Kepler's First Results (2010): Hunting Earth-Size Planets

William Borucki and the Kepler team reported the first results of NASA’s Kepler mission in 2010. Watching about 156,000 stars at once, the space telescope found five new planets in its first six weeks. Kepler went on to confirm 2,681 worlds and showed that planets, including small ones, are everywhere.

The question

Fifteen years after the 1995 discovery of 51 Pegasi b, astronomers knew many giant planets but could not say how common smaller, Earth-size worlds were. Answering that needed a census: stare at a huge number of stars, nonstop, precisely enough to notice an Earth-size planet. Kepler, launched on 6 March 2009 as NASA’s 10th competitively selected Discovery mission, was built to measure how often such planets occur, especially in and near the habitable zone of Sun-like stars.

What the paper found

• A working survey. Kepler monitored more than 156,000 stars simultaneously and almost continuously, with a precision of about 23 parts per million for half of the 12th-magnitude dwarf stars it watched, over 6.5 hours.
• Five new planets in six weeks. Kepler-4b to Kepler-8b range from 0.37 to 1.6 times Jupiter’s radius, with orbits of only 3.2 to 4.9 days.
• A puffball world. Kepler-7b has a density of about 0.17 grams per cubic centimeter, far less than water, making it one of the lowest-density planets then known.
• Many false alarms, carefully removed. In its first three months, more than 200 stars showed transit-like signals, but many turned out to be eclipsing binary stars or blends, not planets.

How they did it

Kepler used the transit method. When a planet crosses in front of its star, as seen from Earth, it blocks a sliver of the star’s light. The depth of that dip gives the planet’s size compared with the star, and the time between dips gives the length of its year. An Earth-size planet crossing a Sun-like star dims it by less than one part in ten thousand, so Kepler watched the same stars nearly without a break, for years.

A dip alone is not proof. Ground-based telescopes, including Keck I in Hawaii, followed up the candidates to rule out impostors and to measure planet masses using the star’s wobble.

Why it mattered

Kepler turned exoplanets from rare individual finds into statistics. Its catalog showed that planets are the rule rather than the exception and that small, possibly rocky planets are common. Its sample also revealed new patterns, such as the Fulton gap in the sizes of small planets. The same transit approach later found the seven Earth-size planets of TRAPPIST-1.

What we know now

Over its lifetime Kepler found 2,681 confirmed planets plus 2,899 candidates, according to NASA. The spacecraft was retired on 15 November 2018 after running out of fuel. Its data still drive studies of how often planets occur.

Its successor, TESS, launched in April 2018 and surveys a sky area about 400 times larger than Kepler’s, focusing on bright nearby stars. As of 3 May 2026, TESS had identified 7,931 candidate exoplanets, 885 of them confirmed. As of October 2026, NASA’s Exoplanet Archive lists more than 6,400 confirmed exoplanets in total.

Who wrote it

William Borucki was Kepler’s founding principal investigator, the scientist responsible for the mission as a whole. The paper lists him with David Koch, Gibor Basri and a large team of mission scientists, and it doubles as the formal introduction of the mission to the scientific community.

Read the original

Borucki, W. J., Koch, D., Basri, G., et al. (2010). “Kepler Planet-Detection Mission: Introduction and First Results.” Science 327, 977-980. doi:10.1126/science.1185402

The journal version may require a subscription. An abstract is free at NASA’s Technical Reports Server.