A 100 percent chance of life?

Karl has written asking about the recent discovery of Gliese 581g, a potentially Earth-like planet:

Planetary Astronomer Mike, are there friendly aliens on that planet waiting for us to greet them?

In particular, he was referencing this article and highly optimistic quote:

http://news.discovery.com/space/earth-like-planet-life.html

"Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say that the chances for life on this planet are 100 percent. I have almost no doubt about it," Steven Vogt, professor of astronomy and astrophysics at University of California Santa Cruz, told Discovery News.

I'll try to put this all in a little perspective. I've been in Pasadena this past week at the largest planetary conference of the year. This single quoted statement has been the source of a whole lot of jokes this week during the conference's off-hours, e.g. "I'd say there's about a 100% chance I'd like fries with that."

Now, acting on sources that are entirely a combination of hearsay and ad hominem arguments, I know some people here who know Vogt, who tell me that he is absolutely the kind of guy who would never, ever say something like this. So, if you're feeling even a little bit generous, it's not too much of a stretch to say that the press fumbled this one. This problem is rampant enough throughout astronomy that NASA actually offers seminars on how to speak to the press in a manner that, among other things, teaches you how to keep them from misquoting you.

A prime example of such an offense is the whole meme about "back in the 70's, scientists used to think we were about to have an ice age!" In truth, of 49 papers published between 1965 and 1979 which predicted global climate change, 42 predicted global warming while only 7 predicted a coming ice age. Why the ice age theory caught on in the mainstream press is not immediately obvious. Perhaps the authors of the 7 ice age papers were just more vocal (notably, 4 of those 7 papers were from just a single author who in later years became a very vocal global warming skeptic), but just as likely is that an imminent ice age simply sounds more sensational than a little spot of toasty weather. Either way, it's hurt the long-term credibility of climate science...but I'll save that for another rant.

As for the whole idea that the planet exists within the "habitable zone", I generally try to avoid that term. The whole concept of such a zone rests around the idea that there's a narrow range of locations at which liquid water can form. While location is one factor which plays into the equation, the properties of the planet tend to be much more important.

Planetary scientists talk about this in terms of the "equilibrium temperature" of a solar system body. Essentially, given the rate at which a body is absorbing heat from the Sun while also dissipating heat back out to space, you can find the temperature you'd expect it to be. Generally this simple equation only works well for big rocks which have no internal energy source or heat redistribution mechanisms. i.e. asteroids, moons, and dead planets like Mercury. Even this very simplified form of finding the temperature has an inherent dependence on the properties of the body, though - namely, its "albedo".

Albedo is just the reflectiveness of an object. Earth's average albedo is 36%, which means it reflects 36% of the incoming sunlight out to space, while 64% is absorbed and actually goes into heating the planet. Using only the equilibrium temperature, we'd expect Earth's average temperature to be -18 degrees Celsius, which is below the freezing point of water. What's been neglected in this formulation is the importance of the infrared absorbers in our atmosphere, better known as the greenhouse effect. Although they're relatively minor constituents by abundance, water vapor, carbon dioxide, and methane are enough to raise Earth's average temperature 33 degrees to a happy 15 degrees Celsius. This is warm enough for liquid water to exist and life to flourish.

The matter gets worse for the case of Venus. You may have seen it hanging low in the Western sky lately, bright enough to tell you it has a very high albedo. In fact, its albedo is 72%. Although Venus is 30% closer to the Sun, working with only equilibrium temperatures we'd actually expect Venus to be a few tens of degrees *colder* than Earth because it reflects so much more of the incoming sunlight. This makes the idea of a habitable zone very difficult to swallow since a planet closer to the Sun is actually predicted to be colder. This too, however, breaks down when you consider the greenhouse effect. Venus' atmosphere, almost 100 times thicker than Earth's and made of 95% carbon dioxide, is enough to raise the temperature nearly 500 degrees Celsius.

