 possible detections in that 43 days—although NASA has only released the data on 306 of the lower priority detections, holding back a balance of about 400 "more interesting" candidates for more detailed study. What has piqued their interest on these, I really want to know….

And the Kepler candidates are not a mix of Hot Jupiters; as the mission was designed for, these potential planets are smaller and with longer orbital periods. In the mix there are a number of Neptune-like possibilities, and a larger number further down the size scale closer to true Earth-like character.

So, in 15 years of observations made from Earth telescopes, a bit over 400 exoplanets. In Kepler's first 43 days of observation: 706.

Okay, the numbers look cool, but I must point out that the 400 (actually closer to 460 as of June 2010) convenctional exoplanet detections are all confirmed to exist, while the 706 from Kepler have yet to be—but that's mostly a matter of time and followup. They may not all end up being confirmed; a number of them will likely turn out to be "false positives". But, I expect many of them will end up being confirmed–and Kepler has another couple of years of left before its mission ends.

How many planets—and how many Earth-sized worlds—will end up being discovered between now and then? Maybe someone should start a Kepler exoplanet poll….

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August 12 2010 conjunction of Venus, Mars, Saturn, with Moon and Mercury

I don't usually blog about things going on in the sky, unless those goings on are things we witness through telescopes or robotic space probes—things that can't be seen with the unaided eye. I thought I'd make an exception in this case.

Mark your calendars for August 12th—and cross your fingers that the weather is clear that evening. Then, plan to be in a spot where you can have an unobstructed view of the western horizon. Check your calendar frequently to remind yourself, and when the date draws near, set your alarm clock….

Okay, what I'm going on about here is an upcoming "conjunction" of objects that will be at its stunning best on this evening. The players: Venus, Mars, Saturn, Mercury, and our own Moon, performing together as an ensemble for a limited engagement, on the stage of the western horizon.

Right now (as of mid June), Saturn (high in the south in Virgo), Mars (in the southwest in Leo, near the bright star Regulus), and Venus (that really stupendously bright thing that will give you second-hand sunburn if you're not careful–low on the western horizon, next to the twin stars of Castor and Pollux, in Gemini) are all strung out in a long, well-spaced line, as if queuing up for some great performance. But, as time goes forward, these three will gradually move closer and closer to each other, gathering toward the western twilight.

Around early to mid August, these three will be in a quite compact little group, at one point forming a nice little triangle. But on August 12, not only will they be in about their tightest grouping of this conjunction, they will be joined by the thin crescent of the Moon, just past the New phase, and that Mercurial planet—what else? Mercury—in one amazing gathering of luminaries.

It's not the end of the world, or a time of great change—at least, not a time of great change CAUSED by the conjunction–but it is a rare and beautiful alignment of celestial bodies that we don't get to see more than once every few years. Hence, I'm blogging about it so you don't miss it!

If this celestial stage act were not reward enough for those who find beauty in nature's rare and wondrous events, August 12th is also the peak of the Perseid Meteor Shower, the most reliable of the annual meteor showers.

So, no excuses: make a plan, put together a nocturnal picnic, search a spot from where you can view the western horizon, and, if you can one, that's away from city lights as much as possible (to make it easier to see those meteors).

Enjoy!

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A limb shot of Mercury's horizon taken by the
MESSENGER spacecraft on January 14, 2008.
Photo Credit "NASA/MESSENGER"

If you can take a name like "Mercury Surface, Space Environment, Geochemistry and Ranging" and craft it into a neat acronym like MESSENGER, then you may have a future working with NASA….

And no, this blog isn't about NASA acronymizations, but rather the heat-resistant robot behind one of them. MESSENGER is the space probe that NASA sent to Mercury to give the Solar System's innermost planet the first up-close look since 1975, when Mariner 10 flew by.

Though MESSENGER's main mission will begin in earnest when it returns to Mercury and finally settles into an orbit around the planet, on March 18th 2011, we were given a tantalizing peak last January 14th when the probe made its initial flyby.

What did this quick, on the fly snapshot tell us that we didn't know before? Well-a lot, considering Mercury has been one of the least understood planets in the Solar System, and was for a long time thought to be similar in character to our own Moon. Mercury is shaping up to be a lot less like Earth's Moon than its gray, cratered, airless appearance would mislead.

One key difference: density-how much material is packed into the planet; or how heavy a standard sized chunk of it would be. Our Moon is a lightweight on this score, with an average density of only 3.4 grams per cubic centimeter, while Mercury weighs in at a hefty 5.427 g/cc-almost as dense as Earth.

Another key difference: magnetic field. Planets like Earth and the Gas Giant worlds (Jupiter et al) generate respectable magnetic force fields, useful for everything from deflecting plasma flowing from the Sun (the "solar wind") to properly directing magnetic compass needles. Venus, Mars, and our Moon do not possess magnetic fields worth mentioning, as it turns out.

Mercury, on the other hand, does. Planetary magnetic fields are believed to be generated by currents in a planet's liquid outer core-like how the electric current in the wire coil of an electromagnet generates a magnetic field. Mercury's magnetic field suggests it still has some activity in its core-molten metals circulating in currents as the core slowly cools off. And speaking of Mercury's core, it appears to comprise 60% of the planet's mass-about twice what is "typical" for Terrestrial (solid) planets.

I've often imagined Mercury to be a cosmic goldmine, with its apparent richness in metals and its density. I wonder if an astronaut could just walk along and pick up chunks of gold from its surface….

Another interesting find by MESSENGER is that some of the flat plains on Mercury may have been formed by volcanoes, long ago. In particular, MESSENGER imaged a number of volcanoes along the edge of the Caloris Basin, a large impact basin-one of the largest in the Solar System, at 1550 kilometers across.

The news coming out of the innermost region of the Solar System makes me giddy. Too bad I have to wait until 2011 for my next look at Mercury. These things take time.

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