Earth-Sized World Found

 Discovery Alert: Earth-Sized World Found Around Ultra-cool Star    

Our galaxy is a jewel box of red stars. More than 70% of the stars in the Milky Way are M dwarfs, also known as red dwarfs. These stars are cool and dim compared with our Sun, but they often blast orbiting exoplanets with high-energy radiation, especially early in their lives. And those ‘‘lives’’ last a long time. Stars like our Sun burn for about 10 billion years before turning into hungry red giants devouring any planets too nearby. M dwarfs keep burning for 100 billion years or more, perhaps offering a foothold for life, and an even longer window for life to develop. An international team using robotic telescopes around the world recently spotted an Earth-sized planet orbiting an ultra-cool red dwarf, the dimmest and longest-lived of stars. When the universe grows cold and dark, these will be the last stars burning. This discovery could lead to the first-ever geological exploration of a world beyond the Solar System.   

Astronomers are reporting the discovery of SPECULOOS-3b, an Earth-sized planet which orbits an ultracool dwarf, a star not much bigger than Jupiter. This is the only second known system such as this. The other one is the well-known TRAPPIST-1, which sports seven Earth-sized planets. The exoplanet SPECULOOS-3 b is about 55 light-years from Earth (really close when you consider the cosmic scale!) and nearly the same size. A year there, one orbit around the star, takes about 17 hours. The days and nights, though, may never end: The planet is thought to be tidally locked, so the same side, known as the dayside, always faces the star, like the Moon to Earth. The night side would be locked in never-ending darkness.  The planet is as big as the Earth, while its star is slightly larger than Jupiter, but much more massive.

In our corner of the galaxy, ultra-cool dwarf stars are ubiquitous. They are so faint that their planetary population is largely unexplored. The SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars) project, led by Michael Gillon at the University of Liège, Belgium, was designed to change that. Ultra-cool dwarf stars are scattered across the sky, so you need to observe them one by one, for weeks, to get a good chance to detect transiting planets. For that, you need a dedicated network of professional telescopes. This is the concept of SPECULOOS.

The star has a surface that is twice as cold as the Sun’s own, and that makes it about 1,000 times dimmer. But the planet is a lot closer to it than Earth is to the Sun. It goes around its star in just 17 hours and it is blasted by 16 times more radiation per second than our planet. It's likely this is only on one side, because orbiting so close, the planet is expected to be tidally locked like the Moon is tidally locked to Earth. One side of SPECULOOS-3b is in perennial night while the other is in constant light. But this gives astronomers an opportunity to study it directly.

Gillon is the lead author of the paper announcing the planet’s discovery, published 15 May, 2024, in Nature Astronomy. The project is a true international endeavour, with partnership with the Universities of Cambridge, Birmingham, Bern, Massachusetts Institute of Technology, and ETH Zürich. ‘‘We designed SPECULOOS specifically to explore nearby ultra-cool dwarf stars in search of rocky planets,’’ Gillon said. ‘‘With the SPECULOOS prototype and the crucial help of the NASA Spitzer Space Telescope, we discovered the famous TRAPPIST-1 system. That was an excellent start!’’

“We can say from our spectra and other observations that the star has a temperature of about 2,800 kelvins, it is about 7 billion years old – not too young, and not too old – and it is moderately active, meaning that it flares quite a lot,” co-author Benjamin Rackham from MIT said. “We think the planet must not have an atmosphere anymore because it would easily have been eroded away by the activity of the host star that’s basically constantly flaring.”

The SPECULOOS-3 star is thousands of degrees cooler than our Sun with an average temperature of about 4,760 F (2,627 C), but it pummels its planet with radiation, meaning there’s likely no atmosphere.  While an atmosphere seems unlikely, there is an even more exciting possibility for this world. The team believes that by observing the planet going behind the star 10 times with JWST, they will be able to confirm whether it has an atmosphere or not. If it doesn’t, they might be able to study its rocks. And that would be a first. “If there’s no atmosphere, there would be no blue sky or clouds – it would just be dark, like on the surface of the moon,” Rackham explained. “And the ‘sun’ would be a big, purplish-red, spotted, and flaring star that would look about 18 times as big as the sun looks to us in the sky.”

Seeing the star, let alone the planet, is a feat in itself. “Though this particular red dwarf is more than a thousand times dimmer than the Sun, its planet orbits much, much closer than the Earth, heating up the planetary surface,” said co-author Catherine Clark, a postdoctoral researcher at NASA’s Jet Propulsion Laboratory in Southern California. “SPECULOOS-3b is the first planet for which we can consider moving toward constraining surface properties of planets beyond the Solar System,” added study co-author Julien de Wit, associate professor of planetary sciences at MIT. “With this world, we could basically start doing exoplanetary geology. How cool is that?”

Understanding the composition of a world in another solar system is extremely important. We could learn how similar or different rocky planets elsewhere are, and whether all of them have the ingredients that might make life. The star is also the most common type of star in the galaxy (70 % of all the stars in the Milky Way) and it is supposed to live 10 times as long as our Sun. Understanding habitability conditions around it will tell us how likely life elsewhere is. 

‘‘We're making great strides in our study of planets orbiting other stars. We have now reached the stage where we can detect and study Earth-sized exoplanets in detail. The next step will be to determine whether any of them are habitable, or even inhabited,’’ said Steve B. Howell, one of the planet’s discoverers at NASA Ames Research Centre in Silicon Valley. While the planet is as big around as Earth, its star is just a tad bigger than Jupiter – but much more massive. The planet receives almost 16 times more energy per second than Earth receives from the Sun. SPECULOOS-3 b is an excellent candidate for follow up observations by the James Webb Space Telescope. Not only might we learn about the potential for an atmosphere and about the surface mineralogy, but it might also help us understand the stellar neighbourhood and our place in it.