On the other side of the matter, there's Jupiter's moon Europa. At a distance 5.2 times greater than the Earth to the Sun, we'd expect this to be a very, very cold place - and at it's surface it most certainly is, with an equilibrium temperature of -170 degrees Celsius. At this location, we'd never expect Europa to be inside the habitable zone. Drill through its approximately 100-km thick ice shell, however, and you'll find an ocean so deep that it contains more liquid water than all the oceans on Earth. Again, the neglected heating term here is the internal energy of Europa generated from the massive tidal forces the moon feels from Jupiter's gravity. The case of Europa has opened astrobiologists to the possibility that Earth-like planets may not be the only place to look for E.T., but moons of large gas giant exoplanets. as well. The field of "exomoons" is fertile ground which has only just recently been opened up.

One more consideration: the method used to detect this specific extrasolar planet - "radial velocity" - can only tell us its minimum possible mass, not its actual mass. Radial velocity is based on the planet gravitationally tugging on its parent star, and we observe the doppler shifts of the star only along our line of sight. In other words, we only see the parent star's movement towards us or away from us. Given the specific amount of doppler shift that we observe, if the planet's orbit were oriented edge-on to our line of sight (i.e. a 90-degree inclination), then its mass should be three times that of Earth. If, however, the orbit's orientation is closer to face-on (i.e. a zero-degree inclination), then most of the tugging would be in the "plane of the sky", with only a small percentage being the to-and-fro motion we observe with doppler shifts. In this case, the total amount of tugging is quite a bit more than what we can observe in the doppler shifts, meaning the planet's mass would be substantially larger than three Earth-masses. (Non-trivial problem for the math-heads out there: find the expectation value of the inclination of a random planet's orbit. It's a neat answer.)

A final note here concerning the timing of this announcement: the Kepler mission results are expected soon. This spacecraft uses a very different technique than the radial velocity method outlined above. Rather, Kepler uses the technique of "transits". It stares at a set field of stars, watching for a specific kind of subtle dip in the brightness of one of them which can only be caused by an orbiting planet eclipsing its parent star. There are several nice things about this technique. First, we know its orbit must be nearly edge-on to produce an eclipse, which it turn means knowing its exact mass using a follow up radial velocity technique, not just its minimum mass. Second, by studying exactly which colors of light are absorbed more than others, we have a spectral fingerprint which allows us to identify the exact constituents of the planet's atmosphere, which in turn gives us a better handle on the planet's true temperature rather than just its equilibrium temperature.

Up until now the Kepler science team has been very hush-hush about the results coming in, but the rumor mill has been churning with whispers that several Earth-like planets are expected to be announced in a matter of months, and most astronomers I know are waiting with bated breath. With such an announcement looming on the horizon, announcing an Earth-like planet now is good opportunity to tap into this building excitement.

A star by any other name...

I've had multiple people ask me about the legitimacy of getting a star named after them or their loved ones. There are several private organizations out there which are more than willing to let you give them money in exchange for just such a commodity, but buyer beware.

In truth, such organizations are usually not outright scams. They generally claim that, for a price, your named star will be placed in a special star catalog which their organization administers. In that sense, they are completely honest.

That said, though, no astronomer is ever going to actually access such a catalog to find a star name, much less turn to their fellow astronomer and declare, "I'm going to use the telescope to observe Todd Jenkins, Jr. tonight. It's an exciting, magnetically variable A-type star in Draco." Some of the less scrupulous star-naming corporations may give you the illusion that astronomers are using the name of your star in the scientific community, but don't be fooled.

Rather, the official, astronomically-recognized names generally come from the International Astronomical Union (the IAU, the same folks who gave Pluto the shaft). The very brightest stars in the sky do have official names: Vega, Arcturus, Fomalhaut, and Capella, just to name a few. Most of these named stars have had their monikers passed down to us from the ancient Arabic astronomers - who kept astronomy thriving as Rome fell - though a few still maintain their much older Babylonian names.

Get past the first few hundred brightest stars, and they begin to take on some less exciting names. Around 1600, Johann Bayer took it upon himself to begin naming stars in a more orderly fashion, such that "Alpha" generally denotes the brightest star in a constellation, "Beta" the second brightest, and so on through the Greek alphabet. Naturally, there are repeats for the brightest stars: Alpha Scorpii, the brightest star in Scorpius, is better known as simply "Antares".

