The potential for discovering Earth-like planets, capable of sustaining organic life, recently improved in the eyes of the scientific community on Monday. Astronomers report that there could be a dizzying number of Earth-size planets in the Milky Way galaxy, likely ranging in the tens of billions.
Following statistical analysis of data collected from NASA’s now-defunct Kepler spacecraft, it appears that 22 percent of Sun-like stars contain Earth-size planets, orbiting habitable regions. As a consequence, many of the planets that reside in these “Goldilocks” orbits have surface temperatures that are suitable for maintaining liquid water – a building block that is critical to establishing life.
The research was conducted by a team based at the University of California, Berkeley, as well as the University of Hawaii, Manoa, with the extraordinary findings published in the Proceedings of the National Academy of Sciences (PNAS).
On the basis that one in five stars are thought to be sun-like, the researchers tentatively posit there to be tens of billions of potentially habitable planets, dotted throughout much of the Milky Way. Erik Petigura, a Berkeley graduate student, who orchestrated the pioneering analytical study into Kepler’s data, ruminates over the likely location of the nearest sun-like star to boast one of these Earth-like planets:
“If the stars in the Kepler field are representative of stars in the solar neighborhood, … then the nearest (Earth-size) planet is expected to orbit a star that is less than 12 light-years from Earth and can be seen by the unaided eye.”
Meanwhile, Andrew Howard, a former post-doc of UC Berkley who is now employed at the Institute for Astronomy at the University of Hawaii, explains that it has been almost two decades since the very first extrasolar planet was identified, around its normal host star. He elaborates that many Earth-size planets have been discovered, but whose orbits are relatively close to their host stars; as a result, the extreme temperatures that these planets are subjected to prevent the existence of life.
Over the last 20 years, astronomers have found over a thousand exoplanets orbiting their host stars. Data retrieved from Kepler’s observations have provided astronomers with around 3,500 additional candidates.
Petigura’s three year research endeavors represents a momentous milestone, and brings astronomers one step closer to achieving one of the principal ambitions behind the Kepler mission – to determine the proportion of sun-like stars that possessed Earth-size planets.
Only last week, Howard, Marcy and the rest of their research crew presented evidence that suggested many of these planets could, in actual fact, be water-baring, rocky planets. They even discovered one planet demonstrating the same density as that of Earth, comprising chiefly of rock and iron deposits; alas, with an estimated surface temperature of around 2,000 Kelvin, it seems unlikely that life will blossom on this scorching Earth-size body.
Although the team estimate there to be an abundance of Earth-size planets, they offer caution in interpreting their results, explaining that the conditions on many of these exoplanets are not conducive to sustaining life. Even Earth-size planets dwelling in the so-called habitable zone, where temperatures lie in an acceptable range, do not guarantee emergence of life.
Geoffrey Marcy, an astronomy professor at Berkeley, explains some of the additional factors that may preclude the development and maintenance of life:
“Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms.”
The American space agency’s Kepler satellite was deployed in 2009 to investigate extra-solar planets that transit in front of their host stars. With each transitory passage, occurring in the same orbital plane as the telescope, the exoplanet fractionally diminishes the amount of returned light from its host star.
With the Kepler team reporting thousands of candidate planets, ranging considerably in size, Petigura began using the Keck telescopes in Hawaii to acquire spectra of many of their host stars. This provides a more accurate interpretation of the brightness of each star, and enables astronomers to calculate the diameters of each transiting planet.
The team looked at 42,000 sun-like stars, which demonstrate similar sizes and temperatures to Sol, alongside 603 candidate planets that are in orbit around them. They discovered a total of ten Earth-size planets, which possessed diameters of between one and two times that of Earth, orbiting at a distance where water would be heated to life-sustaining temperatures.
All of the planets the team defined as potentially habitable were identified around K stars. These K stars are cooler and smaller than our sun. However, according to Petigura, his team’s research study shows that the findings for K stars could be extrapolated to G stars, like Sol.
Unfortunately, due to failure in several of Kepler’s reaction wheels, used for fine pointing of the spacecraft, its mission was compromised. Had Kepler remained fully operational, researchers anticipate that it would have collected sufficient data to detect Earth-size planets in the habitable zones of G-type stars.
From this data, the team created complex algorithms to determine how many Earth-size planets that are likely to exist within the galaxy. By introducing a series of “fake” planets into their computer model, they tested the validity of the algorithms used to make their calculations. This effectively created a census, from which the researchers derived their conclusions.
By James Fenner