Statistics Helps Identify Earth-like Planets in The Galaxy

December 20, 2013

in A World Without Statistics

Planets 600

Astronomy is perhaps the oldest science and the first to systematically collect data for analysis.

For example, the ancient Greek astronomer and mathematician Hipparchus noticed the scatter in Babylonian measurements of the length of the year and wrote about the general problem of combining data to quantify a phenomenon–deciding on the middle of the range. Problems arising from the analysis of astronomical problems continued to fuel the development of statistical methods for centuries–including Legendre’s suggestion of the Least Squares approach and Gauss’ presentation of the normal distribution in their studies of the orbits of comets and planets.

In the 20th century astronomy turned toward physics for insights but important advances in the study of stochastic processes led to new discoveries about the clustering of galaxies and how they are distributed in the universe.

Today, the field of astrostatistics studies important problems like estimating how many Earth-like planets there might be in our Galaxy. It is very hard to find planets orbiting other stars, but since 1995 several thousand have been found by virtue of their tiny effects on the host stars (e.g. Doppler shifts as the planet orbits, or 0.01% diminution of light as it transits in front of the star). A major goal is estimating the parameter dubbed eta-Earth, the fraction of stars with Earth-like planets in Earth-like orbits.

The problem is sensitivity to survey bias: it is easier to detect bigger and more massive planets the size of Jupiter or Neptune, and planets closer to the star (within the orbit of Mercury). Because of this, virtually no planets are known with Earth-size and Earth-orbit … but we can use statistical models and methods to extrapolate from the bigger surveys.

Several researchers have done this in the last few years, and perhaps the most important just emerged at the end of 2013 based on data from the National Aeronautics and Space Administration’s Kepler mission. The results are converging: about 6%±2% of Sun-like stars have Earth-like planets that may be habitable–not too close/hot to the star, not too far/cold from the star.

That means there are billions of “Earths” in the Galaxy with the closest probably only about 10 light-years away. Thus, without statistics we wouldn’t know that the Universe is full of Earth-like planets and we wouldn’t know where the planets are.