Around 1700, Flamsteed took this one step further. He designated stars simply by number in order of West-to-East, and named stars significantly fainter than the 24 letters of the Greek alphabet would allow. Thus, "1 Geminorum" is the most Western star - and approximately the first to rise over the Eastern horizon - in the constellation of Gemini. The 34th most westerly star in Taurus is just "34 Tauri" (bonus points to anyone who knows why this one is special).

After the first few thousand stars, though, the names suddenly become much less interesting. As the limit of naked-eye visibility is reached, telescopes start becoming necessary to see more stars. Large observing campaigns were carried out to catalog and precisely locate ever-dimmer stars, leaving us with decidedly unsexy names such as " HD 209458" or " SAO 151881" (more bonus points to anyone who knows why those stars are special).

There are still a few interesting names in the mix even for these very dim ones, often named after a given astronomer who studied it, such as Barnard's Star and Kapteyn's Star. For the most part, though, it's a desert of notable names.

Now, for comets your prospects are significantly better for getting something actually named after you which the scientific community will recognize. Tradition generally holds that comets are named after their discoverer. Thus, Comet Hale-Bopp was named after its co-discoverers Alan Hale and Thomas Bopp.

However, things have been getting a little dicey for comet hunters as automated robotic surveys find more comets than individuals lately. This has led to several comets all being named "Comet LINEAR" (for the Lincoln Near Earth Asteroid Research program), "Comet NEAT" (for the Near Earth Asteroid Tracking program), and "Comet LINEAR-NEAT".

Currently, your best prospects for getting something up in the sky named after you lie with the minor planets, i.e. the asteroids and Kuiper Belt Objects. Tradition holds that newly discovered minor planets need not merely be named after the discoverer, but that the discoverer may actually choose the name. Originally minor planet names followed the same naming convention as planets:

- (1) Ceres
- (2) Pallas
- (3) Juno
- (4) Vesta

These names are all based in Greco-Roman mythology, while the number preceding them indicates the order in which these objects were discovered. As the number of asteroids began to get into the thousands, novel mythological names were running scarce, leading astronomers to use the names of other famous scientists:

- (2001) Einstein
- (4987) Flamsteed
- (6143) Pythagoras
- (8000) Isaac Newton

Artists, philosophers, and various historical people were also allowed membership into the elite club:

- (4511) Rembrandt
- (5102) Benfranklin
- (5676) Voltaire

As the number of minor planets has now topped the hundreds of thousands, though, even that scheme has worn thin, leading to some fairly creative names over the years:

- (3568) ASCII
- (9007) James Bond
- (13681) Monty Python
- (19383) Rolling Stones
- (82332) Las Vegas

Perhaps most depressing, though, quietly tucked away in the mid 100,000's, you'll find an inconspicuous member deprived of its former glory:

- (134340) Pluto

A moment of silence, friends.

Point being, if you look at the full list, it's a virtual cornucopia of names. My advice: make friends with an astronomer who discovers asteroids.

Answers to the informal quiz, Part 4: The Universe, etc.

What, you say? Over six months since the last post on Dear Planetary Astronomer Mike? Surely that can't be right. Moving on...the final answers to the informal quiz!

• What's the difference between the Solar System, the Galaxy, and the Universe?

These are terms that to the layman might seem interchangeable as simply "big places that the Earth is a part of", but to astronomers these divisions are of paramount importance.

Our solar system consists of one star - the Sun - and all the objects that orbit it. From largest to smallest, these objects include:

- 2 gas giant planets (Jupiter and Saturn)
- 2 ice giant planets (Uranus and Neptune)
- 4 terrestrial planets (Mercury, Venus, Earth, and Mars)
- 5 dwarf planets (Pluto, Eris, Ceres, Haumea and Makemake)
- Several hundred thousand rocky asteroids
- Many thousands (?) of icy/rocky objects in the Kuiper belt
- Millions (?) of comets
- A whole lot of dust

There's no real hard limit to where our solar system ends, although as you travel farther and farther from the center, at some point you're no longer gravitationally bound to the Sun and begin feeling the gravitational pull of other nearby stars. This limit is generally placed somewhere around the 1 light-year mark (the next closest star is 4.2 light years away) and roughly marks the outer edge of the Oort cloud, a massive hypothesized reservoir of our solar system's comets.

