How Many Planets?
“About a quarter of all the stars in the Milky Way galaxy have planets like the Earth in their habitable zone. And that means that there are tens of billions of planets like the Earth just in the Milky Way galaxy. And there are trillions of galaxies like the Milky Way within the observable volume of the Universe. And so there are more planets like the Earth in the observable volume of the Universe than there are grains of sand in all beaches on Earth.”
Director, Department of Astronomy
When searching for planets in deep space, there are three main methods utilised by astronomers.
Radial Velocity (Doppler Spectroscopy)
The most commonly-used method of detecting exoplanets examines a star's radial velocity—the tiny changes in its motion caused by the gravity of an exoplanet. When viewed from a distance this movement produces a recognisable signature in the star's light spectrum. By analysing the change in these signatures using the Doppler Effect, scientists can determine the duration of a planet's orbit and its mass.
Transit photography analyses the dimming of a star's light when an exoplanet passes between Earth and the star. The amount of dimming directly relates to the size of the exoplanet, and if the planet's mass is known then the two values can be used to calculate the planet's density. This allows scientists to differentiate gaseous planets from rocky planets which could harbour life.
A planet's gravitational field may also have an effect on the light of further 'background' stars—while rare, this "microlensing" allows for the detection of exoplanets which cannot be observed with other methods.
Proxima Centauri b
Proxima Centauri b is the closest exoplanet to Earth; it resides within the habitable zone of the red dwarf star Proxima Centauri, only four light years away.
The habitability of the planet itself is debated: due to tidal locking, one side is always facing the parent star—much like how the Moon always faces the same side to Earth. The permanent 'day' side is bombarded by stellar winds 2,000 times as strong as our Sun's, while the opposite 'night' side remains frozen in darkness.
However other scientists at the European Southern Observatory argue that its surface temperature (roughly −39 °C) could allow for water and an atmosphere, and the possibility of hosting life.
With the current rate of technological advancement and the scores of exoplanet discoveries made each year, astronomers are growing ever-confident that a planet with the propensity for life will be discovered in our stellar neighbourhood.
Living Universe explores one such hypothetical planet, Minerva b. It is a rocky planet, with water on its surface and a star, named Minerva, that provides warmth—though it wavers between scorching desert and frigid ice.
Unlike currently-known exoplanets, Minerva b is not tidally locked, allowing for a more stable and balanced climate. This would allow an ecosystem to develop and for alien forms to not only begin evolving, but thrive and expand more extensively in a manner similar to Earth.