From NASA/Kepler via Sky & Telescope : “NASA’s Kepler Finds Outcast Earths”

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From NASA/Kepler


Sky & Telescope

July 13, 2021
Lauren Sgro

Astronomers uncovered four new Earth-mass rogue planet candidates by searching for microlensing events observed with Kepler.

An artist’s impression of a free-floating planet, drifting by its lonesome through the cosmos. Although this depiction shows a Jupiter-like planet, astronomers found four new Earth-mass rogue planet candidates using the Kepler space telescope. Credit: NASA/JPL (US).

No longer part of any stellar system, rogue planets drift aimlessly through space after the tumultuous early stages of planet formation eject them. Now, using NASA’s Kepler telescope, astronomers have announced four new Earth-mass outcast planet candidates.

When a star or planet passes in front of a distant star, it acts like a magnifying lens to temporarily brighten the background star, an effect known as microlensing. Since rogue planets don’t have the luxury of a host star to reveal their presence, they are best detected via microlensing. The smaller the “lens,” the shorter the microlensing event; Earth-mass planets magnify background stars for a couple hours at most, which makes these microlensing episodes hard to find.

In the first search for rogue planets using a space-based observatory, a team lead by Iain McDonald (now at Open University (UK)) used data from a two-month span of the rejuvenated Kepler mission, dubbed K2, to scavenge for microlensing events. K2 was not meant to look at the dense galactic bulge, so the team had to develop new methods to sift through the data. They found 27 microlensing events, five of them brand new. Four of these new events have the shortest duration of all their findings, lasting a little over an hour at most and hinting at the presence of Earth-mass rogue planets. The team presents the results in the July MNRAS.

The Hunt for Rogue Planets

Even though planets cause many microlensing events, most of these worlds are bound to a star – in fact, one of the newly discovered events shows the signature of a bound planet. Previously, astronomers knew of only five super short-lived microlensing episodes (including one we’ve reported previously) thought to be caused by low-mass rogue planets. McDonald’s team has almost doubled that number.

How a gravitational lens temporarily brightens a background star. To search for instances of microlensing, astronomers use light curves, which show how stars change in brightness over time. When they see a burst in the brightness of a star, they know microlensing may be to blame. In this example, a bound planet gives itself away by causing an extra peak in the light curve, in addition to the primary peak caused by the parent star. In the case of free-floating planets, astronomers only see one single peak that lasts for a very short amount of time. Credit: National Aeronautics Space Agency (US), European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU), and K. Sahu (Space Telescope Science Institute (US))

Przemek Mro̒z (California Institute of Technology (US)), a fellow rogue planet hunter, isn’t convinced that all of these planets are actually drifters. “Figuring out whether these objects are indeed free-floating or not is more tricky,” he says. It’s possible, he adds, that some of these planets might be orbiting far from their host star while remaining gravitationally bound. “Their microlensing signature would look like nearly identical to the signal expected from free-floating planets.”

While there’s always a chance that the four new microlensing events could indicate something less interesting, such as bound planets or stellar flares, the fact that they lasted such a short time suggests that free-floating planets are a serious contender. Ground-based observations are needed to confirm these events, but these findings present exciting evidence that an Earth-mass population of rogue planets might wander our galaxy.

“The new results from Kepler confirm our earlier studies [Nature] based on ground-based OGLE observations that such low-mass (Earth-mass) free-floating or widely-orbiting planets are quite common in the Milky Way,” says Mro̒z. If these outcast earths are truly typical in our galactic neighborhood, future telescopes like Euclid and Nancy Grace Roman will be able to detect their signals easily.

See the full article here .


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NASA’s Ames Research Center (US) manages the Kepler and K2 missions for NASA’s Science Mission Directorate. NASA’s Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

In October 2009, oversight of the Kepler project was transferred from the Discovery Program at NASA’s Marshall Space Flight Center, Huntsville, AL, to the Exoplanet Exploration Program at JPL

The loss of a second of the four reaction wheels on board the Kepler spacecraft in May 2013 brought an end to Kepler’s four plus year science mission to continuously monitor more than 150,000 stars to search for transiting exoplanets. Developed over the months following this failure, the K2 mission represents a new concept for spacecraft operations that enables continued scientific observations with the Kepler space telescope. K2 became fully operational in June 2014 and is expected to continue operating until 2017 or 2018.