Hypatia Catalog will help scientists characterize exoplanet systems

Data on stars' chemical compositions will help scientists search for potentially habitable worlds.
By Laurel Kornfeld | Dec 22, 2018
A planetary astrophysicist at the Southwest Research Institute (SwRI) has created a database of stars and star-exoplanet systems that will help scientists characterize exoplanets and focus their search for extraterrestrial life.

Natalie Hinkel named her database after Hypatia of Alexandria, a mathematician and astronomer who lived in the late fourth- and early fifth-centuries, the only known woman scientist of her time.

Composed of thousands of stars, the Hypatia Catalog includes stars and star-planet systems within 500 light years of the Sun observed over the last 35 years. The catalog lists the chemical composition of 6,156 stars, of which 365 are known to have at least one orbiting planet. Also in the database is a list of 72 elements, ranging from hydrogen to lead, that have been found in stars.

These elements are important because some, such as hydrogen, carbon, nitrogen, oxygen, and phosphorus, are building blocks for life.

"At first, scientists focused on temperatures, looking for exoplanets in the 'Goldilocks zone'--neither too close nor too far from the star, where liquid water could exist. But the definition of habitability is evolving beyond liquid water and a cozy temperature," Hinkel said.

To be habitable, a planet needs to be rocky, have an atmosphere, have the necessary chemical elements for life, and undergo geochemical cycles that distribute those elements globally, she noted.

"With current technology, we can't measure the composition of an exoplanet's surface, much less its interior. But we can measure the abundance of elements in a star spectroscopically, studying how light interacts with the elements in a star's upper layers. Using these data, scientists can infer what a star's orbiting planets are made of, using stellar composition as a proxy for its planets," she explained.

Recently, Hinkel and a team of scientists used computer models to determine the water content of the seven planets orbiting the star TRAPPIST-1 and found that several, including one in the star's habitable zone, have compositions ranging from five to 25 percent water. In contrast, Earth's composition is just 0.02 percent water.

She is now working on computer algorithms studying the possibility of orbiting planets chemically influencing their parent stars.

"The Hypatia Catalog and other large databases of stellar chemical abundances open up a new age of exoplanet exploration," Hinkel emphasized.

Open to the public, the catalog is available for free online. An article on the database has been published in the September-October issue of American Scientist.

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