It is 2018, and the Kepler space telescope has officially retired. Analysts are now sifting through its massive haul of data in the hope of confirming the existence of remote planets. As two NASA interns examine a batch of measurements initially discarded by the space agency, they discover what appears to be a “super-Earth.” And it just might be capable of supporting carbon-based life.
Technically, a super-Earth is simply a medium-sized planet with a greater mass than the Earth, regardless of its composition. In fact, a super-Earth might be composed of rocks, metals, ice or gas, but only those planets capable of maintaining liquid water are considered to lie within a star’s circumstellar habitable zone (CHZ) or “goldilocks zone” – the orbital range considered habitable for carbon-based life.
In fact, it is not known for certain what kind of life might be supported by the planet identified by two interns in 2018. Depending on variables such as surface temperature, gravity and terrestrial and atmospheric composition, the environment may or may not resemble anything on Earth. Likewise, indigenous life, where it exists, could have evolved into highly unfamiliar forms.
Of course, our knowledge of the Universe is generally fraught with uncertainty. For example, it is not known whether the Universe – defined as the entire contents of space and time – is infinite or finite. And although the “observable Universe” has a diameter of 93 billion light years, it may form part of a much larger multiverse.
The Universe is truly mysterious and ineffable. Whatever its size and shape – flat, spherical or saddle-like, depending on which theory one subscribes to – it appears to our human eyes an unfathomable vastness. Yet despite our earthly limitations, our mortality and ignorance, there is a sense of universal connection in our wonderment.
The formal study of the Universe dates to the antiquities. Philosophers from Ancient Greece and India conceived of the Universe as geocentric – that is, with the Earth at its center – until centuries later, Nicolaus Copernicus proposed the heliocentric theory, which correctly placed the Earth in orbit around the Sun. Then, in the 17th and 18th centuries, Isaac Newton and his theories of gravity and motion revolutionized physics.
However, most modern physics is based on Albert Einstein’s theory of relativity. As a universal law, relativity explains how objects behave in space and time, and it has radically improved our understanding of stars, planets, black holes and many other cosmological phenomena. And of course, the technological applications of both classic and modern physics have transformed our world.
One such application is space travel. On October 4, 1957, the Soviet Union launched Sputnik 1 into orbit, the world’s first satellite. This historic achievement kickstarted the “Space Race” – an era of Cold War rivalry that propelled space research to sophisticated new levels. The race formally ended with the U.S. Moon landing on July 20, 1969.
The U.S. government agency responsible for meeting the challenges of the Space Race was, of course, the National Aeronautics and Space Administration (NASA). Founded in 1958 as a successor to the National Advisory Committee for Aeronautics (NACA), it remains an independent federal agency with a civilian rather than military or intelligence orientation.
Indeed, NASA continues to lead the world in space exploration. Its manned flight programs over the years have included Mercury, Gemini, Apollo, the Space Shuttle program and the International Space Station, to name but a few. Its unmanned programs have included interplanetary probes and space telescopes.
One of NASA’s current projects is the Exoplanet Exploration Program, an ambitious and ongoing initiative that aims to advance our knowledge of nearby planetary systems and Earth-like planets. Ultimately, the agency hopes that it will answer big questions about the origins of the Universe and the existence of extraterrestrial life.
To date, one of its main research tools has been the Kepler space telescope, launched into a solar orbit on March 7, 2009. Named after German astronomer Johannes Kepler – who famously discovered the laws of planetary motion in the 17th century – the telescope was designed to gather data on Earth-like exoplanets in the habitable zones of local stars in the Milky Way.
Specifically, Kepler used a photometer capable of monitoring around 150,000 sequence stars simultaneously. Over time, scientists observed that the light of some stars periodically dimmed, indicating the passing of a planetary body. This method of detecting planets is known as the “transit method,” and the duration of the dimming can be used to calculate the transiting planet’s radius and other properties.
