A new place to look for Earth-like planets?

Astrophysicist: White dwarfs could be fertile ground for other Earths

In the last decade, planet hunters have discovered hundreds of planets outside the solar system, though it is unclear whether any of them might be habitable. But it could be that the best place to look for planets that can support life is around small, dying stars called white dwarfs.

In a new paper published in The Astrophysical Journal, Eric Agol, a University of Washington professor of astronomy, suggests that potentially habitable planets orbiting white dwarfs could be much easier to find – if they exist – than the other exoplanets located by hunters so far.

White dwarfs are cooling stars believed to be in the final stage of their life. They typically have about 60% of the mass of our sun, but by volume they are only about the size of Earth.


Comparison of a White Dwarf and Earth
Originally born hot, they eventually become cooler than the sun and emit a tiny fraction of its energy. This means that the habitable zones (often called the goldilocks zone’) for their planets are significantly closer than Earth is to the sun.

“If the planet is close enough to the star, it could have a stable temperature long enough to have liquid water at the surface – if it has water at all – and that’s a big factor for habitability.” Agol said.

A planet so close to its star could be observed using an Earth-based telescope as small as 1 meter across. This is possible because as a planet passes in front of the star, it dims the light of the white dwarf.

White dwarfs evolve from stars like our sun. When a star’s core is no longer able to produce nuclear reactions that convert hydrogen to helium, it starts burning hydrogen outside the core. That causes the transformation to a red giant, with a greatly expanded outer atmosphere that typically envelops and destroys any planets as close as Earth.

The star eventually sheds its outer atmosphere, leaving a glowing, gradually cooling, core. This core is what is considered to be a white dwarf. Its surface temperature is a relatively cool 5,000 degrees Celsius (9,000 degrees Fahrenheit). The star produces heat and light in the same way as a dying fireplace ember, though this “ember” could last for 3 billion years.

After the red giant sheds its outer atmosphere, the more distant planets that were beyond the reach of the red giant’s atmosphere could begin to migrate closer to the white dwarf, Agol said. It’s also very possible that new planets could form from the ring of debris left behind by the star’s transformation.

A planet would have to move very close to the white dwarf to be habitable however, perhaps 550,000 to 2 million miles from the star. That’s less than 1 percent of the distance from Earth to the sun which makes it substantially closer than Mercury is to our sun.

“Standing on such a planet, the star would appear slightly larger than our sun, because it is so close, and slightly more orange, but it would look very, very similar to our sun,” Agol said.

The planet also would be tidally locked, which means the same side would always face the star and the opposite side would always be in darkness. The likely areas for habitation, he said, might be toward the edges of the light zone, nearer the dark side of the planet.


Inside A White Dwarf Star
Sirius B is the nearest white dwarf to us at a distance of about 8.5 light years (51 trillion miles). It is believed to once have been five times more massive than the sun, but now it has about the same mass as the sun packed into the same volume as Earth.

Agol is proposing a survey of the closest 20,000 white dwarfs. Using a 1 meter ground telescope, he said, one star could be surveyed in 32 hours of observation. If there is no tell-tale dimming of light from the star during that time, it means no planet orbiting closely enough to be habitable is passing in front of the star. Ideally, the work could be carried out by a vast network of telescopes which would make successive observations of a white dwarf as it progresses through the sky.

“This could take a huge amount of time, even with such a network,” he said.

The work could also be accomplished by larger specialty telescopes, such as the Large Synoptic Survey Telescope that is planned for operations later this decade in Chile. If it turns out that the number of white dwarfs with potential Earthlike planets is very small – say one in 1,000 – the LSST still would be able to track them down efficiently.

Finding an Earth-like planet around a white dwarf could provide a meaningful place to look for life, Agol said. But it could also be a potential lifeboat for humanity if Earth becomes uninhabitable.

“Those are the reasons I find this project interesting,” he said. “And there’s also the question of, ‘Just how special is Earth?’”