Next up: the galaxy. From a very dark location away from city lights, you can often make out the structure of our galaxy - the Milky Way - as a faint band of light across the sky. The Romans named this the "Via Lactea", literally the "Way of Milk", and forms the root of the word galaxy.

Our Sun is just one star of roughly 300 billion found in the Milky Way Galaxy. In addition to all those billions of stars - each of which could have many planets - there's another several billion solar masses worth of gas and dust from which new stars are constantly forming, and old stars are constantly replenishing. Lying at the exact center of this giant "star city" is a supermassive black hole, calculated to be roughly 3 million times more massive than our own Sun.

Even more massive than all of our galaxy's stars, gas, dust, and the central black hole put together, though, is our galaxy's supply of dark matter. As stated in a previous post, we don't really know what dark matter is exactly, but we know it's there. The mass of our galaxy's dark matter is currently estimated to be at least 1 trillion times the mass of our Sun.

Finally, the universe. It's everything...literally. Anything that exists, exists within our universe. We know there exist many, many billions of galaxies - each with many billions of stars - which stretch out across a cosmic web-like structure. Between these web-like filaments, each made of thousands of galaxies, are gigantic voids where little matter exists at all. The assumption is this void-and-filament structure came initially from microscopic density fluctuations just a few seconds after the Big Bang which has been ballooning outwards ever since.

• What is a star?

A star is nothing more than a ball of gas which is massive enough to produce sufficient internal pressure to start hydrogen fusion. As mentioned in the last post, our Sun is somewhat average on the mass scale of stars, though there tends to be a whole lot more small stars than large stars. For a more detailed examination of the life of stars, see this post.

• How are planets different than stars?

The big difference here is that, going by the above answer, planets *don't* have enough mass to produce sufficient internal pressure to start hydrogen fusion. Our solar system's largest planet, Jupiter, is still quite far from being a star. In fact, Jupiter would need to be 80 times more massive to produce enough internal pressure to start hydrogen fusion at its core.

Now, there is an intermediate group of objects known as "brown dwarfs", which aren't quite stars, and aren't quite planets, either. If Jupiter were only 13 times more massive it could fuse deuterium, an uncommon isotope of hydrogen (even though it still couldn't fuse regular old hydrogen).

So, a brown dwarf can shine like a star for a little while, but the problem is deuterium is uncommon. Once a brown dwarf uses up what little deuterium it has in a matter of a couple million years, that's it...it just cools down like a planet from then on. (Note that a couple million years is nothing compared to the several billion years our Sun will last, or even the trillions of years some small red stars will last.)

• Where do the stars go during the day?

Why, they're still there, of course! Just because the sky is lit up with sunlight during the day doesn't mean that the stars have "gone" anywhere. If you carefully point a telescope at the brighter stars in the middle of the day, you can actually make them out in the clear blue sky! (Cautionary because-our-lawyers-told-us-we'd-better note: do not ever, ever, ever point a telescope at the Sun!).

On days with very clear blue skies, you can even spot the planet Venus completely unaided without any telescope. It looks like a little white dot hanging in the daylit sky...the trick is to know exactly where to look.

• What's the farthest human beings have ever traveled in space?

In spite of all the sci-fi you may have seen, humans just haven't traveled that far from the planet that brought them into existence. To this day, the Moon is the most distant object humans have ever reached - only about 250,000 miles away. Compare that to the nearest planet, Venus, which even at its closest approach to Earth is over 100 times farther than the Moon.

Answers to the informal quiz, Part 3: The Sun

I know it's been a while since my last post, but research and conferences have been taking up all of my time lately...at least I've got a new first-author paper submitted to the journals! Nonetheless, I extend my deepest apologies to my loyal readers.

So with my mea culpa out of of the way, let's get to the next set of answers!

• Compare the Sun to the stars.

So, this is a bit of a trick question. The standard answer is that the our Sun is an "average-to-small" star. In some sense, this is true - stars can range from 100 times more massive to 10 times less massive than our Sun. They can also be 8 times hotter, or 2 times cooler. Finally, they can be 1,000,000 times brighter or 10,000 times dimmer than our Sun. So, in this sense, our Sun is in the middle of these ranges (though a little on the smaller side).

