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Mysterious population of rogue planets spotted near the center of our galaxy

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Artist’s impression of a free-floating planet.

Enticing evidence has been uncovered for a mysterious population of “rogue” (or “floating”) planets, planets that may be alone in deep space, unrelated to a host star. The results include four new findings consistent with planets of Earth-like masses, published today (July 6, 2021) in Monthly notices from the Royal Astronomical Society.

The study, led by Iain McDonald of the University of Manchester, UK (now based at Open University, UK) used data obtained in 2016 during the K2 mission phase of the telescope NASA’s Kepler Space Station. During this two-month campaign, Kepler monitored a crowded field of millions of stars near the center of our Galaxy every 30 minutes for rare gravitational microlens events.

The study team found 27 candidate short-lived microlens signals that varied over time scales ranging from one hour to 10 days. Many of them had already been observed in data obtained simultaneously from the ground. However, the four shortest events are new findings that correspond to planets of similar masses to Earth.

These new events do not show a longer accompanying signal than one would expect from a host star, suggesting that these new events may be floating planets. Such planets may have originally formed around a host star before being ejected by gravitational force from other heavier planets in the system.

Predicted by Albert Einstein 85 years ago as a result of his general theory of relativity, the microlens describes how the light from a background star can be temporarily amplified by the presence of other stars in the foreground. This produces a short burst of brightness that can last anywhere from a few hours to a few days. About one in a million stars in our Galaxy is visibly affected by the microlens at any given time, but only a few percent of these are expected to be caused by planets.

Kepler was not designed to find planets using microlenses, nor to study the extremely dense star fields of the Inner Galaxy. This meant that new data reduction techniques had to be developed to find signals in the Kepler dataset.

Iain notes, “These signals are extremely difficult to find. Our observations pointed an elderly and diseased telescope with blurry vision at one of the most densely populated parts of the sky, where there are already thousands of bright stars of varying brightness and thousands of asteroids roaming our field. . From this cacophony, we try to extract tiny, characteristic illuminations caused by the planets, and we only have one chance to see a signal before it disappears. It’s about as easy as searching for a firefly’s single wink in the middle of a freeway, using just a cell phone.

University of Manchester co-author Eamonn Kerins also comments: “Kepler has achieved what it was never designed to do, providing new provisional evidence for the existence of a population of land mass floating planets. . Now he’s passing the baton to other missions that will be designed to find such signals, signals so elusive that Einstein himself believed they were unlikely to ever be observed. I am very happy that ESA’s upcoming Euclid mission can also join this effort as an additional scientific activity to its main mission. “

Confirming the existence and nature of the floating planets will be a major focus for future missions such as NASA’s Nancy Grace Roman Space Telescope, and possibly ESA’s Euclid mission, both of which will be optimized for research. signals from microlenses.

Reference: “Kepler K2 Campaign 9 – I. Short-lived candidate events of the first space survey for planetary microlenses ”by I McDonald, E Kerins, R Poleski, MT Penny, D Specht, S Mao, P Fouqué, W Zhu and W Zang, July 6 2021, Monthly notices from the Royal Astronomical Society.
DOI: 10.1093 / mnras / stab1377


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