The first Earth-like exoplanet discovery could be made in less than a year (Image: NASA/JPL/Caltech/R. Hurt)

Can the rate of past discoveries be used to predict future ones? We may soon find out. Two researchers have used the pace of past exoplanet finds to predict that the first habitable Earth-like planet could turn up in May 2011.

In 1965, Intel co-founder Gordon Moore observed that the number of transistors that fit on a chip doubles about once every two years – a trend now known as Moore’s law. Samuel Arbesman of Harvard Medical School in Boston wants to see if scientometrics – the statistical study of science itself – can similarly be used to not only study past progress but also to make predictions.

He and Greg Laughlin of the University of California, Santa Cruz, are testing the idea with exoplanets. Over the past 15 years or so, the pace of planet discoveries has been accelerating, with some 490 planets now known. “It is actually somewhat similar to Moore’s law of exponential growth,” Arbesman says.


Habitability metric

To predict when astronomers might find the first planet similar in size to Earth that also orbits far enough from its star to boast liquid water, the team scoured the discovery records of 370 exoplanets.

They focused on two basic properties needed for habitability: a planet’s mass and its surface temperature. They used these two factors to assign each planet a ‘habitability metric’ ranging from 0 to 1, where 0 was uninhabitable and 1 is close to Earth’s twin.

A rough estimate of each planet’s habitability was then plotted against the date of its discovery. Using different subsets of the 370 planets, the researchers made curves from the individual points and extrapolated the curves to find when a planet would be found with a habitability of 1. They then analysed the range of discovery dates to determine which would be most probable.

Their calculations suggest there is a 50 per cent chance that the first habitable exo-Earth will be found by May 2011, a 75 per cent chance it will be found by 2020, and a 95 per cent chance it will be found by 2264.

Change of pace

In fact, exoplanet researchers have made forecasts of the future informally, plotting the mass of planets against the date of discovery to see how the field is progressing. “We’ve done that for many years at conferences,” says Eric Ford of the University of Florida in Gainesville. “The new aspect of this paper is putting an uncertainty on those predictions and unfortunately the uncertainty is quite large.”

One source of uncertainty is how factors like changes in funding and the development of new techniques and technology can alter the pace of discovery. “Like the stock market, past returns are no guarantee of future performance,” Ford says.

“There are always these complex factors of how science is actually done,” Arbesman agrees. But he says the forecasting technique could still prove useful, even if these factors are not accounted for directly. In part, that is because new technologies tend to take a while to ramp up, so they may not lead to sharp jumps in the number of discoveries made.

Previously, Arbesman has quantified how the ease of discovering new mammalian species, chemical elements, and asteroids affects the rate of their discovery. New species and asteroids are more difficult to find the smaller they are, and indeed larger ones are found first. For chemical elements, the opposite is true, since the bigger they are, the rarer and more unstable they tend to be.

Journal reference: PLoS ONE (in press)