However, there are far, far more small, cool, dim stars than big, hot, bright stars...the vast majority of stars in our universe are small little red dwarf stars. In fact, the distribution is so lopsided that the mass of all the small stars put together is many times larger than the mass of all the big stars put together (though exactly how many times is still debated). So, in that sense, our Sun is actually bigger, hotter, and brighter than most other stars.

• Why does the Sun shine?

Deep in the core of the Sun, hydrogen atoms are packed under such great pressure and temperature - being gravitationally compressed by the outer layers of the Sun - that nuclear fusion ensues. This is similar to how a hydrogen bomb works. In our Sun, four hydrogen atoms will collide together to eventually make one helium atom.

However, the resulting helium atom weighs just slightly less than the four hydrogen atoms that went into it. This tiny bit of missing mass is actually converted into pure energy via Einstein's famous equation E=mc^2. This energy, which leaves the nucleus of the atom as an extremely energetic gamma ray photon (a particle of light), bounces around the interior of the Sun countless times, getting absorbed and re-emitted by surrounding atoms and losing just a little energy to them each time. By the time it reaches the surface of the Sun - a process which takes, on average, about 1 million years - the photon has lost enough energy to emerge, on average, as yellow light...and that's why the Sun looks yellow.

This is also what makes movies like "The Core" utter nonsense. If the core of the Sun suddenly stopped undergoing fusion, we wouldn't really notice the effects until a million years later, once all the photons had managed to escape.

• What happens to the Sun at night?

Okay, this was a softball question: the Sun is still there, it's just shining on the other half of the Earth. At any given time, it's always day for one half of the Earth, and night for the other half.

Still, I've heard some pretty wacky wrong answers for this, like the idea that space itself is lit up during the day, and dark at night. Meanwhile, Socrates thought that every night the Sun passed through a giant hole in the middle of the Earth. Those wacky ancient Greeks...

• What is the sun made of?

Mostly hydrogen (about 75% by mass), some helium (about 24%), and about 1% everything else (carbon, oxygen, iron, etc.). The Sun's hydrogen is primordial - it comes from the big bang. The helium is partially from the big bang, and partially manufactured from the Sun turning hydrogen into helium. That 1% of everything else comes from exploded stars which seeded the interstellar gas cloud from which the Sun formed.

Note that this also goes for the planets and everything on them...So, all that hydrogen locked up in the water in your body came from the big bang, while all the carbon, oxygen, iron, etc., in your body originally came from ancient exploded stars.

If you're a romantic, you can say, "we are all stardust." Meanwhile, if you're a pessimist, you can say, "we are all nuclear waste products."

• What causes a solar eclipse?

This occurs whenever our Moon passes between the Earth and the Sun and blocks out the sunlight. When the Moon only blocks out part of the Sun, it's a partial solar eclipse. If the Moon manages to block out the entire disc of the Sun - though this is only visible in a narrow range of locations - it's a total solar eclipse (which just happened in Shanghai a couple weeks ago).

Now, it doesn't happen every New Moon because the Moon's orbit around the Earth is tilted 5 degrees to the orbit of the Earth around the Sun. Only when the orbits line up during New Moon (about once every 6 months), does the Moon block out the Sun's light...otherwise it passes a couple degrees below or above the Sun from our perspective.

Just by chance, both the Moon and the Sun span about half a degree on the sky, so they have to be lined up just right for a total solar eclipse to occur, and it's only visible at just the right location on Earth. This wasn't always the case - in Earth's past, the Moon used to be quite a bit closer to us, meaning it appeared quite a bit larger in our sky.

Currently, the Moon is moving away from Earth at a rate of about 1.5 inches per year. This is all because of the tides...the difference in the Moon's gravity felt on the Earth raises two bulges on the Earth which rotate roughly once per day (anyone who's lived near the ocean knows the cycle of low-tide and high-tide). These act as a very subtle brake on Earth's rotation, just like slightly depressing the brakes in your car.

This causes the Earth's rotation to slow down ever so slightly - this is why every now and then you'll hear about the powers-that-be inserting a leap second to keep the clocks accurate. So, the days on Earth are getting slightly longer...but all this rotational energy the Earth is losing has to go somewhere. The moon ends up absorbing it, causing it's orbit to slightly spin-up, which makes it's orbit slightly wider every year.

