A new algorithm developed at the Southwest Research Institute uses the chemical composition of stars to fast-track the discovery of exoplanets – planets outside of our solar system.
Planetary astrophysicist Natalie Hinkel created a database of stars and their elemental compositions and, with colleagues, crafted code that looks for complex patterns in the stellar data to determine which combination of elements is common to stars that host planets.
“Instead of looking through huge all-sky telescope surveys hoping to find a signature of a planet, [my team] can fast-track the discovery and characterization of planets by providing a prediction score” based on the amount of carbon, oxygen, iron, and sodium present, Hinkel said.
The algorithm, which is publicly available following its publication in the Astrophysical Journal, has looked at more than 4,200 stars and assessed their likelihood of hosting planets. Hinkel’s team identified about 360 potential giant-planet host stars that have more than a 90 percent probability of hosting a giant exoplanet.
Exoplanets are usually found using the “transit method,” with telescopes aimed at stars recording sudden dips in their brightness, said Knicole Colon, astrophysicist at NASA Goddard. “If that occurs on a regular schedule, it indicates that a planet is passing between the star and the telescope, blocking some of the light.”
The other common method is Doppler spectroscopy – also known as the radial-velocity method or, colloquially, the wobble method – which measures the movement of a host star under the gravitational pull of an orbiting exoplanet.
While the transit and wobble methods have helped astrophysicists confirm more than 4,000 exoplanets to date, the data is limited by day-night cycles, Colon said.
“What is great about [Hinkel’s] algorithm is that it allows us to go back and look at this archival data and use it to prioritize stars, because there are a lot of stars in the sky. We need to know of better ways to look for them, and this is one,” Colon said.
At first, the rate of exoplanet discovery was slow. In 2000, just 32 had been confirmed by scientists, but numbers started to increase dramatically after 2009, when NASA launched the Kepler space telescope – a spacecraft designed to search for alien planets in our part of the Milky Way.
In April 2018, NASA launched its Transiting Exoplanets Survey Satellite, which is expected to uncover at least 20,000 additional exoplanets during a two-year all-sky search.
Hinkel’s database – the Hypatia Catalog – is the largest database of stars and their elements within 500 light years of our sun and uses both Kepler and ground telescope data. At last count, Hypatia had stellar elemental data for 6,193 stars, 401 of which are known to host planets. The database also catalogs 73 stellar elements from hydrogen to lead.
“Exoplanets are cool because it’s essentially exploring the possibility of life in outer space. We are trying to figure out if life could be there or if life could flourish there based on the cycling of the elements,” Hinkel said, noting that some known exoplanets have orbits different from our solar system, with planets moving in tight circles and others with temperatures that go from extremely hot to extremely cold.
Colon said exoplanets help inhabitants of Earth learn more about its place in the universe.
“Our solar system is made up of eight major planets and then a bunch of minor planets, and so far, [NASA] hasn’t been able to find anything similar – not even a scaled-down analogy of our system,” Colon said. “We are trying to figure out how common we are, with the ultimate goal of figuring out if the planet has to be like Earth to sustain life.”