In 2017 two interns were working at NASA’s Goddard Space Flight Center, a pioneering research laboratory in Maryland. Adina Feinstein was a graduate student at the University of Chicago. Makennah Bristow was a University of North Carolina in Asheville student. Part of their work included sifting through Kepler data to find new exoplanets.
Specifically, Feinstein and Bristow were searching for stars with at least three observable transit events – the level of proof needed to claim the existence of a new planet. One star had caught their eye, but the data showed two transits and not three. Subsequently, however, their data turned out to be flawed.
Partway into its mission, Kepler had encountered a problem. The spacecraft used four reaction wheels to aim its photometer at the sky, but on July 14, 2012, one of them failed. Kepler continued operating with three wheels. Then, on May 11, 2013, a second one broke, disabling the spacecraft. The future of the mission was suddenly unclear.
NASA consulted with the wider community of space scientists and came up with a plan. Using its two working reaction wheels, Kepler would continue to observe stars and search for planets, albeit with restrictions. For example, it would now focus its gaze on fainter, smaller red dwarf stars.
In May 2014 NASA approved the K2 “Second Light” mission extension, which consisted of discreet astronomical observing “campaigns,” each one lasting three months. Between campaigns, the spacecraft re-aimed its photometer at a different section of the sky. And scientists feared that reorienting the craft might cause temporary problems with its measuring instruments.
In a press release published by NASA in January 2019, astrophysicist Dr. Geert Barentsen explained the significance of the problem. He said, “Reorienting Kepler relative to the Sun caused miniscule changes in the shape of the telescope and the temperature of the electronics, which inevitably affected Kepler’s sensitive measurements in the first days of each campaign.”
To correct for these errors, the algorithms responsible for processing Kepler’s data simply overlooked the first days of any given observation period. Of course, the missing third transit was concealed in the first days of the Kepler’s fourth observation. And eventually, the scientists figured out how to amend their software and fix the errors without discarding any data.
According to NASA astrophysicist Joshua Schlieder, this generated an extended list of potential planets that required checking manually. “We eventually reran all data from the early campaigns… to get a list of candidates, but these candidates were never fully visually inspected,” he said. “Inspecting, or vetting, transits with the human eye is crucial because noise and other astrophysical events can mimic transits.”
To process the new, corrected data, NASA reached out to the public. Exoplanet Explorers is a citizen science project that asks participants to sift through Kepler data and identify new planets using the transit method. The project is available online at Zooniverse, one of the most popular collaborative science platforms in the world.
“The Zooniverse is the world’s largest and most popular platform for people-powered research,” states its website. “This research is made possible by volunteers – hundreds of thousands of people around the world who come together to assist professional researchers… Zooniverse research results in new discoveries, datasets useful to the wider research community, and many publications.”
Of course, citizen science is by no means limited to astronomy. To date, collaborations between researchers and the public have contributed to multiple fields, such as biology, ecology and conservation. For example, citizen science apps released in the wake of the BP oil spill in 2010 enabled users to share data about slicked wildlife, thus contributing to the cleanup effort.
In May 2017 volunteers from Exoplanet Explorers identified the missing third transit hidden in the new Kepler data. And their lively online discussions subsequently alerted Feinstein and her associates to the discovery. “That’s how missed it – and it took the keen eyes of citizen scientists to make this extremely valuable find and point us to it,” she said.
The discovery led to several subsequent observations of the star system using various apparatuses. These included NASA’s Infrared Telescope facility and its Keck II telescope at the W. M. Keck Observatory, both in Hawaii, as well as the agency’s Spitzer Space Telescope. Data from Gaia mission, a project of the European Space Agency (ESA) also contributed to their analyses.
The star system in question is situated in the constellation of Taurus, 226 light years away in K2-288. It contains two faint, cold M-type stars. The brighter of the two is approximately half as large as our Sun; the dimmer is approximately one third. The distance between them is 5.1 billion miles – the equivalent of three round-trip journeys from our Sun to Saturn.
