Transiting Exoplanet
Exoplanets are planets that lie outside of our solar system and orbit their own central star. Since these other star/planet systems are very far away, and since planets are very faint compared to stars, the most popular method that astronomers use to search for exoplanets is the transit method. During a transit, the exoplanet crosses between the star and our telescope, causing the light that we detect from that star to dim very slightly. This dimming can be detected in light curves as a tiny decrease in brightness. The depth, width, and frequency of this dip can be used to calculate a few key properties of the transiting planet, such as its size, its period and the shape of its orbit.
WASP-17b
Below is an observed light curve for the transiting exoplanet WASP-17b. WASP-17b is what is known as a "hot Jupiter". This means it is a large, massive planet like Jupiter, but is orbiting close to its host star, with a very short orbital period. Hot Jupiters make up a good portion of all discovered transiting exoplanets because their size, the large corresponding dip in brightness, and their frequent transit events make them easiest to detect. The video scans over time (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes. In this video, about twelve days are covered, and 3 transits can be heard as quick dips in brightness. This light curve was observed continuously from space at high cadence by NASA’s TESS satellite, so there are no seasonal gaps in coverage, and the light curve is not very different from the phased version below.
The next video is the phased light curve for WASP-17b, the observed transiting exoplanet. WASP-17b was determined to orbit its host star with a period of 3 days and 17 hours. That is the length of a year on WASP-17b! With this orbital period, and the properties of its host star, WASP-17b is calculated to be about half as massive as Jupiter with almost twice the radius, so it is a very low density planet. In the video, you should hear two full periods, each consisting of the baseline brightness of the star, and one quick dip in brightness that corresponds to the transit of WASP-17b. The video scans over phase (x-axis) and modulates pitch based on magnitude (y-axis). Lower pitch represents dimmer magnitudes.
Astronomers may also learn something about an exoplanet's atmosphere during a transit. Some light from the star passes through the atmosphere of the planet during the transit, and is absorbed. The difference between the in-transit and out-of-transit spectrum of the star creates a new transmission spectrum of the planetary atmosphere that can be analyzed to determine the atomic and molecular elements in it. Atmospheric composition, along with orbital size and stellar temperature, is important for determining whether an exoplanet may be able to support life. Below is a sonified video of a model of the transmission spectrum of WASP-17b, which provides a good fit to the actual observed colors across the transmission spectrum. This video scans across a plot of brightness measured in flux or intensity of light (y axis) versus wavelength (x axis), where lower pitch represents weaker flux. The spectrum is in the infrared wavelength range of the electromagnetic spectrum, moving from 10,000 to 50,000 Angstroms. The infrared region is where certain spectral signatures that define composition are easiest to measure. Broad, strong bands of absorption by molecules in the planet’s atmosphere, primarily from water vapor, cause deep decreases in brightness in several regions.
The spectrum model is from Sing et al. 2016. The light curve is from NASA’s Transiting Exoplanet Survey Satellite (TESS).