So, in the distant past, total eclipses were much more common. Likewise, in the distant future total eclipses will no longer occur - the Moon's apparent size will just be too small to block out the entire Sun. It's a rather happy circumstance we were all born at a time in Earth's history when the two brightest celestial objects are the same size on the sky.

Answers to the informal quiz, Part 2: The Moon

Back with more answers to the informal quiz...this time: mysteries of our Moon.

• Why does the Moon go through different phases?

The most common mistake here is that the phases are caused by Earth's shadow falling on the surface of the Moon, thus producing the "dark" part. It turns out moon phases have nothing to do with Earth's shadow at all. If it did, then the moon would always have to be "behind" Earth (on the other side of the Earth from the Sun) in order for Earth to cast its shadow there. The only time you see Earth's shadow on the Moon is during a lunar eclipse.

So, the real answer: At any given time, exactly half of the moon is lit up - the half of it that faces the Sun. The other half of it is dark. The same goes for Earth (and all the planets), actually...that's why at any given time half of the Earth is experiencing daytime, while the other half is experiencing night.

Now, as the moon orbits the Earth, we see different amounts of the lit half depending on the geometry. During a full moon, we only see the lit half. During a new moon, we don't see any of the lit half. During, say, a crescent, we only see a small fraction of the lit half. Hopefully this diagram will help, since describing the geometry in words is a bit tough:


In the above diagram, we're looking top-down on the Moon-Earth system - that's meant to be the North Pole in the middle of the Earth figure (thought to be more accurate I should've put less ice). From this perspective, everything moves counter-clockwise: the Earth's rotation, and the Moon's motion in its orbit around the Earth. Sunlight is streaming in from the right, illuminating the right side of the Earth. Note that as the Moon moves round the Earth, its right side always stays illuminated. The pictures in each of the boxes are the phases we would see on Earth for each of the positions of the Moon in its orbit, based only on what percentage and which side of the Moon's lit surface we can see.

• Why is the Moon bright?

I asked this one just to highlight that the moon does not produce its own light. Rather, we only see it because it reflects sunlight.

That said, I've received some answers that the "Moon is made of very reflective stuff". Turns out that isn't actually true, it's quite dark. Its "albedo" - in other words, what percentage of incoming light it reflects - is only round 11%. That's roughly the same reflectivity as a blackboard.

Its surface is made of relatively unweathered basaltic lava. If you've every seen a relatively fresh lava field, you know it's pretty dark stuff. I've promised Stephanie I'll do a follow-up on this one once all the answers are up...

• Can you see the Moon during the day?

Absolutely! Some people are still amazed by this when they see the moon in the clear blue daytime sky.

As shown in the diagram for the moon phases, a good chunk of the Moon's orbit takes it quite close to the Sun from Earth's perspective, which would mean seeing it in the daytime sky near the Sun. The caveat here is that the phases seen around that part of the orbit only show a very small percentage of the Moon's illuminated half. You can't actually "see" a New Moon, since none of the illuminated half is visible from Earth...but once it gets into a part of its orbit to produce a decent-sized crescent, seeing it in the daytime sky is no problem.

• Does the Moon rotate?

Okay, so I put this one here because I actually got into a heated argument with my 5th grade science teacher, who absolutely insisted the Moon does not rotate. Just for the record, I was right.

If you watch carefully over time, you'll notice the Moon always keeps the same face pointed towards us. I'm not talking about the illuminated half here, but the actual surface topography - we always see the side with the "man-in-the-moon" image (just illuminated from different angles). Many people use this as justification for why the Moon does not rotate. However, the Moon is also orbiting us. If you picture the geometry in your head, in order for it
to always keep the same face pointed towards us, it must rotate at the exact same rate as it orbits. In other words, it orbits once every 27.3 days, and it rotates once every 27.3 days.

Now, is this just a coincidence? No, not at all. I promised Chris I'll do a follow-up on this question, too.

• Why does the moon appear larger on the horizon?

The common wrong answer here is that the atmosphere magnifies the image of the Moon, a theory also espoused by my 5th grade science teacher (how they let this person teach science, I'll never know). This is not correct...in fact, it slightly shortens the image in the vertical direction.

