An exoplanet reveals Earth's fate – a rocky planet orbiting a white dwarf that survived its star's red giant phase

Does Earth have a chance? It is still debated whether our planet will survive the end of the sun or be swallowed by it. An Earth-like exoplanet orbiting a white dwarf now offers hope. The planet survived the expansion of its sun-like star into a red giant, even though it orbited the star at a close distance to Earth. This could mean that Earth could also survive this stage, as astronomers report.

As our Sun approaches the end of its life cycle, it will continue to expand and become a red giant. In about six to seven billion years, this red solar giant will engulf Mercury and Venus and reach Earth's orbit. But what about the Earth? Obviously, our planet would already be a dead, waterless desert world by this point. Could she at least avoid being eaten?

The answer to this is controversial. On the one hand, the Sun will expand to reach Earth's orbit. On the other hand, increased stellar winds push planets further outward – including Earth's orbit. If this happens faster than the Sun grows, Earth could survive the red giant phase. But so far, astronomers have discovered only a few gas giant exoplanets that orbit white dwarfs and thus survived their star's red giant phase — but no Earth-like planets.

A rocky planet around the remains of a burned-out star

Now that has changed: Astronomers led by Qiming Zhang of the University of California, San Diego have discovered an Earth-like exoplanet that resembles a future version of our planet. The planet, located about 4,000 light-years away, orbits the burned-out remains of a sun-like star that once had a mass of one or two solar masses. However, this star has already passed the red giant stage, shed its crust and is now a white dwarf weighing only about 0.5 solar masses.

Astronomers discovered this system in July 2020 when it moved in front of a background star. This made it possible to observe the star and its planets in backlight and estimate their mass thanks to the gravitational lensing effect. However, because the first observations were too inaccurate, Zhang and his team took another look at the system, named KMT-2020-BLG-0414, in 2023 using the high-resolution optics of the ten-meter Keck-II telescope in Hawaii.

An exoplanet with the same orbit as Earth.

These observations revealed that the exoplanet, called KB200414Lb, is about 20% larger than Earth and weighs about 1.9 Earth masses, making it similar to Earth's slightly larger sibling. However, this exoplanet's orbit is more interesting: It is currently orbiting the white dwarf at a distance of about 2.3 AU, Zhang and his team determined. If we apply this to our solar system, this planet would be approximately at the level of the asteroid belt between Mars and Jupiter.

But before its star burned up, exoplanet KB200414Lb may have been following an orbit very similar to our Earth's: “Taking into account the expansion of the orbit during the final stages of the central star, this Earth-like planet may have had an orbit originally.” About one astronomical unit,” the astronomers report. In other words, this planet orbits its star at the same distance that our Earth orbits the sun.

…and survived the end of his sun

Thus, the exoplanet shows us a possible future scenario for Earth and gives us hope for its survival. Because the planet KB200414Lb and its star prove that Earth can emerge victorious from the race between the expanding Sun and the outward shift of Earth's orbit. In about eight billion years, our planet could still exist and have survived the Sun's red giant phase.

“Whether life on Earth would be able to survive at this stage is somewhat questionable. But the most important thing is that the Sun does not have to swallow the Earth if it becomes a red giant,” explains co-author Jessica Lu of the University of California, Berkeley. “KB200414Lb has also survived its star's red giant phase.” (Natural Astronomy, 2024; doi: 10.1038/s41550-024-02375-9)

Source: University of California Berkeley

September 27, 2024 – Nadia Podbrigar