Exoplanets Close to Earth's Size Are Surprisingly Common

A recently-finished study of Kepler data is providing a new glimpse into the Milky Way's exoplanetary composition. Planets about one to four times the size of Earth appear to be typical around Sun-like stars. But researchers say we don't know yet whether they have the potential to harbour life.

This latest survey, which was conducted by Geoffrey Marcy and colleagues, shows that small planets — those about one to four times the size of Earth — are quite common around Sun-like stars. This is surprising because our solar system does not feature this particular class of planet, which are known as sub-Neptune-mass exoplanets; our sun hosts no planets slightly larger than Earth and smaller than Neptune (which is 3.9X the radius of Earth (3.9 R_e), and has 17 times its mass).

Once again we're finding that our solar system is not typical.

The Latest Census

Here's what Marcy and his team discovered:

Of the 4,000 planets known around other stars, the most common are sub-Neptune-mass exoplanets (1-4 R_e). Kepler data shows that small planets are the most common, as 26% of Sun-like stars feature planets one to two times the radius of Earth with orbital periods under 100 days.

Exoplanets Close to Earth's Size Are Surprisingly Common

Importantly, Marcy estimates that 11% of sun-like stars have planets one to two times the size of Earth that receive about one to four times the incident stellar light that our Earth enjoys.

Exoplanets Close to Earth's Size Are Surprisingly Common

The team also determined that planets larger than 1.5 times the size of Earth decrease in density with increasing radius, likely because they have rocky cores and an envelope of hydrogen and helium gas — and possibly water. Planets smaller than 1.5 R_e are almost exclusively comprised of a rocky core, and are thus denser than larger planets.

Exoplanets Close to Earth's Size Are Surprisingly Common

Location of Habitable Zones 'Uncertain'

Despite these numbers, Marcy remains hesitant in declaring these planets a boon to alien life hunters. The concluding section of his paper, which now appears in PNAS, is titled "Habitable Zone: Humility and Hubris," where he writes:

We have no evidence of microbial life at any orbital location within our solar system beside the Earth. We have no empirical information about microbial life as a function of orbital distance from our Sun or from any other star. We also have no evidence of multicellular life around any other star, nor evidence of intelligent life.

Thus, we have no empirical knowledge about the actual domain of habitable zones, for any type of life, around any type of star. Moreover we have virtually no theoretical underpinnings about exobiology. We still do not know how biology started on Earth. We do not know the mechanisms that caused a transition from chemistry to biology, nor do we know the biochemical steps that spawn proteins, RNA, DNA, or cell membranes although there has been recent progress. Indeed, we still have a poor definition of life.

Subsequently, Marcy says we can't predict if Mars, Europa, or Enceladus have habitable environments any better than "we can predict the weather in our hometown a week in advance."

Serious words considering that Marcy works at SETI Berkeley. What's needed, he says, is a door-to-door census of biology among a sample of nearby stars. This will help us determine the true domain of habitability around stars. Marcy says this census can be carried out in three ways: (1) by searching for life in our own solar system, (2) by space-based telescopes that perform chemical analyses of rocky planets, (3) and by searches for transmissions from extraterrestrial intelligences.

Read the entire study at PNAS: "Occurrence and core-envelope structure of 1—4x Earth-size planets around Sun-like stars."

Top image: Vadim Sadovski/Shutterstock; other images Marcy et. al. | PNAS

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