In the past ten years, the discovery of extrasolar planets has accelerated immensely. So far, 4,424 exoplanets have been confirmed in 3,280 star systems, and a further 7,453 are awaiting confirmation. Previously, most of these planets were gas giants, with about 66% similar to Jupiter or Neptune, while another 30% were giant rock planets (also known as “super-earths”). Only a small fraction of confirmed exoplanets (less than 4%) are similar in size to Earth.
However, according to new research from astronomers working at NASA Ames Research Center, it is possible that Earth-sized exoplanets are more abundant than previously thought. As they showed in a recent study, there could be twice as many rocky exoplanets in binary star systems that are obscured by the glare of their parent stars. These findings could have a drastic impact on the search for potentially habitable worlds, since about half of all stars are binary systems.
For their study, the research team examined 517 stars harboring exoplanets identified by NASA’s Transiting Exoplanet Survey Satellite (TESS) during its three-year period of operation. When compared with data from the twin telescopes at the Gemini International Observatory and the WIYN 3.5-meter telescope at the Kitt Peak National Observatory, they found that over 100 of these stars likely had a binary companion.
Artist’s impression of the Transiting Exoplanet Survey Satellite (TESS). Photo credit: NASA’s Goddard Space Flight Center
The paper describing their results was accepted for publication in the Astronomical Journal. Dr. Kathryn Lester, a postdoctoral fellow at NASA Ames Research Center, led the research effort with support from colleagues from NASA Ames, the US Naval Observatory, NASA Exoplanet Science Institute, NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab). , the Lowell Observatory, and Georgia State and Standford Universities.
The problem with transits
To date, the vast majority of confirmed exoplanets (approx. 75%) have been discovered using the transit method (also known as transit photometry). This consists of observing stars for periodic drops in brightness that may be the result of a planet passing (transiting) in front of their face 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 point in time.
Unfortunately, binary companions have always been a challenge when it comes to spotting exoplanets in transit. Transit photometry requires star systems to be observed from the edge so that exoplanets can be discovered. But in binary star systems in which two stars orbit each other, there are regular drops in brightness and are the result of the eclipse of a companion.
As a result, it can be very difficult to spot smaller exoplanets orbiting closer to their stars, where astronomers expect to find rocky planets in the stars’ Circumolar Habitable Zone (HZ). Instead, using the transit method with binary star systems will likely only reveal gas giants and / or planets that have distant orbits from their parent stars. For this reason, Dr. Lester and her colleagues are figuring out whether some of the stars that host the exoplanet were actually binary stars.
In search of stellar companions
The team relied on a technique called speckle imaging, which combines large numbers of short exposure images and analyzes them to greatly improve 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 also found that 29 TOI stars that had generated false positive results in the past also had stellar companions. Dr. Lester recently said in a press release from NOIRLab:
“With the 8.1-meter telescopes at the Gemini Observatory, we received extremely high-resolution images of exoplanet host stars and discovered stellar companions at very small distances … Since around 50% of the stars are in binary systems, we could discover – and the chance to study – many earth-like planets. “
The next step was to compare the exoplanets discovered in these systems with the sizes of the exoplanets discovered in single star systems. From this, the team was able to show that the TESS space probe was able to identify both Jupiter and Neptune-like (“large”) exoplanets as well as super-earths and Earth-like (“small”) exoplanets that orbit individual stars only large planets in binary star systems.
These results suggest that there may be a population of Earth-sized exoplanets in binary systems that have gone undetected by missions such as TESS, Kepler, and other exoplanet surveys based on transit photometry. Scientists have suspected for some time that small planets in binary star systems are missing during transit surveys because of possible interference from a companion star.
However, this new study provides the first observational support for this suspicion while also showing what type of exoplanets are affected. It is also important because the transit method has so far been considered to be the most effective means of detecting exoplanets – it accounts for 3343 of the 4424 confirmed exoplanets. But if these results are correct, there could be up to 1,600 rocky exoplanets that have been missed during transit surveys.
This means that in the future, astronomers will have to rely on a variety of observation techniques before they can conclude that a binary system has no Earth-like planets. Like Dr. Lester said:
“With around 50% of the stars in binary systems, we could miss the discovery – and the chance to study many Earth-like planets. Astronomers need to know whether a star is a single or a binary star before claiming that no small planets exist in this system. If it is a single one, it could be said that there are no small planets. But when the host is in a binary star, you don’t know if a small planet is obscured by the companion star or does not exist at all. To find out, one would need more observations with a different technique. “
“We have shown that it is more difficult to find Earth-sized planets in binary star systems because small planets are lost in the light of their two parent stars,” added Dr. Steve Howell, director of speckle imaging work at NASA Ames, added a co-author on paper. “Your transits are ‘filled in’ by the light of the companion star. This is an important finding in the exoplanet work. The results will help theorists to create their models for the formation and evolution of planets in binary star systems. “
An illustration of the Kepler-47 circumbinary planetary system. Photo credit: NASA / JPL Caltech / T. Pyle
Another aspect of the study was that Dr. Lester and her colleagues analyzed the distance between binary companions in systems in which TESS discovered large planets. They found that pairs that host exoplanets are typically further apart than binary pairs that do not know any exoplanets. This could be interpreted as an indication that no planets form around stars that have nearby stellar companions.
In the future, this could be used to put additional restrictions on where astronomers should look for rocky planets. Dr. Lester and her team’s speckle imaging study also illustrates how exoplanet studies move from exoplanet discovery to characterization. In addition to characterizing the atmospheres and surface environments of exoplanets, there is also the crucial task of characterizing planetary systems.
By knowing which types of stars are most likely to support rocky exoplanets, astronomers and astrobiologists can narrow down their search for planets that are best suited for “life as we know it.”
Further reading: NOIR Lab