The list of planets that could be habitable just keeps getting bigger and bigger – and we’re not just talking about Mars. Increasingly, it’s looking like some worlds outside the Solar System (exoplanets) we previously deemed inhospitable may be rather friendlier than they appeared. Now, yet another class of exoplanets can be added to the list.
Scientists from the University of Washington have claimed that Earth-like, rocky planets that are tidally locked to low-mass stars – which means their same face always points towards the star – could support life thanks to having a magnetic field. Previously, it was thought impossible for such planets to have magnetic fields, owing to their proximity to their host star, leaving them open to an intense barrage of radiation that would hamper the formation of life.
The stars in question typically range in size from the size of our Sun down to one-tenth that size. The smaller you get, the smaller the star’s habitable zone becomes, and thus the closer a planet must orbit to be habitable. This increases the chance of tidal locking.
“The question I wanted to ask is, around these small stars, where people are going to look for planets, are these planets going to be roasted by gravitational tides?” said lead author Peter Driscoll in a statement.
The team found that tidal heating – caused by the stretching and shrinking of a world's core by its orbit around another body – can actually drive a magnetic field, rather than hinder it. In our own Solar System, we know that tidal heating is responsible for producing activity on Jupiter’s moon Io, the most volcanically active body in our system.
Io (color composite image shown) gets its volcanic activity from the push and pull of Jupiter's gravity. Galileo Project/JPL/NASA.
This latest research combined models of thermal evolution of planetary interiors by Driscoll, with models of orbital interactions and heating by co-author Rory Barnes. It explains somewhat counterintuitively how the more tidal heating a planet’s mantle experiences, the more heat it dissipates and the cooler the core becomes. This core cooling is the dominant way we know a magnetic field can form around a planet.
As long as the planet remains in a noncircular orbit, tidal heating will continue – and thus these planets can keep magnetic fields for their entire lifetimes. Of course, there are many other pre-requisites that make a planet habitable, but we know that magnetic fields are useful in protecting from radiation.
“These preliminary results are promising, but we still don’t know how they would change for a planet like Venus, where slow planetary cooling is already hindering magnetic field generation,” Driscoll added in the statement. “In the future, exoplanetary magnetic fields could be observable, so we expect there to be a growing interest in this field going forward."