Look: Astronomers Find a Potentially Habitable Exoplanet With a Violent Past
Astronomers say they’ve found another potentially habitable exoplanet orbiting a red dwarf star.
Wolf 1069b is about the same mass as Earth, so its gravity should be pretty similar. Don’t picture bright, sunny plains and beaches, though. Any alien life forms on its surface must live in an unending dim red daylight. And everything astronomers know about Wolf 1069b so far points to a lonely world with a violent past.
The exoplanet’s discoverers published their findings in the journal Astronomy & Astrophysics.
Wolf 1069b’s slight tug on its parent star gave its presence away to astronomer Diana Kossakowski, of the Max Planck Institute for Astronomy, and her colleagues. They used the Calar Alto Observatory in Spain to watch for slight changes in the spectra of the star’s light — the telltale sign of a star wobbling along as a planet danced around it. Astronomers call that slight stellar wobble radial velocity, and it happens because a planet’s gravity pulls ever so slightly on the star, causing the star to move just a tiny bit around its shared center of gravity with the planet.
Over the last few decades, astronomers have used radial velocity to discover thousands of exoplanets orbiting other stars. They can’t see the planet itself, but they can measure its effect on the star to learn its mass and how long it takes to complete an orbit. And that was enough for Kossakowski and her colleagues to realize that Wolf 1069b is between 1 and 1.5 times as massive as our Earth — and firmly in the habitable zone, the region around a star where temperatures should be just right for liquid water on a planet’s surface.
There’s a catch, though. Wolf 1069 is a red dwarf — a small, dim, reddish-hued star — which keeps its habitable zone much closer than larger, hotter stars like our Sun do. Kossakowski and her colleagues calculate that Wolf 1069b orbits its star at about a 1/15th the distance between Earth and the Sun, making a full lap every 15.6 Earth days. And because it’s so close, the planet is probably tidally locked to its star, meaning that the same side of the planet is always facing Wolf 1069, bathed in dim red daylight.
That would make Wolf 1069b a weird place to live, but not an impossible one. When Kossakowski and her colleagues modeled the temperature on the planet’s surface — based on how much energy it should get from its baleful red star — they found that the night side of the planet would be frozen and inhospitable, but most of the daylight side should be habitable, with temperatures averaging around 55 degrees Fahrenheit.
Digging Into the Details
But that estimate assumes Wolf 1069b has an Earth-like atmosphere, and so far astronomers don’t have enough information about any exoplanet orbiting a red dwarf to say what their atmospheres are made of, or even whether they have atmospheres. Red dwarfs are notoriously restless stars, prone to flinging powerful solar flares out into space and bathing nearby planets in extremely unhelpful amounts of charged particles and radiation. That sort of thing tends to be pretty tough on an atmosphere, and it’s possible that any world orbiting close to a red dwarf may find its atmosphere stripped away early on.
One thing might help: a magnetic field. And Kossakowski and her colleagues say Wolf 1069b just might have one. They can’t measure it directly, but when they simulated how planetary systems form around red dwarfs, they noticed that about 5 percent of the time, a series of violent collisions between embryonic planets leaves just one survivor in the inner star system. The force and heat of those collisions would be enough to melt a whole planet and leave behind a core of churning liquid metal exactly like the one that powers Earth’s magnetic field.
And after four years of watching Wolf 1069, Kossakowski and her colleagues say they’re sure there’s nothing else Earth-sized within 10 AU of the star (an AU, or Astronomical Unit, is the distance between Earth and the Sun). So it’s possible that Wolf 1069b may be all alone in its star system, and that it may have a magnetic field.
Why It Matters
Wolf 1069 b is the latest of about a dozen Earth-mass planets astronomers have discovered in the habitable zones of their stars in the last 30 years. Most of them orbit red dwarf stars like Wolf 1069. In part, that’s because red dwarfs are the most common type of star in the galaxy, accounting for about 75 percent of all known stars. But it’s also because planets are easier to detect around smaller stars if you’re using radial velocity, and around dimmer stars if you’re using the transit method (which measures the slight dimming of the star as a planet passes between the star and Earth).
If you’re looking for alien worlds where life might find a foothold, the habitable zone is the obvious — though not the only — place to look. And a planet with approximately Earth’s gravity also makes a good place to start. So far, however, only about a quarter of a percent of the 5,000 alien worlds we’ve discovered are Earth-sized planets in the habitable zone.
Some models of how star systems form suggest that habitable zone-dwelling, Earth-sized worlds should be pretty common in the universe, especially around stars similar to our Sun — they’re just harder for us to actually find.
Wolf 1069b doesn’t transit its star, so it won’t be possible to directly image its atmosphere the way the James Webb Space Telescope has done with exoplanet gas giant WASP-39b. But Kossakowski and her colleagues say that more simulations could reveal something about what the planet’s climate might be like with different kinds of terrain; an ocean world is going to have very different weather patterns than one that’s mostly land, for instance.
The astronomers are also searching the skies around Wolf 1069 with more sensitive instruments, looking for even smaller exoplanets on daringly close orbits around the star. What they find — or don’t find — could help them figure out how likely it is for the new exoplanet to have a magnetic field, and therefore an atmosphere.