What I Learned Today

Today I attended a fascinating talk by Dr. Michelle Thaller about the Spitzer infra-red telescope and the search for exoplanets. I love hearing about the ongoing discoveries of planets orbiting around other stars. This is cutting-edge observational science! The first exoplanet was detected in 1995; before that, they were only hypothetical.

Spitzer is an IR telescope that orbits the Sun, lagging behind the Earth in its orbit. This lets it observe out away from the IR signal of the Earth and Moon. Dr. Thaller opened the talk with some fun (and fascinating even if you’ve seen an IR camera before!) demos showing how in IR, you can see through some things (black plastic bags) but not others (optically transparent glasses). She noted that the Earth’s atmosphere is opaque in IR, which helps explain both the greenhouse effect and why you need a space-based telescope to observe the universe in IR. More than that, since dust is opaque optically but often transparent in IR, Spitzer has given us our first views deep into the center of our own galaxy (dust blocked optical telescopes’ view into the Milky Way). We subsequently learned that we live in a barred spiral galaxy (previously thought to be just a spiral).

Spitzer doesn’t have the resolution to pick out individual planets orbiting other stars, but it can detect a swept-out gap in a stellar disk that can indicate where a planet has formed. That can guide more detailed investigations for exoplanets, such as astrometry and radial-velocity studies. You can browse a catalog of discovered exoplanets, sorted by their method of discovery or an even more attractive atlas of the planets and their stars. We’re currently up to 326 (from 1, only 13 years ago!). You can follow along with the latest planetary discoveries at PlanetQuest, and even download a desktop/Dashboard widget tracking the exoplanet tally.

The talk was exceedingly well timed. Just yesterday, it was announced that the first ever images of exoplanets had been recorded: Fomalhaut b by Hubble and three planets around HR 8799 by ground-based telescopes Keck and Gemini, using adaptive optics (see more pictures here). The full scientific papers are available here (AAAS subscription needed for full text PDF):

• Optical Images of an Exosolar Planet 25 Light-Years from Earth by Kalas et al.
• Direct Imaging of Multiple Planets Orbiting the Star HR 8799 by Marois et al.

Despite this pile of planetary discoveries, the hunt is still on for “Earth-like” planets: similar to our home world in terms of mass, size, temperature, and atmospheric composition. It’s bound to happen soon!

Many of us have a mental image of Mercury that’s black-and-white, like 1950’s TV. We liken it to the Moon: a pale, devastated landscape pock-marked with staccato craters and blanketed with the finest grey dust imaginable. But this view, it turns out, is somewhat mistaken–probably influenced by the classic view at right, captured by Mariner 10 in 1975. Mariner 10 did take some color images as well, but they are lower resolution, and only about half of the planet was imaged in color, so they get less press.

Enter MESSENGER, the NASA mission that just executed its second close flyby of Mercury, on its way to a 2011 orbital insertion. With its more capable imaging system, MESSENGER has captured close approximations of “true color” for Mercury, although as always this isn’t as simple as snapping a photograph (CCDs just don’t respond to color the way the human eye does!).

Further, false color is often more scientifically useful than the our mineralogically impoverished RGB views. MESSENGER recently released a color movie that pans across the surface of the planet in fascinating detail. Color differences here yield clues to compositional differences. Note also that you can see a definite change in resolution; later in the movie, the crater edges are less crisp and somewhat fuzzy. I assume that this is because it was imaged during a flyby, not in regular orbit; the spacecraft was only 200 km from the surface at closest approach, but then would have been moving rapidly away. The movie has been adjusted to correct for the geometric distortion, but the reduced resolution remains. Check it out!

Yesterday, I attended the first day of the Astrobiology Science Conference, or AbSciCon. The day began with a great talk by Lord Martin Rees, who is the Astronomer Royal in England. He wrote a book called “Just Six Numbers” about six parameters of our solar system and Earth that have allowed for conditions conducive to the existence of life (and human beings). He was introduced by Paul Davies, who wrote “The Goldilocks Enigma”. Both books are now candidates for inclusion in my to-read list.

I next attended some talks about the galactic habitable zone. While I’ve read about the “habitable zone” inside our solar system (largely determined by the temperature range within which water exists as a liquid), this was the first time I’d encountered its galactic counterpart. In the galaxy, the constraints relevant for habitability (specifically, the creation of planets) involve the probability of a nearby, disruptive, supernova (so you don’t want to be too close to the galactic center, where stellar density is high) and the availability of metals for forming planets, which are more available where stellar density is high since they’re created by stars (so you don’t want to be too far away from the core). Our Sun is at 8.5 kiloparsecs from the galactic center, although it apparently wobbles in and out a bit, and there’s enough uncertainty in the measurement that it’s more like 7.5 to 8.8 kpc.

The discovery of exoplanets (planets outside our solar system, orbiting other stars) is, in my opinion, one of the foremost scientific discoveries of the past decade or so. It sounds like science fiction, but it’s real (we’re up to 287 exoplanets so far!). So far we’ve predominantly found only Jupiter-sized planets that are close to their host stars (and very, very hot), but most expect that this is because those are the planets that are easiest to detect. The hunt is on for Earth-sized planets that reside in their star’s habitability zone. In particular, a five-year study is beginning to collect 100,000 observations of Alpha Centauri B in hopes of detecting terrestrial planets. No planets have yet been detected in the complex (triple star!) Alpha Centauri system. However, in planet formation simulations, 42% of the Earth-sized planets that formed fell into the habitable zone around Alpha Centauri B. Therefore, if there are planets there, they might be very interesting to study (and much closer than many of the other stellar systems with planets). There are also reasons that planets in a binary or triple star system would be less likely to exist (e.g., gravitational disruptions could prevent them from accreting), but it seems like a good place to look.

But maybe our own location isn’t always so habitable, either. It’s been observed that if you plot the number of extant species as a function of time on Earth (a biodiversity curve), there is a certain cyclicity to the peaks and troughs. Fourier analysis identifies frequencies that have a strong correlation with the signal. It was previously thought that there was a 26-27 Myr periodicity, but this is now viewed as an artifact of the sampling rate (through time) of the curve. After the recent revision of the geological time scale, a stronger signal is found with a period of 62 Myr. So, what might be happening to cause biodiversity to peak and fall every 62 Myr? There are a lot of ideas, including the Nemesis theory of a companion star repeatedly passing through and disrupting the solar system, a sharp increase in the number of mantle plumes in the Earth, solar nuclear oscillations, and, intriguingly, oscillations in the position of the solar system with respect to the galactic plane. We seem to rise up (“north”) of the galactic plane and dip down (“south”) with a period of about 64 Myr, which could be a potential match. When we head north, we move in “front” of the galaxy as it travels through the intergalactic medium, exposing us to more of the incoming cosmic rays, which are known to have negative effects on life. Three of the five known mass extinctions in history coincide with a peak of this vertical oscillation (one is already quite firmly believed to have been caused by a meteor impact and therefore need not fit with the cosmic ray periodicity). A less exotic explanation for the cyclicity is that sea level changes have affected how well fossils are preserved, therefore making it appear that there are fewer of them when preservation rates are low. In fact, strontium isotope ratios, which indicate the degree of rock weathering and erosion going on, seem to have a strong 59 My periodicity. I’d say that the jury’s still out on this one.