The reason is solely a matter of perspective. The diameter of the Moon spans roughly 1/2 of a degree of angular size in the sky, no matter whether it's on the horizon or over your head. We don't realize how large 1/2 degree is when the Moon is overhead because there are no terrestrial objects in our immediate vision for comparison. Only when it's on the horizon do we have a basis for comparison to everyday objects like trees and buildings.

If you don't believe this explanation, measure it! Hold your pinky finger at arm's length when the moon is on the horizon and note its comparative size. Then, later, try this again when it's overhead...you'll find it's the same.

• Is there gravity on the moon?

Yes, there is! As seen in this video which I posted before, things fall on the surface of the moon, albeit with only 1/6 the gravitational force found at the surface of the Earth. I've had many students who somehow acquire the incorrect notion that no atmosphere means no gravity. For an amusing anecdote related to this misconception, read this.

Answers to the informal quiz, Part 1: Earth

Okay, one week has passed, and I've been getting lots of feedback from the informal quiz. Since there's quite a few questions, I'll post the answers in parts, starting with the question about Earth. In addition to the correct solutions, I'll also post the most common mistakes. Without further ado, then, here are the answers many have been eagerly awaiting...

• Why does the Earth experience seasons?

By far the most common mistake here involves distance. Many people believe Earth is closer to the Sun in summer than in winter, thus making it hotter or colder, respectively.

This seems like a reasonable explanation, but breaks down when you consider that the Northern Hemisphere and Southern Hemisphere experience opposite seasons. When it's summer in the USA and Europe, it's winter in Australia, and vice versa. It runs into more trouble when you consider the Earth is actually closest to the Sun in January, and farthest in July...exactly the opposite of what you'd expect for the Northern Hemisphere if this explanation were true.

The real answer here involves the Earth's "axial tilt". The axis about which the Earth rotates is at a constant tilt of 23.5 degrees. This means at certain times of the year, the Northern Hemisphere is more directly facing the Sun, while 6 months later when the Earth is on the other side of the Sun, the Southern Hemisphere is more directly facing the Sun. Note the diagram:


I've marked the equator and the axis of rotation in red. The hemisphere which more directly faces the Sun is the one which experiences summer. The hemisphere angled away from the Sun is the one experiencing winter.

Another common "almost right, but not quite" answer I've heard several times involves the tilt, but in the wrong way. Those folks maintain that because the Earth is tilted, one hemisphere is closer to the Sun than the other. However, this effect is minuscule, since the tilt only accounts for a difference in distance to the Sun of a couple thousand kilometers, while the average distance to the Sun is 150 million kilometers. Again, seasons have nothing to do with distance, it's all about angles.

• Why is there a 24 hour day-night cycle?

This one is relatively easy. The Earth rotates about its axis every 24 hours, bringing the Sun into view for half of that time period.

(Now, technically, the Earth makes a full 360 degree rotation in only 23 hours, 56 minutes...but because the Earth has also traveled about 1 degree in its orbit around the Sun during that time, the Earth must rotate an extra 1 degree to bring the Sun back to the same relative position. Thus, the 4 extra minutes.)

Most folks get this one, but there are still some who maintain that the Earth doesn't rotate at all, and just orbits once around the Sun per day. Woe is them.

• Why is the sky blue?

In spite of being a question asked by every 5-year-old in existence, this is without doubt the most frequently incorrect answer.

I've gotten simply wrong responses such as "the sky is simply reflecting the ocean". In the middle of a continent far from any ocean, though, the sky is still blue. I've also gotten the "almost correct" response that the atmosphere is refracting the sunlight. It does involve the atmosphere interacting with sunlight, which is good...but still not quite right.

The correct answer here has to do with scattering, specifically - "Rayleigh scattering". When light has a wavelength close to the size of a gas molecule it's passing near, there's a good chance the light will essentially "bounce" off the molecule and start heading in a different direction.

Moreover, the chance of scattering is also very dependent on wavelength - it scales as 1 over the wavelength to the fourth power. In other words, short wavelengths are much more likely to scatter than long wavelengths. Our eyes interpret the different wavelengths of light as different colors. Red light's wavelength is roughly twice as long as blue light, so blue light is 2 to the 4th power = 16 times more likely to scatter than red light.

