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Study casts doubts about the existence of a potential “Earth 2.0”

Some astronomers are questioning the existence of what could be the most Earth-like planet that has been found outside the solar system, based on a reexamination of archival data.

Kepler 452 b was discovered by NASA's Kepler space telescope and announced in 2015. At that time it seemed that everything astronomers expected in an analogue of Earth: a little larger and more massive than our planet, and in a habitable orbit of 385 days around a star remarkably similar to our sun.

But approximately 1,000 light-years away, Kepler 452b is too weak to facilitate follow-up studies. Its apparent existence is based solely on the data collected during the main mission of Kepler, which was developed from 2009 to 2013 before being interrupted by the equipment malfunctioning. During this period, the spacecraft continually looked at a single point in the sky, waiting for any of the stars to dim almost imperceptibly from the shadows of the planets that passed by their faces. Such "transits" are how Kepler found the vast majority of his planets; but many things besides the planets can make the stars dim slightly, leading to many more false alarms than discoveries of new worlds. For any candidate planet to be confirmed as genuine, it should be observed in transit at least three times. Due to its long orbital period, Kepler 452 b barely met that minimum criterion before the telescope's main mission ended, but a series of other more technical tests convinced the Kepler team that the planet had a 99 percent chance of being real.

a new analysis, reported last month in a document accepted in The Astrophysical Journal researchers take an extremely statistical approach to considering any given planetary candidate, averaging the errors of the entire span of the Kepler mission , and of each instrument as a whole. In the process, they say they learned to better distinguish a true planetary signal from false astrophysical alarms or instrumental noise. Armed with a deeper understanding of Kepler's quirks, astronomers argue that they can more easily identify where and how small defects in the spacecraft could compromise the data. The authors used this approach to re-examine Kepler's data of more than 100,000 stars, hoping to find ways to more quickly confirm strong planetary candidates and increase the probabilities of validating the limits. Their massive reanalysis showed that the worlds with orbits of less than 200 days were very easy to confirm, because the transits of these planets were repeated enough to show a clear trend away from any instrumental noise or astrophysical background. But the authors found that confirmation of small, relatively Earth-sized planets with longer periods proved more difficult due to the premature end of Kepler's primary mission. With this in mind, they set their sights on one of the most marginal objectives under these restrictions: Kepler 452 b.

"The reason why my co-authors and I focused on Kepler 452 b is because it is the longest period, [and the] the weakest signal that has been confirmed so far," says study co-author Jeff Coughlin, scientist of the SETI Institute working in the Kepler mission.

Coughlin and his colleagues ran the Kepler 452 b data through several new simulations under its refined signal-to-noise ratio threshold, paying particular attention to possible contamination by minor faults in the spacecraft's instruments. They discovered that the planet has up to 92 percent and a probability as low as 16 percent to be real. "There is no evidence that it is not a planet," says Coughlin. "But it's not 99 percent." (However, he adds that he personally believes that there is "more than 50 percent chance" that it is real).

Some candidates can be further verified using another technique that looks for "wobble" "On the star caused by the gravitational pull of a body in orbit, but Kepler 452 b is too distant and too small for that." The great hope of saving it as a candidate is the Hubble Space Telescope, which could be used to monitor the next transit of the supposed planet, which is expected by the end of this month. "We know when the next transit will occur, within a margin of error of a couple of hours", says Natalie Batalha, Kepler's former co-researcher and mission scientist, and co-author of the discovery document Kepler 452 b.

Batalha argues that Coughlin and his co-authors are simply highlighting a problem too common and well known with many exoplanet claims: Studies often only explain astrophysical phenomena, not problems with instruments that can degrade the quality of the data. and some potential problems with the study of Coughlin and his colleagues, particularly his statistical approach (rather than case by case) to analyze Kepler's data. By averaging what is and is not a good candidate signal, he says, the study ignores the factors that made Kepler 452b so promising in the first place.

Batalha says that the method used by Coughlin and his colleagues does not take into account Kepler's real problem areas, which were discovered and defined through years of hard work. For example, some parts of their detectors worked better than others. The Kepler instrument has 21 segmented detectors, each assigned to a certain region of the sky full of stars, and the segment used to detect Kepler 452 b was known to produce clearer and clearer data than other segments that proved problematic, she says. "When you calculate the average reliability of Kepler's entire catalog of planetary candidates, all the detectors are averaged," he says. This does not take into account the real strengths of the Kepler 452 b signal, artificially reducing the confidence of detection. The initial detection actually has a one in 3000 chance of being in error, he adds. "You want to use the best possible data processing, and the discovery document has a slightly cleaner data set."

When asked for a comment, a NASA spokesperson called Kepler 452 b "an example of the scientific method in work and how science is an evolving process as new information challenges us. reviewing our thinking and refining our hypotheses We welcome the debate as it leads to our continued growth in knowledge of exoplanet science and other fields of astronomy. "

In an email sent to Kepler scientists working on the project, representatives of NASA's Exoplanet Science Institute (who administer the institute's "Exoplanet Archive" catalog) wrote that Kepler 452b should maintain its status as Planet for now: "This new document has not definitively demonstrated that the Kepler 452 b signal has an instrumental origin." Although external reliability remains high, internal reliability is lower than that stated in the original document of 2015. For now, the Exoplanet Archive has chosen to retain Kepler 452 b within the table of Confirmed Planets until a more definitive rebuttal is published. "

The only instrument that works now could resolve this debate, and Coughlin says that" still be a challenge "-it's Hubble. But the next expected transit of Kepler 452 b is April 18, too early for astronomers to get a precious interval of time in the tight schedule of the space observatory. The next transit (if the planet is really there) will not happen until May 8, 2019. Until then, the true state of the planet will remain uncertain, and astronomers will probably continue to guess Kepler's few other finds.


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