Final Act for NASA’s Kepler Planet Hunter: Unearthing a Triad of Alien Worlds

NASA Kepler Space Telescope Final Observing Campaign

The final observing campaign of NASA’s Kepler Space Telescope lasted only a month. As the spacecraft began to run low on attitude control fuel, it couldn’t maintain its position long enough to collect useful observations. In the end, astronomers only had about seven days of high-quality data. A research team worked with a group of citizen scientists and professional astronomers and found three planets in the last bit of data. Credit: NASA/JPL-Caltech (K. Walbolt)

Astrophysicists and citizen scientists have discovered three exoplanets, considered to be among the last observed by NASA’s retired Kepler space telescope. Throughout its mission, Kepler observed hundreds of thousands of stars and contributed to the identification of over 2,600 confirmed exoplanets. Despite facing mechanical issues, Kepler persevered and continued to uncover new celestial bodies until its final days.

A team of astrophysicists and citizen scientists have identified what may be some of the last planets NASA’s retired Kepler space telescope observed during its nearly decade-long mission.

The trio of exoplanets – worlds beyond our solar system – are all between the size of Earth and Neptune and closely orbit their stars.

”These are fairly average planets in the grand scheme of Kepler observations,” said Elyse Incha, a senior at the University of Wisconsin-Madison. “But they’re exciting because Kepler observed them during its last few days of operations. It showcases just how good Kepler was at planet hunting, even at the end of its life.”

A paper about the planetary trio led by Incha was published in the May 30, 2023 issue of the journal Monthly Notices of the Royal Astronomical Society.

NASA Retires Kepler Space Telescope

This illustration depicts NASA’s exoplanet hunter, the Kepler space telescope. The agency announced on October 30, 2018, that Kepler has run out of fuel and is being retired within its current and safe orbit, away from Earth. Kepler leaves a legacy of more than 2,600 exoplanet discoveries. Credits: NASA/Wendy Stenzel/Daniel Rutter

Kepler launched in March 2009. The mission’s initial goal was to continuously monitor a patch of sky in the northern constellations Cygnus and Lyra. This long period of observations allowed the satellite to track changes in stellar brightness caused by planets crossing in front of their stars, events called transits.

After four years, the telescope had observed over 150,000 stars and identified thousands of potential exoplanets. It was the first NASA mission to find an Earth-size world orbiting within its star’s habitable zone, the range of distances where liquid water could exist on a planet’s surface.

In 2014, the spacecraft experienced mechanical issues that temporarily halted observations. The Kepler team devised a fix that allowed it to resume operations, switching its field of view roughly every three months, a period called a campaign. This renewed mission, called K2, lasted another four years and surveyed over 500,000 stars.

When Kepler was retired in October 2018, it had aided the discovery of over 2,600 confirmed exoplanets and many more candidates.

Kepler’s Final Image

The “last light” image taken on September 25, 2018, represents the final page of the final chapter of Kepler’s remarkable journey of data collection. The blackened gaps in the center and along the top of the image are the result of earlier random part failures in the camera. Due to the modular design, the losses did not impact the rest of the instrument. Credit: NASA/Ames Research Center

K2’s final campaign, number 19, lasted only a month. As the spacecraft began to run low on attitude control fuel, it couldn’t maintain its position long enough to collect useful observations. In the end, astronomers only had about seven days of high-quality data from Campaign 19.

Incha and her team worked with the Visual Survey Group, a collaboration between citizen scientists and professional astronomers, to scan this dataset for exoplanets. The citizen scientists hunted for signals of transiting worlds over all Campaign 19’s light curves, which record how monitored stars brightened or dimmed.

“People doing visual surveys – looking over the data by eye – can spot novel patterns in the light curves and find single objects that are hard for automated searches to detect. And even we can’t catch them all,” said Tom Jacobs, a former U.S. Navy officer and Visual Survey Group team member. “I have visually surveyed the complete K2 observations three times, and there are still discoveries waiting to be found.”

Jacobs and others found one transit for each of the three planet candidates, each orbiting a different star, in the high-quality dataset.

After their initial discovery, Incha and her team also went back and looked at the lower-quality data from the rest of Campaign 19 and found one additional transit each from two of the three stars flagged in the visual search.

“The second transits for those two planet candidates helped us confirm their discovery,” said Andrew Vanderburg, an assistant professor of physics at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology (MIT) in Cambridge. “No one had found planets in this dataset before, but our collaboration was able to find three. And we’re really pushing up against the last few days, the last few minutes, of observations Kepler collected.”

NASA TESS in Space

Illustration of NASA’s Transiting Exoplanet Survey Satellite (TESS) at work. Credit: NASA’s Goddard Space Flight Center

Using the transit information, Incha and her team calculated the worlds’ potential sizes and orbital periods. The smallest planet, K2-416 b, is about 2.6 times Earth’s size and orbits its red dwarf star about every 13 days. K2-417 b, just over three times Earth’s size, also orbits a red dwarf star but completes an orbit every 6.5 days. The final, unconfirmed planet, EPIC 246251988 b, is almost four times Earth’s size and orbits its Sun-like star in around 10 days. (The first two planets take their name from the K2 era of the mission, the last from the Ecliptic Plane Input Catalog (EPIC) of stars in the K2 fields.)

NASA’s Transiting Exoplanet Survey Satellite (TESS), which launched in April 2018, also uses the transit method, surveying large swaths of sky at a time. During August and September 2021, TESS observed the patch of space containing the three new Kepler planets. Astronomers were able to detect two more potential transits for K2-417 b.

In October 2018, after nine years in space, NASA’s Kepler Space Telescope reached the end of its mission. Earlier that year (April 2018), NASA’s Transit Transiting Exoplanet Survey Satellite launched, taking the planet-hunting torch.

“In many ways, Kepler passed the planet-hunting torch to TESS,” said Knicole Colón, the TESS project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who worked on the Kepler mission for several years. “Kepler’s dataset continues to be a treasure trove for astronomers, and TESS helps give us new insights into its discoveries.”

Reference: “Kepler’s last planet discoveries: two new planets and one single-transit candidate from K2 campaign 19″ by Elyse Incha, Andrew Vanderburg, Tom Jacobs, Daryll LaCourse, Allyson Bieryla, Emily Pass, Steve B Howell, Perry Berlind, Michael Calkins, Gilbert Esquerdo, David W Latham and Andrew W Mann, 30 May 2023, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stad1049

NASA’s Ames Research Center in California’s Silicon Valley managed 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 in Boulder, Colorado, operated the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

TESS is a NASA Astrophysics Explorer mission led and operated by MIT and managed by Goddard. Additional partners include Northrop Grumman, based in Falls Church, Virginia; NASA Ames; the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science Institute in Baltimore. More than a dozen universities, research institutes, and observatories worldwide are participants in the mission.

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