This Hubble Space Telescope image shows the “last hurrah” of a star like our sun, the outer layers of gas being cast off and leaving behind the burned out white dwarf, the white dot in the center. NASA/European Space Agency |
In a new paper published March 29 in The Astrophysical Journal Letters,
Eric Agol, a University of Washington associate professor of astronomy,
suggests that potentially habitable planets orbiting white dwarfs could
be much easier to find – if they exist – than other exoplanets located
so far.
White
dwarfs, cooling stars believed to be in the final stage of life,
typically have about 60 percent of the mass of the sun, but by volume
they are only about the size of Earth. Though born hot, they eventually
become cooler than the sun and emit just a fraction of its energy, so
the habitable zones for their planets are significantly closer than
Earth is to the sun.
“If
a 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, as the planet passes in front of,
and dims the light from, the white dwarf, he said.
White
dwarfs evolve from stars like the sun. When such a star’s core can no
longer produce nuclear reactions that convert hydrogen to helium, it
starts burning hydrogen outside the core. That begins the transformation
to a red giant, with a greatly expanded outer atmosphere that typically
envelops – and destroys – any planets as close as Earth.
Finally
the star sheds its outer atmosphere, leaving the glowing, gradually
cooling, core as a white dwarf, with a surface temperature around 5,000
degrees Celsius (about 9,000 degrees Fahrenheit). At that point, the
star produces heat and light in the same way as a dying fireplace ember,
though the star’s ember could last for 3 billion years.
Once
the red giant sheds its outer atmosphere, more distant planets that
were beyond the reach of that atmosphere could begin to migrate closer
to the white dwarf, Agol said. New planets also possibly could form from
a ring of debris left behind by the star’s transformation.
In
either case, a planet would have to move very close to the white dwarf
to be habitable, perhaps 500,000 to 2 million miles from the star.
That’s less than 1 percent of the distance from Earth to the sun (93
million miles) and substantially closer than Mercury is to the sun.
“From
the 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, so 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.
The
nearest white dwarf to Earth is Sirius B at a distance of about 8.5
light years (a light year is about 6 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 20,000 white dwarfs closest to Earth.
Using a 1-meter ground telescope, he said, one star could be surveyed in
32 hours of observation. If there is no telltale dimming of light from
the star in that time, it means no planet orbiting closely enough to be
habitable is passing in front of the star so that it is easily
observable from Earth. Ideally, the work could be carried out by a
network of telescopes that 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
same work could be accomplished by larger specialty telescopes, such as
the Large Synoptic Survey Telescope that is planned for operations
later this decade in Chile, of which the UW is a founding partner. If it
turns out that the number of white dwarfs with potential Earthlike
planets is very small – say one in 1,000 – that telescope still would be
able to track them down efficiently.
Finding
an Earthlike planet around a white dwarf could provide a meaningful
place to look for life, Agol said. But it also would be a potential
lifeboat for humanity if Earth, for some reason, 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?’”
Agol’s work is funded by the National Science Foundation.
