In the past decade, the discovery of extrasolar planets has accelerated immensely. To date, 4,424 exoplanets have been confirmed in 3,280 star systems, with another 7,453 awaiting confirmation. So far, most of these planets have been gas giants, with about 66% being similar to Jupiter or Neptune, while another 30% have been giant rocky planets (aka. “Super-Earths). Only a small fraction of confirmed exoplanets (less than 4%) have been similar in size to Earth.
However, according to new research by astronomers working at NASA Ames Research Center, it is possible that Earth-sized exoplanets are more common than previously thought. As they indicated in a recent study, there could be twice as many rocky exoplanets in binary systems that are obscured by the glare of their parent stars. These findings could have drastic implications in the search for potentially habitable worlds since roughly half of all stars are binary systems.
For the sake of their study, the research team examined 517 exoplanet-hosting stars that were identified by NASA’s Transiting Exoplanet Survey Satellite (TESS) during its three years in operation. When compared to data from the twin telescopes of the international Gemini Observatory and the WIYN 3.5-meter Telescope at Kitt Peak National Observatory, they found that over 100 of these stars likely had a binary companion.
An artist’s rendition of the Transiting Exoplanet Survey Satellite (TESS). Credit: NASA’s Goddard Space Flight Center
The paper that describes their findings has been accepted for publication in the Astronomical Journal. Dr. Kathryn Lester, a postdoctoral researcher at NASA Ames Research Center, led the research effort with the assistance of colleagues from NASA Ames, the U.S. Naval Observatory, the NASA Exoplanet Science Institute, the NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab), the Lowell Observatory, as well as Georgia State and Standford University.
The Trouble with Transits
To date, the vast majority of confirmed exoplanets (roughly 75%) have been discovered using the Transit Method (aka. Transit Photometry). This consists of observing stars for periodic dips in their brightness, which can be the result of a planet passing in front of their face (transiting) relative to the observer. Like its predecessor, Kepler,TESS relies on the Transit Method to determine the presence of exoplanet systems around thousands of stars at any given time.
Unfortunately, binary companions have always been challenging when it comes to detecting transiting exoplanets. Transit Photometry requires that star systems being observed edge-on in order for exoplanets to be detected. But in binary star systems, where two stars orbit each other, dips in luminosity are a regular occurrence and are the result of one companion eclipsing the other.
As a result, it can be very difficult to spot smaller exoplanets that orbit closer to their stars, which is where astronomers expect to find rocky planets in the stars’ circumsolar Habitable Zone (HZ). Instead, using the Transit Method with binary star systems is likely to reveal only gas giants and/or planets that have distant orbits from their parent stars. This is why Dr. Lester and her colleagues set out to determine whether some of the exoplanet-hosting stars were in fact binaries.
Searching for Stellar Companions
The team relied on a technique called Speckle Imaging, where large numbers of short exposure images are combined and analyzed to greatly enhance the resolution of ground-based telescopes (similar to interferometry). Of the 517 TESS Objects of Interest (TOIs) they examined, they found that 73 exoplanet host stars that had previously appeared as a single point of light actually had a stellar companion.
They similarly found that 29 TOI stars that had produced false positives in the past also had stellar companions. Said Dr. Lester in a recent NOIRLab press release:
“With the Gemini Observatory’s 8.1-meter telescopes, we obtained extremely high-resolution images of exoplanet host stars and detected stellar companions at very small separations… Since roughly 50% of stars are in
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