- 3244 - EXOPLANETS - new techniques for discoveries? Using the “transit method” for detecting Exoplanets, the Kepler Space Telescope has examined over 530,000 stars and discovered over 2,600 exoplanets in nine years. TESS, the successor to Kepler, is still active, and has so far identified over 1800 candidate exoplanets, with 46 confirmed.
------------------ 3244 - EXOPLANETS - new techniques for discoveries?
- The transit method detects the drop in light intensity when the planet passes in front of the star. Every method of planet detection has an inherent selection bias. A single transit in front of a star is not enough to be considered a planet detection. At least two transits are needed so an orbit can be defined.
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- With the transit method, detection is biased towards planets with short orbital periods. It’s also biased towards finding larger planets, which block more starlight causing a bigger dip in brightness.
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- Only a handful of well-characterized transiting exoplanets are known to have orbital periods longer than about 30 days.
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- Since the TESS space telescope only looks at most sections of the sky for 27 days, it’s biased toward detecting planets that complete two transits in that time period, meaning it’s likely to find planets close to their stars. And those planets are bound to be hot ones; too hot for liquid water or for life.
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- The Kepler mission was similar, and if you visit NASA’s Exoplanet Catalog, terrestrial planets like Earth make up a tiny part of the catalog. But hidden in all of the TESS data are detections of single transits: planets that are too far from their stars to complete two transits while TESS is watching. Those planets are further away from their stars, and cooler than the typical hot planets we find closer to their stars.
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- The team of astronomers at Warwick University are developing a way to find some of those planets lost in the TESS data. Their first find is “NGTS-11 b” that orbits a star that’s about 620 light years away. It’s five times closer to its star than Earth is to the Sun, and its orbit is only 35 days.
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- “NGTS-11b” has a temperature of only 160°C, cooler than Mercury and Venus. Although this is still too hot to support life as we know it, it is closer to the “Goldilocks zone” than many previously discovered planets which typically have temperatures above 1000°C.
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- NGTS is an array of twelve, small robotic telescopes designed to find exoplanets of Neptune size or smaller around bright stars. It can monitor stars for months on end, and is very precise.
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- This original transit appeared just once in the TESS data and then they found it again a year later with NGTS. NGTS has twelve state-of-the-art telescopes, which means that we can monitor multiple stars for months on end, searching for lost planets. The dip in light from the transit is only 1% deep and occurs only once every 35 days, putting it out of reach of other telescopes.
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- Astronomers observed a star with one of NGTS’s twelve ‘scopes for a total of 79 nights, and 105,642 exposures. They employed an algorithm to comb through all that data, looking for the right light curve that signaled a second transit of the suspected exoplanet. Finally, on the night of October 24th 2019, NGTS spotted it.
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- After detecting the second transit with NGTS, they turned to other follow-up observations. They used the ESO’s Leonard Euler telescope, and the HARPS spectrograph on the ESO’s 3.6 m telescope to get radial velocity measurements. All of that data ruled out a low-mass companion star as the cause of the transit.
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- After a detailed analysis of all that data, astronomers were able to characterize “NGTS-11 b” more completely. It’s roughly the same size and mass as Saturn. It’s 0.81 Jupiter radii, and .034 Jupiter masses. It’s still hot compared to Earth, at about 160 Celsius (320 F), but much cooler than Venus and Mercury.
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- There are hundreds of single transits detected by TESS that will be monitored using this method. This will allow us to discover cooler exoplanets of all sizes, including planets more like those in our own Solar System. Some of these will be small rocky planets in the “Goldilocks zone” that are cool enough to host liquid water oceans and potentially extraterrestrial life.
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- By reviving another technique capable of combining specialized gamma-ray telescopes to one giant virtual instrument, scientists have even measured the diameters of individual stars hundreds of light-years away.
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- The team used the four VERITAS telescopes (Very Energetic Radiation Imaging Telescope Array System) in the US as one combined instrument to determine the size of Beta Canis Majoris which is a blue giant star located 500 light-years from the sun, and Epsilon Orionis, a blue supergiant star located 2,000 light-years from the sun.
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- The Stellar Intensity Interferometry technique, demonstrated for the first time nearly 50 years ago, could be a secondary use for other gamma-ray observatories as well, including the upcoming Cherenkov Telescope Array (CTA).
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- “Interferometry’ has been widely successful in achieving the angular resolution needed to spatially resolve stars and has demonstrated the capability to perform optical intensity interferometry measurements with an array of many telescopes that in turn will help to improve our understanding of stellar systems.
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- Usually, the VERITAS telescopes monitor the sky for faint blue flashes of Cherenkov light that are produced when gamma rays from the cosmos hit Earth's atmosphere. However, these observations are limited to dark moonless hours. The team used time during which VERITAS cannot perform its normal observations in December 2019.
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- Modern electronics allow us to computationally combine light signals from each telescope. The resulting instrument has the optical resolution of a football-field-sized reflector.
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- The team observed both stars for several hours. The measurements resulted in angular diameters of 0.523 milli-arcseconds for Beta Canis Majoris and 0.631 milli-arcseconds for Epsilon Orionis.
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- A milli-arcsecond is about the size of a penny atop the Eiffel Tower in Paris as seen from New York.
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- The measured values for both stars are in good agreement with previous measurements with the same technique made with the Narrabri telescopes in the 1970s The Narrabri telescopes were the first instruments performing stellar measurements using Stellar Intensity Interferometry and were operating from 1963 to 1974.
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- The scientists have proven that dozens of telescopes could be combined using modern electronics. This could prove an interesting option for the future Cherenkov Telescope Array. It will be the world's largest gamma-ray observatory.
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- CTA will feature gamma-ray telescopes in three size classes, DESY is responsible for the medium-sized telescopes. CTA will employ up to 99 telescopes with kilometer baseline in the southern hemisphere and 19 telescopes with several hundred-meter baselines in the Northern hemisphere.
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- Intensity interferometry could not only enable scientists to determine the diameters of stars, but also to image stellar surfaces, and to measure the properties of systems like interacting binary stars, rapidly rotating stars, or the pulsation of Cepheid variables, among others.
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- August 6, 2021 EXOPLANETS - new techniques for discoveries? 3244
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