Life More Likely to Exist on Older Milky Way Planets

An artist’s impression of Proxima b, a planet in the Proxima Centauri system that scientists believe may be capable of supporting life. (European Southern Observatory)

(CN) — Planets formed during the earlier days of the Milky Way have the greatest likelihood of developing life, according to a study released Monday.

Researchers say these early planets are more likely to have a magnetic field, rocky terrain and exhibit tectonic activity, making them more closely resemble Earth than many of their newer counterparts. Planets that formed more recently often lack these features because of a finite supply of the interstellar matter needed to create them.

Exoplanets have attracted much attention recently because of the possibility that they may harbor life. These celestial bodies orbit distant stars, far outside of our solar system, and out of reach for even our most distant probes.

To date, NASA has confirmed over 4,158 exoplanets in our galaxy using the Kepler Space Telescope. The closest exoplanets to Earth orbit the star Proxima Centauri, 4.2 light-years away. Using state-of-the-art technology, the trip would take approximately 6,300 years. By breaking the laws of physics, you can get that down to just over four.

The catalog of known exoplanets has grown significantly in recent years, but scientists are still unable to characterize many of these bodies. The compositional variation of these exoplanets and how the galactic reservoir of available material impacts their creation is still largely unknown.

For a planet to be considered Earth-like it must have a rocky surface, with a size about 0.5 to 1.5 times that of Earth and remain in the habitable zone of its orbiting star — the sweet spot allowing for the existence of liquid water.

Scientists now believe plate tectonics, the movement of the giant slabs of rock dividing the Earth’s crust that causes earthquakes, is another crucial factor. Plate tectonics play an important role by allowing a planet to regulate its own temperature over billions of years.

“Plate tectonics act as a kind of thermostat for the Earth creating the conditions which allow life to evolve. The Earth has a lot of iron in its core, and we had assumed that this would be necessary for tectonic development,” said Craig O’Neill, lead author and director of the Macquarie Planetary Research Centre in Sydney, Australia. “However, we found that even planets with little iron may develop plate tectonics if the timing is right. This was completely unexpected.”

Researchers ran detailed simulations for the study using the Australian National Computational Infrastructure, a group of supercomputers often used in cutting edge astrophysical, geophysical and medical research. The parameters selected by the researchers were ran through the ASPECT geodynamics code, which simulates problems in thermal convection inside a planet’s mantle layer.

They found early planets were more likely to develop plate tectonics favorable to the emergence of life. The raw materials available today to develop new astrological bodies significantly differs from than that of the early universe, due to matter coalescing into the stars and planets we see now, while other material has been expelled through supernovas.

“Planets which formed later may not have developed plate tectonics, which means that they don’t have this built-in thermostat. This doesn’t just affect the surface temperature, this means that the core stays hot, which inhibits the development of a magnetic field. If there’s no magnetic field, the planet is not shielded from solar radiation and will tend to lose its atmosphere. So, life becomes difficult to sustain. A planet needs to be lucky to have the right position and the right geochemistry at the right time if it’s going to sustain life,” said O’Neill.

Galactic chemical evolution models are used to calculate how the building blocks of planetary bodies evolve over time. With the help of these models, scientists found trends in the availability of critical elements like iron content relative to silicate, and heat-producing elements such as uranium, thorium and potassium. These trends suggest heat-producing elements are becoming less common, while the ratio of iron to silicate is growing due to increased supernova activity.

Planets that formed early in the Milky Way tend to have low iron to silicate ratios, leaving them with smaller cores and greater amounts of these heat producing elements. Such small-core planets show increased plate tectonic behavior, along with weaker surface faults due to their lower gravity.

“It looks like the optimum conditions plate tectonics existed for planets forming early in the galaxy’s lifespan, and may be unlikely to easily recur,” added O’Neill. “For life, maybe that was as good as it gets.”

O’Neill will present the findings and more at the annual Goldschmidt conference, the world’s main geochemistry summit, hosted by the Geochemical Society and the European Association of Geochemistry.

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