Theme 4: Searching for Life beyond the Solar System

Lecture 12: Searching for planets orbiting other stars I.

TEEMING WITH PLANETS Astronomers are closing in on identifying distant worlds that may have the right conditions to support life. Techniques for detecting “exoplanets” are becoming more sophisticated, and over 400 have been discovered so far — 30 in October alone. This year brought two particularly intriguing finds. One is Gliese 581d, orbiting a star at a distance that could indicate surface temperatures not so different from Earth’s. Astronomers also discovered a “waterworld” composed mostly of H2O, which would be a prime candidate for extraterrestrial life if it were just a little farther from its sun.

The discovery of Earth-like planets, with water and moderate temperatures, is now so likely that the Vatican held a conference of astrobiologists this year to discuss the theological repercussions of extraterrestrial life. “If biology is not unique to the Earth, or life elsewhere differs biochemically from our version, or we ever make contact with an intelligent species in the vastness of space, the implications for our self-image will be profound,” said Chris Impey, a professor of astronomy at the University of Arizona.

Discovering that we have company in the universe, in fact, might open our eyes to what’s important on Earth.

William Falk is the editor in chief of The Week magazine.

December 28, 2009

Lecture 12: Searching for planets orbiting other stars I: Properties of Light

1. How could we study distant habitats ? 2. The nature of light - spectrum, spectral lines3. Using light’s properties to do remote sensing

Light & Matter interact in 4 basic ways

The nature of light• Light - electromagnetic waves that have:

Wavelength

Frequency

Speed

… and

Energy

The nature of visible light

• Obeys the inverse-square law:

The nature of visible light

White light is a mixture of the colors;

monochrome light behaves like waves of the same wavelength.

The nature of visible light

• Visible light: a form of electromagnetic energy / radiation that our eyes are sensitive to.

The Spectrum

• Can tell us temperature:

a thermal radiation spectrum is a continuous spectrum of light that depends only on the temperature of the object that emits it.

The Spectrum

• Thermal spectrum:

the spectrum of the Sun is roughly similar to a thermal spectrum.

The Spectra of Stars

• The Sun vs. a smaller, cooler star (M-star),• The wavelength at which a star’s spectrum peaks, reveals the star’s surface temperature:

Using Spectra for Remote Sensing

• Forming spectral lines in the spectrum

Using Spectra for Remote Sensing

• Measuring spectral lines in the spectrum

Electron Orbits in Atoms

Electron Orbits in AtomsPlots of electron density shapes of 1s, 2p and 3d orbitals:

Atoms and Spectral Lines• Spectral lines correspond to the energy of a transition an electron makes between two distinct states.

Molecules and Spectral Lines• Spectral lines of molecules also correspond to the energy to transit between distinct states

The Spectra of Stars

• The Sun vs.a smaller, cooler star (M-star)

The Spectra of Planets

• Mars

Main points to take home:

1) Visible light: form of electromagnetic energy (radiation) to which our eyes are sensitive.

2) Spectrum: the amount of light of any given wavelength, emitted or reflected by an object.

3) Thermal spectrum: a simple spectrum that depends only on the object’s temperature.

4) Spectral lines: in emission or absorption; every atom and molecule has a specific set.