Earth-sized planets common in the Milky Way

Artist's impression of an Earth-like exoplanet

There are billions of Sun-like stars in our galaxy, 17 percent of which have close-orbiting planets not much bigger than Earth. Image: ESO/M. Kornmesser.

  • NASA’s Kepler space mission aims to detect Earth-like planets
  • New analysis shows that early Kepler analyses missed over 30 planets
  • It’s now thought 17% of Sun-like stars have planets not much bigger than Earth

AN ANALYSIS OF THE FIRST three years of data from NASA’s Kepler mission, which already has detected thousands of potential exoplanets, contains good news for those searching for habitable worlds outside our Solar System.

It shows that 17 percent of all Sun-like stars have planets one to two times the diameter of Earth orbiting close to their host stars, according to a team of astronomers from the University of California, Berkeley, and the University of Hawaii at Manoa.

This estimate includes only planets that circle their stars within a distance of about one-quarter of Earth’s orbital radius – which would be well within the orbit of Mercury if it were in our Solar System. This is the current limit of Kepler’s detection capability.

Further evidence suggests that the fraction of stars having planets the size of Earth or slightly bigger orbiting within Earth-like orbits may amount to 50 percent.

The team – UC Berkeley graduate student Erik Petigura, former UC Berkeley post-doctoral fellow Andrew Howard, now on the faculty of the Institute for Astronomy at the University of Hawaii, and UC Berkeley professor of astronomy Geoff Marcy – reported its findings on Wednesday (Australian time) at a session on the Kepler mission during the American Astronomical Society meeting in Long Beach, California.

Not necessarily habitable

Planets one to two times the size of Earth are not necessarily habitable. Painstaking observations by Petigura’s team show that planets two or three times the diameter of Earth are typically like Uranus and Neptune, which have a rocky core surrounded by helium and hydrogen gases and perhaps water. Planets close to a star may even be water worlds – planets with oceans hundreds of kilometres deep above a rocky core.

Nevertheless, planets between one and two times the diameter of Earth may well be rocky and, if located within the Goldilocks orbital zone – not too hot, not too cold, just right for liquid water – could support life.

“Kepler’s one goal is to answer a question that people have been asking since the days of Aristotle: What fraction of stars like the Sun have an Earth-like planet?” said Howard. “We’re not there yet, but Kepler has found enough planets that we can make statistical estimates.”

Plot of Kepler and TERRA exoplanets

Using a computer program called TERRA, scientists have sifted extra exoplanets (red dots) out of the existing Kepler data (grey dots). Image by Erik Petigura and Geoff Marcy, UC Berkeley, and Andrew Howard, Institute for Astronomy, University of Hawaii.

Finding planets in the ‘noise’

The estimates are based on a better understanding of the percentage of big Earth-size planets that Kepler misses because of uncertainties in detection, which the team estimates to be about one in four, or 25 percent.

To find planets, the Kepler telescope captures repeated images of 150,000 stars in a region of the sky in the constellation Cygnus. The data are analysed by computer software – the “pipeline” – in search of stars that dim briefly as a result of a planet passing in front, called a transit.

For planets as large as Jupiter, the star may dim by 1 percent, or one part in 100, which is easily detectable. A planet as small as Earth, however, dims the star by one part in 10,000, which is likely to be lost in the data ‘noise’, Petigura said.

The missing worlds

So Petigura spent the past two years writing a software program called TERRA, which is very similar to Kepler’s pipeline. The team then fed TERRA simulated planets to test how efficiently the software detects Earth-size planets.

After carefully measuring the fraction of planets missed by TERRA, the team corrected for this and then plugged in real Kepler observations freely available on the Internet. They identified 119 Earth-like planets ranging in size from nearly six times the diameter of Earth to the diameter of Mars. Thirty-seven of these planets were not identified in previous Kepler reports.

The analysis confirmed that the number of planets increases as the size decreases, which Howard and the Kepler team reported last year. Perhaps 1 percent of stars have planets the size of Jupiter, while 10 percent have planets the size of Neptune.

Adapted from information issued by the University of California, Berkeley.

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