The transiting planet – which is known as K2-288Bb – is approximately half as big as Neptune, or 1.9 times the size of Earth. There is potential for liquid water on the planet, but its composition – gas or rock – is not known. However, we do know that it has a relatively short year. Every 31.3 days, the planet completes a single orbit of the fainter star.
A public presentation concerning the discovery was held on January 7, 2019, at the 233rd meeting of the American Astronomical Society in Seattle, Washington. “It’s a very exciting discovery due to how it was found, its temperate orbit and because planets of this size seem to be relatively uncommon,” said Feinstein.
In fact, K2-288Bb lies within the so-called “Fulton Gap.” Named after Benjamin Fulton, who first identified it, the Fulton Gap or Radius Gap refers to a category of planets that are 1.5 to 2 times the size of Earth. Such planets appear to be relatively rare, making the discovery of K2-288Bb significant.
Meanwhile, on October 30, 2018, Kepler expended its last fuel reserves. A short time later, on November 15, on the anniversary of the death of Johannes Kepler in 1630, mission control sent the spacecraft a “goodnight” command, permanently deactivating it. After nine years of service, Kepler has so far contributed to the discovery of more than 2600 planets.
The types of planets observed by Kepler vary greatly in terms of mass, distance from host star, radii, habitability and composition. In addition to super-Earths, the telescope identified numerous “hot Jupiters” – large, hot, gaseous planets with orbital periods of less than 10 days. And Kepler also picked up so-called “circumbinary planets” – planets which orbit two stars.
With Kepler in retirement, the hunt for exoplanets is currently being led by a new telescope, the Transiting Exoplanet Survey Satellite (TESS). Launched on a Falcon 9 rocket on April 18, 2018, TESS is predicted to discover more than 20,000 new exoplanets, significantly expanding our list of alien worlds.
Specifically, TESS will be using wide-field cameras to gather data on bright local stars and nearby planets, including their size, mass and orbit. Those exoplanets deemed worthy of further investigation will then be scrutinized with several other powerful telescopes, both in space and on the ground.
One of them, the James Webb Space Telescope (JWST) is currently under development by NASA. Named after James E. Webb, NASA’s administrator during the Apollo program, the telescope is designed to be even more powerful than the Hubble Space telescope. Ultimately, it will be capable of observing some of the most remote phenomena in the universe.
Meanwhile, NASA interns Feinstein and Bristow went on to collaborate on an article published by The Astronomical Journal, a world-class, peer-reviewed scientific journal belonging to the American Astronomical Society. Published in January 2019, the article was titled “K2-288Bb: A Small Temperate Planet in a Low-mass Binary System Discovered by Citizen Scientists.”
Describing how the planet was found, the authors praised the usefulness of citizen science. “K2-288 is the third transiting planet system identified by the Exoplanet Explorers program,” wrote Feinstein and her colleagues. “Its discovery exemplifies the value of citizen science in the era of Kepler, K2, and the Transiting Exoplanet Survey Satellite.”
Indeed, citizen scientists have been analyzing Kepler data for nearly a decade now. Starting in December 2010, the Planet Hunters project – which is also hosted by Zooniverse and is presently processing TESS data – discovered 69 planetary candidates initially overlooked by NASA. Among them was a Neptune-sized planet found by a British amateur astronomer and subsequently named Threapleton Holmes B.
Later, citizen scientists from Exoplanet Explorers scoured K2 data to find no less than six super-Earths orbiting a Sun-like star system called K2-138. Further analysis suggested that five of them are likely to be gaseous; one might be rocky. Observed by Kepler during its 12th campaign, the system is situated in the constellation of Aquarius, nearly 600 light years from Earth.
Ultimately, whatever the future of the Earth, exoplanets appear to be abundant throughout the Universe. Our present technological limitations mean we are only capable of observing them from a great distance. But, one day, perhaps, our descendants might be able to travel to them. Until then, we can only gaze into the night sky and wonder.