The search for a second Earth within the Milky Way is underway, but a recently discovered exoplanet may not be the ideal candidate. This planet, with a mass around 1.9 times that of Earth, orbits its star at about twice the distance between Earth and the Sun. However, the star in question is a white dwarf, meaning any potential life on the exoplanet likely perished during the star's red giant phase.
Despite this, the discovery remains exciting, offering a glimpse into the future of our Solar System and Earth's potential fate once the Sun dies and evolves into a white dwarf. Led by astronomer Keming Zhang from the University of California, the research highlights the power of microlensing, a technique used to detect elusive Earth-like planets throughout the galaxy.
What Are White Dwarfs?
White dwarfs are the remnants of stars like our Sun, which run out of hydrogen to fuse in their cores. This instability causes them to expand into red giants. Eventually, these stars shed their outer layers and their cores collapse into dense, bright objects, white dwarfs, which glow from the residual heat of the collapse and can take trillions of years to cool down.
The red giant phase is dramatic, with the star’s outer atmosphere expanding to hundreds of times its original size. Some projections suggest that in around 5 billion years, the Sun could grow to engulf Mercury, Venus, and possibly Earth. Whether our planet will survive this remains uncertain, but the discovery of this Earth-like world orbiting a white dwarf hints that survival could be possible.
"The simplest explanation is that the planet survived through the red giant host star," Keming Zhang told ScienceAlert.
'Microlensing' Unveils Earth-Like Exoplanet Around White Dwarf
The discovery of this system was made possible by a phenomenon known as microlensing, which occurs due to a gravitational quirk and the alignment of celestial objects. The white dwarf system is located around 4,200 light-years away, while a much larger, bright star situated about 26,100 light-years away briefly aligned along the same line of sight from Earth.
"The white dwarf lens was nearly perfectly aligned with the background source star during the event, causing it to be magnified by over 1,000 times," Zhang explained.
"For these rare ultra high magnification microlensing events, a companion as small as a terrestrial planet could significantly affect the magnification pattern, enabling us to accurately infer the lens configuration across a wide range of masses and orbital separations."
This technique enabled researchers to calculate the mass and orbital distance of the Earth-like exoplanet.
White Dwarf Discovery Offers Hope For Earth's Future Survival
The white dwarf has about half the mass of the Sun, indicating that it was once similar in size to the Sun before it expelled its outer layers. The current distance between the Earth-like exoplanet and the white dwarf suggests that the planet once orbited at a distance similar to Earth's orbit around the Sun (about 1 astronomical unit) before being pushed farther out as the star died.
"The planet's current orbit of 2.1 astronomical units is around exactly where you would expect to find planet Earth after the Sun has become a white dwarf," Zhang told ScienceAlert.
"Our discovery suggests some of the models predicting against Earth's survival may be too pessimistic. At the end of the day Earth may just narrowly escape being engulfed similar to our discovered system."
By the time the Sun enters its red giant phase, life on Earth will either be extinct or vastly different. In about a billion years, the Sun may become hot enough to evaporate all of Earth's water, rendering the planet uninhabitable.
By then, humans may have discovered a way to sustain life elsewhere.
"As the Sun becomes a red giant, the habitable zone will move to around Jupiter and Saturn's orbit, and many of these moons will become ocean planets," Zhang explained. "I think, in that case, humanity could migrate out there."
The team's findings have been published in Nature Astronomy.
