Discovering exoplanets is a difficult task. Given the challenges, it’s amazing that we even found some. But astronomers are smart, so there are currently more than 4,300 confirmed exoplanets. They range from small Mercury-sized worlds to planets larger than Jupiter, but most of them have one thing in common: They orbit near their home star.
This is not because most planets orbit close to their stars, but because our observations have a tendency towards closely orbiting planets. The most common way to discover exoplanets is through the transit method. Here a planet passes in front of our view of a star, which makes the star slightly darker. In principle it is a simple idea, but in practice it is difficult. The brightness of stars varies by itself due to things like flares and star spots. To confirm a planet you have to see a repeating pattern in the way a star darkens, and that means you have to observe multiple transits of a planet. With a planet orbiting the star every few days or months, you can confirm a planet pretty quickly. But if a planet orbits a star every few years, it can take a decade or more to get observations.
The observed flicker of a star during the transit of an exoplanet. Image credit: EPIC
Despite this tendency, we have learned a lot about planetary systems. We know that there are “hot Jupiters” that orbit close to their stars within a few days, and carbon worlds that would look very different from Earth. We also now know that our solar system, with its rocky inner planets and gaseous outer planets, is not typical of most star systems. Little is known about planets with orbits that last a year or more. But that’s starting to change, as seen in a recent article.
Most of the exoplanets known to us were found by the Kepler mission. While Kepler found some exoplanets with orbital times greater than 100 days, they are more difficult to confirm. Since the transit method only gives the size of a planet relative to its star, we cannot use the Kepler data to determine the mass of a planet. This is particularly problematic for planets the size of Jupiter. Because the weight of a large planet causes it to compress more, a Jupiter mass planet and a brown dwarf 30 times its size can be roughly the same size. Determining the mass of these exoplanets is the goal of the Giant Outer Transiting Exoplanet Mass Survey, GOT’EM Survey for short.
In this latest work, the team used a different method to study exoplanets known as the radial velocity method. When a planet orbits its star, it pulls the star gravitationally, causing the star to wobble slightly. When a star wobbles towards and away from us, the star’s light is shifted a little towards blue and red in a regular pattern due to the Doppler effect. This method is particularly useful for large planets and can be used to measure the mass of a planet because the amount of wobble depends on the mass of the planet.
The relative speed measure of Kepler-1514. Photo credit: Dalba, Paul A., et al
The team focused on an exoplanet called Kepler-1514b, which orbits its star every 218 days. It was discovered in 2016 and is known to have a diameter about 10% larger than that of Jupiter. Using one of the telescopes at the Keck Observatory in Hawaii, they made radial velocity measurements of the star Kepler-1514 and found that the exoplanet has a mass of about 5.3 Jupiters.
Studies like this will be useful for future missions like the Nancy Grace Roman Space Telescope, slated for 2025, which attempts to directly image large exoplanets. If we know the size and mass of the GOT’EM worlds, we can estimate their temperature and brightness, which can be compared to future observations.
Reference: Dalba, Paul A. et al. “GOT’EM (Giant Outer Transiting Exoplanet Mass) survey. I. Confirmation of an eccentric, cool Jupiter with an Earth-sized planet inside that orbits Kepler-1514. “ArXiv preprint arXiv: 2012.04676 (2020).
Reference: Morton, Timothy D. et al. “False positive probabilities for all Kepler objects of interest: 1284 newly validated planets and 428 likely false positives.” The Astrophysical Journal 822.2 (2016): 86.
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