White Dwarfs Could Support Life. So Where are All Their Planets?

Artist's view of old white dwarfs surrounded by planetary debris. Credit: University of Warwick/Dr Mark Garlick

Astronomers have found plenty of white dwarf stars surrounded by debris disks. Those disks are the remains of planets destroyed by the star as it evolved. But they’ve found one intact Jupiter-mass planet orbiting a white dwarf.

Are there more white dwarf planets? Can terrestrial, Earth-like planets exist around white dwarfs?

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What’s the Best Way to Find Planets in the Habitable Zone?

This artist's illustration of Kepler 22-b, an Earth-like planet in the habitable zone of a Sun-like star about 640 light years (166 parsecs) away. Credit: NASA/Ames/JPL-Caltech

Despite the fact that we’ve discovered thousands of them, exoplanets are hard to find. And some types are harder to find than others. Naturally, some of the hardest ones to find are the ones we most want to find. What can we do?

Keep working on it, and that’s what a trio of Chinese scientists are doing.

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If Earth is Average, We Should Find Extraterrestrial Life Within 60 Light-Years

Illustration: Assortment of exoplanets
Astronomers have detected thousands of planets, including dozens that are potentially habitable. (NASA Illustration)

In 1960, while preparing for the first meeting on the Search for Extraterrestrial Intelligence (SETI), legendary astronomer and SETI pioneer Dr. Frank Drake unveiled his probabilistic equation for estimating the number of possible civilizations in our galaxy – aka. The Drake Equation. A key parameter in this equation was ne, the number of planets in our galaxy capable of supporting life – aka. “habitable.” At the time, astronomers were not yet certain other stars had systems of planets. But thanks to missions like Kepler, 5523 exoplanets have been confirmed, and another 9,867 await confirmation!

Based on this data, astronomers have produced various estimates for the number of habitable planets in our galaxy – at least 100 billion, according to one estimate! In a recent study, Professor Piero Madau introduced a mathematical framework for calculating the population of habitable planets within 100 parsecs (326 light-years) of our Sun. Assuming Earth and the Solar System are representative of the norm, Madau calculated that this volume of space could contain as much as 11,000 Earth-sized terrestrial (aka. rocky) exoplanets that orbit within their stars’ habitable zones (HZs).

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When did the First Continents Appear in the Universe?

Continents might be necessary for life, especially complex life. This image shows super-continent Pangaea during the Permian period (300 - 250 million years ago). Credit: NAU Geology/Ron Blakey

On Earth, continents are likely necessary to support life. Continents ‘float’ on top of the Earth’s viscous mantle, and heat from the planet’s core keeps the mantle from solidifying and locking the continents into place.

The core is hot because of the presence of radioactive elements that came from neutron star collisions. It should be possible to calculate when the first continents formed in the Universe.

So that’s what one researcher did.

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Is it Time for a New Definition of “Habitable?”

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser

Things tend to move from the simple to the complex when you’re trying to understand something new. This is the situation exoplanet scientists find themselves in when it comes to the term ‘habitable.’ When they were discovering the first tranche of exoplanets, the term was useful. It basically meant that the planet could have liquid water on its surface.

But now that we know of over 5,000 confirmed exoplanets, the current definition of habitable is showing its age.

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Forget the Habitable Zone – We Need to Find the Computational Zone

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser

Astronomers are currently searching for signs of life in the “habitable zones” of nearby stars, which is defined as the band around a star where liquid water can potentially exist. But a recent paper argues that we need to take a more nuanced and careful approach, based not on the potential for life, but the potential for computation.

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Even the Calmest Red Dwarfs are Wilder than the Sun

An artist's conception of a violent flare erupting from the red dwarf star Proxima Centauri. Such flares can obliterate atmospheres of nearby planets. Credit: NRAO/S. Dagnello.
An artist's conception of a violent flare erupting from the red dwarf star Proxima Centauri. Such flares can obliterate atmospheres of nearby planets. Credit: NRAO/S. Dagnello.

There’s something menacing about red dwarfs. Human eyes are accustomed to our benevolent yellow Sun and the warm light it shines on our glorious, life-covered planet. But red dwarfs can seem moody, ill-tempered, and even foreboding.

For long periods of time, they can be calm, but then they can flare violently, flashing a warning to any life that might be gaining a foothold on a nearby planet.

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Do Red Dwarfs Provide Enough Sunlight for Plants to Grow?

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser

To date, 5,250 extrasolar planets have been confirmed in 3,921 systems, with another 9,208 candidates awaiting confirmation. Of these, 195 planets have been identified as “terrestrial” (or “Earth-like“), meaning that they are similar in size, mass, and composition to Earth. Interestingly, many of these planets have been found orbiting within the circumsolar habitable zones (aka. “Goldilocks zone”) of M-type red dwarf stars. Examples include the closest exoplanet to the Solar System (Proxima b) and the seven-planet system of TRAPPIST-1.

These discoveries have further fueled the debate of whether or not these planets could be “potentially-habitable,” with arguments emphasizing everything from tidal locking, flare activity, the presence of water, too much water (i.e., “water worlds“), and more. In a new study from the University of Padua, a team of astrobiologists simulated how photosynthetic organisms (cyanobacteria) would fare on a planet orbiting a red dwarf. Their results experimentally demonstrated that oxygen photosynthesis could occur under red suns, which is good news for those looking for life beyond Earth!

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Venus is Like an Exoplanet that’s Right Next Door

Venus' thick clouds mean that only radar imaging can reveal surface details. Image Credit: NASA/JPL-Caltech

We’re lucky to have a neighbour like Venus, even though it’s totally inhospitable, wildly different from the other rocky planets, and difficult to study. Its thick atmosphere obscures its surface, and only powerful radar can penetrate it. Its extreme atmospheric pressure and high temperatures are barriers to landers or rovers.

It’s like having a mysterious exoplanet next door.

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Earth-Sized Planet Found At One of the Lightest Red Dwarfs

Artist’s conception of a rocky Earth-mass exoplanet like Wolf 1069 b orbiting a red dwarf star. If the planet has retained its atmosphere, chances are high that it would feature liquid water and habitable conditions over a wide area of its dayside. Image Credit: NASA/Ames Research Center/Daniel Rutter

Astronomers have found another Earth-sized planet. It’s about 31 light-years away and orbits in the habitable zone of a red dwarf star. It’s probably tidally locked, which can be a problem around red dwarf stars. But the team that found it is optimistic about its potential habitability.

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