16. Brave New Worlds

    • According to the nebular hypothesis, the Sun and planets formed out of a single collapsing, rotating cloud of interstellar gas and dust called the solar nebula. This hypothesis provides a natural explanation for the highly regular pattern of the planet and satellite orbits.

    • Conservation of angular momentum in gravitational collapse suggests that the Sun initially rotated much more rapidly than it does now.

    • Spiral nebulae were once thought to be young stars enveloped by nascent planetary systems, but they are now known to be distant galaxies, each containing roughly 100 billion stars.

    • The youngest stars in our Milky Way Galaxy are surrounded by dusty planet-forming disks, initially discovered by their infrared radiation, and detected in large numbers and great detail by the Hubble Space Telescope and the Spitzer Space Telescope.

    • Vast interstellar clouds of gas and dust are the incubators of large numbers of newborn stars, many of them embedded in the material from which they arose and surrounded by flattened, rotating planet-forming disks.

    • At least two planets with a mass comparable to that of the Earth were discovered orbiting a cold, dark pulsar.

    • The first unseen planets circling ordinary Sun-like stars were inferred from the tiny, periodic Doppler wavelength shifts of their parent starís spectral lines, caused by the motion of the orbiting planet. They became known as hot Jupiters, since they revolve unexpectedly close to their star and have masses comparable to that of Jupiter.

    • Hundreds of planets have been found circling nearby stars, including many multi-planet systems. These newfound planets are known as extrasolar planets, or exoplanets for short.

    • Most of the exoplanets have been indirectly inferred from the miniscule, periodic velocity changes they create in their parent star, but many have also been inferred from the brief, periodic dimming of starlight when they pass in front of, or transit, their star as viewed from Earth.

    • The orbital motions of a few exoplanets have been confirmed by direct observation of the moving planets.

    • The Kepler mission is searching for Earth-size planets that reside in the warm habitable zone where liquid water can exist on the planetís surface and living things might be present.

    • The COROT mission has detected a super-Earth that is about 4.8 times as massive as the Earth, but revolves at just four times the parent starís radius. The worldís best ground-based telescopes are actively searching for exoplanets and have detected several super-Earthís with a mass between those of the Earth and Jupiter.

    • The atmospheres of transiting exoplanets are being investigated using the Hubble Space Telescope, the Spitzer Space Telescope, and ground-based infrared telescopes.

    • Water vapor, carbon dioxide and methane have been detected in the atmosphere of one hot Jupiter, named HD 189733b.

Copyright 2010, Professor Kenneth R. Lang, Tufts University