Discovery suggests: Earth may escape after Sun turns into red giant

The hunt for a second Earth is going on somewhere out there in the Milky Way. Along the way, there is one planet that has caught the attention of astronomers.

The planet has a mass of about 1.9 times that of Earth, orbiting its host star at a distance about twice that of Earth from the Sun… Only, the host star is a white dwarf. That means any life that might have existed on the exoplanet was likely wiped out before or during the star's transformation into a red giant. Why did they discover this strange exoplanet?

Use stars to calculate stars

The work, led by astronomer Keming Zhang of the University of California, shows the potential of this method of detection — called microlensing — to locate Earth-like planets that are hard to find elsewhere in the galaxy.

This system was discovered due to an anomaly in the gravity and position of objects in space called a microlensing. The white dwarf system is about 4,200 light years away. We have another very large, bright star about 26,100 light years away, lined up along the same line of sight from Earth.

As light from the more distant star passes the white dwarf, its path is bent by its gravitational field, creating a magnifying effect. The white dwarf's lens is almost perfectly aligned with the background source star during this event, causing it to be magnified more than 1,000 times.

Did the Earth escape after the Sun turned into a red giant?

"For these rare ultra-high magnification microlensing events, a small companion object like a planet can significantly influence the magnification pattern, allowing us to accurately infer the mass and orbital distance," Zhang explains .

This allowed the researchers to not only determine the mass and orbital distance of the Earth-like exoplanet, but also determine the presence of a brown dwarf orbiting the white dwarf, an object about 30 times the mass of Jupiter.

A brown dwarf is a strange object that falls somewhere between a star and a planet. It is too big to be a planet, but too small to be a star—just big enough to fuse deuterium (an isotope of hydrogen) in its core, but not big enough to fuse hydrogen into a star.

The white dwarf has a mass about half that of the Sun, suggesting that its initial mass was quite close to the Sun before it completely disintegrated. And the current orbital distance between the exoplanet and the white dwarf suggests that it was once about the same distance as Earth from the Sun, before being pushed farther away when the star died.

An interesting discovery

But the discovery is exciting nonetheless: it's like getting a glimpse into the future of the Solar System and the fate of Earth after the Sun collapses and completes its evolution into a white dwarf.

White dwarfs are what stars like the Sun become when they reach the end of their lives. When they run out of hydrogen fuel to fuse their cores, they lose control between the thrust of the fusion reactions in their core and the pressure of gravity on the outside. As a result, they swell to enormous size. This is the red giant phase.

Eventually, the star will have completely expelled its outer material and its core will collapse under gravity to form a dense object. Its bright light will not be produced by nuclear fusion, but by the heat left over from its collapse. That hot core is a white dwarf, and it will take trillions of years to cool to complete darkness.

The red giant phase is pretty incredible. The star's outer atmosphere can expand to hundreds of times its original size. This gives us a good idea of ​​what the Sun will be like in about 5 billion years. During its red giant phase, it is predicted that the Sun will swell to the orbit of Mars, engulfing Mercury, Venus, and Earth in the process.

We don't know what the discovery of the exoplanet will mean for our planet. Its destruction is a possibility. But this new discovery of an Earth-like exoplanet orbiting a white dwarf suggests that it is possible to survive the inflation of its host star.

We and the Earth can both escape.

Zhang analyzed: "The planet's current orbit is 2.1 astronomical units (1 astronomical unit is the distance from Earth to the Sun), which is exactly where you would expect to find planet Earth after the Sun becomes a white dwarf.

Models are currently divided on whether the Earth can avoid being engulfed by the swelling Sun, because we do not know exactly how fast the Sun will lose mass as it becomes a red giant. Our discovery therefore suggests that some previous predictions that the Earth cannot survive may be too pessimistic. On the day of the Sun's death, the Earth may still narrowly escape being engulfed, similar to the white dwarf system we have just discovered.'

Life on Earth by the time the Sun transitions to its red giant phase may be long gone or very different . The Sun is getting hotter and brighter over time, just not enough for us to notice in our lifetimes. But in about a billion years, the Sun will be so hot that all the water on Earth will evaporate. The world that remains on Earth will be inhospitable to life.

But perhaps by then, we'll have figured out how to create life elsewhere. "When the Sun becomes a red giant, the habitable zone will move to the orbits of Jupiter and Saturn, and many of the moons of these two planets will become more hospitable to life. I think, in that case, humans could migrate out there," Zhang suggests .