Sunday, October 10, 2010

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:
"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.

Monday, May 24, 2010

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.

Tuesday, February 16, 2010

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 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.