So far, astronomers have confirmed the existence of 4,301 extrasolar planets in 3,192 star systems. Another 5,650 candidates are waiting for their confirmation. In the years to come, next-generation astronomers will be able to use next-generation telescopes to observe many of these exoplanets directly and limit their potential habitability more severely. In time this could lead to the discovery of life beyond our solar system!
The only problem is we know what to look for in order to find evidence of life. According to a new study by an interdisciplinary team of scientists from the University of California at Santa Cruz (UCSC), radioactive elements could play a role in the habitability of planets. Future studies of rocky exoplanets should therefore look for specific isotopes that indicate the presence of long-lived elements such as thorium and uranium.
On Earth, internal warming caused by the slow decay of heavy radioactive thorium and uranium drives plate tectonics, which may be required to maintain the Earth's magnetic field. This field protects the earth from radiation and cosmic rays and prevents our atmosphere from being stripped off. Because of this, exoplanet hunters and astrobiologists believe that magnetic fields could be critical to habitability.
The magnetic field and electrical currents in and around the earth create complex forces that have an immeasurable impact on daily life. Photo credits: ESA / ATG medialab
In fact, it was the work of astronomy and astrophysics professor Natalie Batalha with the Astrobiology Initiative at UCSC that sparked the interdisciplinary collaboration that led to this study. As she explained in a UCSC Newscenter publication, the planetary dynamo was linked to habitability in several ways:
“It has long been speculated that the internal heating powers plate tectonics that create carbon cycling and geological activities like volcanism that creates an atmosphere. And the ability to maintain an atmosphere is related to the magnetic field, which is also powered by internal heating. "
The carbon cycle refers to the process by which carbon dioxide (CO2) is introduced into the atmosphere through volcanic activity and bound by interacting with various minerals (resulting in carbonates). This process is critical to maintaining stable temperatures over time, ensuring that complex, long-term evolutionary processes can take place.
In addition, fast-moving charged particles (also known as solar wind) can steadily erode a planet's atmosphere in the absence of a magnetic field. This happened on Mars, where the disappearance of its magnetic field (about 4.2 billion years ago) resulted in much of its atmosphere and surface water being lost to space – causing it to become cold, parched and irradiated environment was today.
Illustration shows three versions of a rocky planet with different internal warming from radioactive elements. Photo credit: Melissa Weiss / UCSC
On Earth, the magnetic field is the result of convection in its liquid outer core, which creates a dynamo effect when it rotates in the opposite direction as Earth. With internal warming through slow decay, the earth's radioactive elements are essential to maintain this effect. As Francis Nimmo, professor of earth and planetary sciences at UCSC and first author of the study, explained in a publication by the UCSC Newscenter:
“We found that different planets accumulate different amounts of these radioactive elements, which ultimately drive geological activity and the magnetic field. So we took a model of the earth and went up and down the amount of internal radiogenic heat generation to see what happened. "
They found that there is a critical balance when it comes to radioactive elements and radiogenic warming. Too much, more than what the earth experiences, would have a number of negative effects on potential habitability. Since thorium and uranium are primarily found in the mantle on Earth, an excess of them would cause the mantle to act as an insulator.
This would prevent the molten outer core from giving off enough heat to maintain convective motion. Another consequence is that too much radiogenic warming would lead to far greater volcanic activity, which could trigger events at the extinction level. On the other hand, too little radioactive heat would not lead to volcanism, which would lead to a geologically “dead” planet (also known as a stagnant lid).
A diagram of some well-known stars nearby. It is estimated that approximately 14,600 stars are within 100 light years of the Sun. Photo credit: Andrew Z. Colvin / Wikipedia
Francis Nimmo, professor of Earth and Planetary Sciences at UCSC and lead author of the study, stated:
“If you change just that one variable, you go through these different scenarios, from geologically dead to terrestrial to extremely volcanic with no dynamo. Now that we see the important effects of varying the amount of radiogenic warming, the simplified model we used should be verified by more detailed calculations. "
Heavy elements like thorium and uranium are created by the merging of neutron stars, which are extremely rare occurrences. The availability of these elements in a circumsolar disk from which planetary systems are formed would therefore depend on the proximity of these rare events. This would lead to considerable variability between planetary systems.
To determine the range of variability, Nimmo and colleagues consulted europium measurements of nearby stars made by astronomers. Europium is easy to spot in stellar spectra and is created by the same processes that create thorium and uranium. Europium can therefore be used to determine how frequently these other two elements occur in nearby stars and planets.
By feeding these measurements into his models of radiogenic warming, Nimmo and the research team were able to produce a series of estimates for radioactive elements in nearby star systems. While the Earth and the solar system are in the center of this area, many of the stars studied had either half as much europium as the sun, while others had up to twice as much.
NASA's James Webb Telescope, featured in this artist's conception, provides more information on previously discovered exoplanets. After 2020, many more next-generation space telescopes are expected to build on what they discover. Photo credit: NASA
As Batalha explained, this could be another important constraint for determining whether a planet is "potentially habitable":
"It's a complex story because both extremes have habitability implications. You need enough radiogenic warming to keep plate tectonics going, but not so much that you turn off the magnetic dynamo. Ultimately, we're looking for the most likely places in life. The Abundance of uranium and thorium seem to be key factors, possibly even another dimension for the definition of a Goldilocks planet. "
The importance and variability of radiogenic warming open up many new opportunities for exoplanet studies and astrobiology. For starters, astronomers can use spectroscopy to measure the abundance of europium in stars and infer the levels of radiogenic elements on planets that orbit them. Next-generation instruments like the James Webb Space Telescope (JWST), slated to hit the market next year, are well suited for this task.
The study, entitled "Radiogenic Warming and Its Impact on Rocky Planet Dynamics and Habitability," appeared in the Astrophysical Journal Letters on November 10th.
Further reading: UCSC, The Astrophysical Journal Letters