How Can We Know if We’re Looking at Habitable exo-Earths or Hellish exo-Venuses?

How can astronomers tell exo-Earths and exo-Venuses apart? Polarimetry might be the key. Image Credits: NASA

The differences between Earth and Venus are obvious to us. One is radiant with life and adorned with glittering seas, and the other is a scorching, glowering hellhole, its volcanic surface shrouded by thick clouds and visible only with radar. But the difference wasn’t always clear. In fact, we used to call Venus Earth’s sister planet.

Can astronomers tell exo-Earths and exo-Venuses apart from a great distance?

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Watch This 12-Year Timelapse of Exoplanets Orbiting Their Star

Four faint exoplanets orbit star HR8799, which is represented by a star-shaped icon in the center. (The star itself was removed from the video because its glare is so intense that it blocks out the surrounding planets.) Credit: Jason Wang/Northwestern University.

Back in 2008, astronomers made a big announcement: for the first time, they had taken pictures of a multi-planet solar system, much like ours, orbiting another star. At the time, the star system, named HR8799 was known to have three planets, but follow-up observations a year later revealed a fourth world.

Astronomers have continued to watch this intriguing star system, and now, using observations from the last 12 years, astrophysicist Jason Wang has put together a time lapse video showing the orbital motions of the four planets.

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It’s Already Hard Enough to Block a Single Star’s Light to See its Planets. But Binary Stars? Yikes

Binary stars are common and imaging their planets will be a challenge. How can astronomers block all that light so they can see the planets? This artist's illustration shows the eclipsing binary star Kepler 16, as seen from the surface of an exoplanet in the system. Image Credit: NASA

Detecting exoplanets was frontier science not long ago. But now we’ve found over 5,000 of them, and we expect to find them around almost every star. The next step is to characterize these planets more fully in hopes of finding ones that might support life. Directly imaging them will be part of that effort.

But to do that, astronomers need to block out the light from the planets’ stars. That’s challenging in binary star systems.

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Could Next-Generation Telescopes See That Earth Has Life?

In this image, Earthshine lights up the dark portion of the lunar surface. Image Credit: NASA

While the Earth absorbs a lot of energy from the Sun, a lot of it is reflected back into space. The sunlight reflected from Earth is called Earthshine. We can see it on the dark portion of the Moon during a crescent Moon. The Farmer’s Almanac said it used to be called “the new Moon in the old Moon’s arms.

Earthshine is one instance of planetshine, and when we look at the light from distant exoplanets, we’re looking directly at their planetshine without it bouncing off another object.

If distant astronomers were looking at Earthshine the way we look at exoplanet shine, would the light tell them our planet is rippling with life?

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By Blocking the Light From a Star, Webb Reveals the Dusty Disk Surrounding It

These coronagraphic images of a disk around the star AU Microscopii, captured by Webb’s Near-Infrared Camera (NIRCam), show compass arrows, scale bar, and color key for reference. Image Credit: SCIENCE: NASA, ESA, CSA, Kellen Lawson (NASA-GSFC), Joshua E. Schlieder (NASA-GSFC) IMAGE PROCESSING: Alyssa Pagan (STScI)

AU Microscopii is a small red dwarf star about 32 light-years away. It’s far too dim for the unaided human eye, but that doesn’t diminish its appeal. The star has at least two exoplanets and hosts a circumstellar debris disk.

It’s also young, only about 23 million years old, and it’s the second-closest pre-main sequence star to Earth. The JWST recently imaged the star and its surroundings and found something surprising.

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Worlds Bustling With Plantlife Should Shine in a Detectable Wavelength of Infrared

Artist's rendering of a super-Earth-type exoplanet, TOI 1452 b. Credit: Benoit Gougeon, Université de Montréal.

Future historians might look back on this time and call it the ‘exoplanet age.’ We’ve found over 5,000 exoplanets, and we’ll keep finding more. Next, we’ll move beyond just finding them, and we’ll turn our efforts to finding biosignatures, the special chemical fingerprints that living processes imprint on exoplanet atmospheres.

But there’s more to biosignatures than atmospheric chemistry. On a planet with lots of plant life, light can be a biosignature, too.

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TESS has Found A Second Earth-Sized World in This System. Exoplanet Science is Maturing

Newly discovered Earth-size planet TOI 700 e orbits within the habitable zone of its star in this illustration. Its Earth-size sibling, TOI 700 d, can be seen in the distance. Credit: NASA/JPL-Caltech/Robert Hurt

For planet-hunters, finding an Earth-sized exoplanet must be special. NASA estimates there are about 100 billion planets in the Milky Way, but the large majority of the 5,000+ exoplanets we’ve found are extremely inhospitable. So finding one that’s similar to ours is kind of comforting.

In this case, it’s even more interesting because it’s the second Earth-sized planet orbiting the same star.

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The Webb Has Confirmed its First Exoplanet, and it’s the Same Size as Earth.

This artist's illustration shows the exoplanet LHS 474 b, the first exoplanet detected by the James Webb Space Telescope. Image Credit: NASA, ESA, CSA, L. Hustak (STScI)

The James Webb Space Telescope is the most powerful telescope ever launched into space. That power has led to a string of observational successes: ancient galaxies, obscured star-forming regions, and an exoplanet atmosphere. Now the telescope has identified its first exoplanet, and it’s a rocky planet the same size as Earth.

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Giant Exoplanet is Spiraling Inward to its Doom

Kepler 1658b may spiral in to its aging star in the distant future.
Kepler 1658b may spiral in to its aging star in the distant future. Credit: Gabriel Perez Diaz/Instituto de Astrofísica de Canarias

“Death by star” is a fate awaiting most planets in star systems. That includes our Sun, Venus, and Mercury a few billion years from now. And, astronomers now see that same fate awaiting Kepler-1658b. It’s a hot Jupiter exoplanet orbiting an evolved F-type yellow-white dwarf star about 2600 light-years away from Earth.

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Are Planets Tidally Locked to Red Dwarfs Habitable? It’s Complicated

habitable exoplanet interstellar message
Artist's impression of a habitable exoplanet orbiting a red dwarf star. The habitability of the planets of red dwarf stars is conjectural (Credit ESO/M. Kornmesser public domain)

Astronomers are keenly interested in red dwarfs and the planets that orbit them. Up to 85% of the stars in the Milky Way could be red dwarfs, and 40% of them might host Earth-like exoplanets in their habitable zones, according to some research.

But there are some problems with their potential habitability. One of those problems is tidal locking.

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