So, imagine incoming sunlight coming from the Sun and passing through our atmosphere. Remember, the sun emits every colors of the rainbow, it's just that when the colors are all combined, they appear to us as white light. Now, the red light is more likely to make it through unhindered, while the blue light gets scattered everywhere and appears to an observer on the ground to be originating from a direction other than the Sun. Hopefully this diagram will help explain the concept:



As you can see, most colors on the red side of the spectrum appear to be coming from the direction of the Sun. The blue light (and a bit of the green), however, appear to be coming from elsewhere in the sky. This process happens all over the sky, so it appears that blue light is coming from everywhere.

This also explains why our sun appears slightly more yellow than it would from space. Some of the blue gets scattered out of our line of sight towards the Sun.

• Why are sunsets red?

The "blue sky" answer above also explains this question. At sunset (and sunrise), sunlight has to pass through far more atmosphere to make it to an observer on the ground, vastly increasing the chances of scattering even the not-so-blue light. Only the very reddest light makes it to the observer without getting scattered...thus, a red sunset.

• How does the Earth compare to other planets?

So, this is meant as just an open-ended question...I only ask as there seem to be quite a few folks who think that Earth is the largest planet.

Within our own Solar System, planets can be divided into two groups - the inner, terrestrial rocky planets (Mercury, Venus, Earth, and Mars) and the outer gas giant planets (Jupiter, Saturn, Uranus, and Neptune). Earth is the largest of the inner rocky planets, but many times smaller than the outer gas giants. So, it could be said to fit somewhere in the middle, albeit slightly on the smaller side.

On the other hand, if you take the ~300 planets known around other stars, Earth is quite dwarfed. Just about all of these "extrasolar" planets are massive gas giants - many larger than Jupiter - and most of which are found orbiting incredibly close to their parent star. Within that population, Earth is no more than a puny wet rock.

Of course it's quite likely that this known collection of extrasolar planets are not representative of the population of planets as whole. The problem is that our observing techniques for detecting these distant worlds are severely biased towards only detecting very large planets which orbit very close to their parent star. The hope is that with improving technology, we'll start detecting Earth-like planets in a matter of a few years, particularly with space-based missions such as Kepler.

• Why does a feather fall slower than a bowling ball?

The common mistaken belief here is to think that because the bowling ball is heavier, it falls faster.

It turns out that the only thing which slows down the feather is increased air resistance. In essence, it has a much lower terminal velocity than the bowling ball. (Terminal velocity is the fastest a given object of a given shape can fall - it's the speed at which the Earth's gravitational acceleration is perfectly balanced by the force exerted by air friction.)

Take away the air, and everything - no matter its weight - falls at the same rate. Check out this video from one of the Apollo moon landings - you'll see that in the absence of any air, a hammer and a feather fall at exactly the same rate.

Now, technically the bowling ball, with a larger mass, feels a stronger gravitational force. However, because it has a larger mass, it's inertia is also greater...in other words, you need more force to get it going. It turns out that its increased gravitational force perfectly balances its increased inertia, resulting in all things falling at the same rate (in a vacuum).

An informal quiz...

Based solely on my experiences teaching undergrad students, I've found there's quite a few commonly held misconceptions about astronomy. See if you can answer these seemingly intuitive astronomy questions. (No looking 'em up on the interwebs! That's cheating!)

Answers will be posted in one week...if you really don't want to wait until then, email me and I'll send you the answer key.

Earth:

• Why does the Earth experience seasons?
• Why is there a 24 hour day-night cycle?
• Why is the sky blue?
• Why are sunsets red?
• How does the Earth compare to other planets?
• Why does a feather fall slower than a bowling ball?

Moon:

• Why does the moon go through different phases?
• Why is the moon bright?
• Can you see the moon during the day?
• Does the moon rotate?
• Why does the moon appear larger on the horizon?
• Is there gravity on the moon?

Sun:

• Compare the Sun to the stars.
• Why does the Sun shine?
• What happens to the Sun at night?
• What is the sun made of?
• What causes a solar eclipse?

Universe, etc:

• What's the difference between the Solar System, the Galaxy, and the Universe?
• What is a star?
• How are planets different than stars?
• Where do the stars go during the day?
• What's the farthest human beings have ever traveled in space?