The work on Kepler actually began in 1983. The technique used by Kepler to detect exoplanets is known as “Transit Photometry”. When a planet passes in front of a star, there is a slight dip in the brightness of the star. By observing the dips in the brightness over time, a telescope could measure the frequency and intensity of the dips. These measurements, could, in turn, tell the orbital period and size of the planets. After years of improving the instruments needed to capture these observations, NASA greenlit the Kepler Space Telescope in 2001. It took another 8 years to build and deploy the astronomical instrument. Kepler began its operations by simultaneously monitoring 170,000 stars for dips in brightness, in the constellations of Lyra and Cygnus.
A stellar career
Within a year of its launch, Kepler had identified five exoplanets. Now, nine and a half years later, Kepler has identified a staggering confirmed 2,662 exoplanets. An additional 2,244 exoplanet candidates have been identified, that can potentially be confirmed by future observations. Kepler has studied 530,506 stars, collected 678 GB of data, which has been used to publish 2,946 scientific papers. The feat was achieved over two missions, called K1 and K2, each consisting of multiple observation campaigns of particular points in the sky.
Kepler is by far the most prolific planet-hunting telescope so far. When Kepler was being designed, astronomers had not identified a single planet outside our own solar system. There are some problems with the approach used by Kepler. The plane of the system, where the planets orbit the stars, has to be along the line of sight from Earth. The planet has to pass between the star and Earth for the instruments to capture the dips in brightness. With these considerations, the number of planets found by Kepler is an indication of how common planets are in the universe.
According to the data gathered by Kepler, between 20 to 50 per cent of all the stars that we can see in the night sky have small, rocky exoplanets, similar to the size and composition of the Earth, in the habitable zones of their host stars. The habitable zone, or the Goldilocks zone, is just the right distance from the star for liquid water to exist on the surface – a prerequisite for life as we know it on Earth. Kepler gathered so much data that the professional astronomers could not keep up with monitoring all the stars. The scientists released the raw data to the public, to crowdsource the discoveries. One of the interesting objects tagged by citizen scientists turned out to be one of the most mysterious stars known to science. Although Kepler has been shut down and is drifting in a safe orbit going away from the Earth, the study of the data continues. Scientific discoveries based on the data collected by Kepler is expected to continue for at least another ten years.
Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington said, “As NASA’s first planet-hunting mission, Kepler has wildly exceeded all our expectations and paved the way for our exploration and search for life in the solar system and beyond. Not only did it show us how many planets could be out there, but it also sparked an entirely new and robust field of research that has taken the science community by storm. Its discoveries have shed a new light on our place in the universe, and illuminated the tantalizing mysteries and possibilities among the stars.”
The biggest findings of Kepler
In 2013, Kepler identified the first exoplanet that was about the same size and composition as the Earth, 400 light years away towards the constellation of Cygnus. Kepler-78b orbits have a year that lasts just 8.5 Earth days and is so close to the host star that its surface is a raging inferno. In 2016, 3,700 light years away, and again, in the Cygnus constellation, Kepler discovered an exoplanet orbiting two suns. The skies of this planet would appear similar to Tatooine from Star Wars. Kepler-1647b is a circumbinary planet, but scientists informally refer to the category as “Tatooine planets”. Finding planets in orbits around two stars is significantly more difficult because the timing of the transits can vary. The planet takes three Earth years to circle around the twin stars, which is among the longest orbital periods on record. In 2017, Kepler made one of its blockbuster discoveries. It found seven Earth-sized exoplanets in orbit around a single ultracool dwarf star.
The Trappist-1 system is one of the most interesting extrasolar systems known to science. The system is under intense scrutiny, as all the seven planets can potentially have liquid water on the surface, given the right conditions. Follow up observations revealed that the planets of the system indeed had massive amounts of liquid water on the surface – several times that as the oceans of Earth. The massive amount of data gathered by Kepler has been processed in various ways, including AI and crowdsourcing. Kepler-90i was the first exoplanet to be discovered by a neural network based on Google’s TensorFlow, using the data gathered by Kepler.
K2-138, a system with at least five planets, was discovered by citizen scientists, through a crowdsourcing initiative known as Exoplanet Explorers. Scientists published a research paper based on the results.
Towards the end of its life, in October 2018, Kepler discovered the first exomoon, a moon in orbit around an exoplanet. There were smaller and larger dips in the amount of light coming from the star, which allowed the scientists to identify the exomoon. Kepler-1625 is 8,000 light years away, towards the Cygnus constellation. Data from the Hubble space telescope was used to confirm the finding.
Over just 9 years, Kepler managed to significantly advance our understanding of the universe. It might seem like a no-brainer that there are more planets than stars in the galaxy, but Kepler actually found the evidence for this. Kepler also showed that the small rocky planets are very common in the galaxy. The most common type of planet discovered by Kepler is of an intermediate size between Earth and Neptune. No planet of this size exists within our own Solar System. Astronomers would have had no idea that this kind of planets existed if it were not for Kepler. Since Kepler was trying to identify planets by continuously observing stars, it gathered a ton of information about the stars themselves. The observations helped scientists better understand the history and structure of our galaxy. Over a period of nine years, Kepler was able to spot stars that were in the process of going Nova. The space telescope was able to document 61 supernovae, from the earliest stages.
William Borucki, who started working on the Kepler mission in 35 years ago, said, “Now that we know planets are everywhere, Kepler has set us on a new course that’s full of promise for future generations to explore our galaxy.”