The search for a second Earth is happening somewhere out there in the Milky Way. In this quest, there is one planet that has captured the attention of astronomers.
This planet has a mass about 1.9 times that of Earth, orbiting its host star at a distance twice that of Earth from the Sun… However, the host star is a white dwarf. This implies that any life that may have existed on this exoplanet could have been wiped out before or during the phase when the host star evolved into a red giant. Why was this peculiar exoplanet discovered?
Using Stars to Calculate Stars
The research led by astronomer Keming Zhang from the University of California demonstrates the potential of this discovery method. It employs a technique known as microlensing to locate Earth-like planets that are challenging to find elsewhere in the galaxy.
This system was detected due to gravitational anomalies and the positioning of objects in space, referred to as microlensing. The white dwarf system is approximately 4,200 light-years away. In addition, there is another very large and bright star located about 26,100 light-years away, aligned along the same line of sight from Earth.
As light from the more distant star passes through the white dwarf, its path is bent by gravitational fields, creating a magnification effect. The lensing effect of the white dwarf is nearly perfectly aligned with the background source star in this event, leading to an amplification of over 1,000 times.
Will Earth escape after the Sun becomes a red giant?
Zhang explains: “For these rare high-magnification microlensing events, a small companion object like a planet can significantly affect the magnification pattern, allowing us to accurately infer its mass and orbital distance.”
This enables researchers to not only determine the mass and orbital distance of the Earth-like exoplanet but also to confirm the presence of a brown dwarf orbiting the white dwarf, an object with a mass approximately 30 times that of Jupiter.
Brown dwarfs are intriguing objects that sit between stars and planets. They are too large to be classified as planets but too small to be considered stars—just large enough to fuse deuterium (a hydrogen isotope) in their cores, but not large enough to fuse hydrogen to become a star.
White dwarfs have a mass about half that of the Sun, indicating that their initial mass was quite close to that of the Sun before they completely disintegrated. The current orbital distance between the exoplanet and the white dwarf suggests it was once at the same distance from its star as Earth is from the Sun before being pushed farther away as the star died.
An Exciting Discovery
Regardless, this discovery is fascinating: it offers 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 the stars like our Sun that they become when they exhaust their life cycle. When they run out of hydrogen fuel for nuclear fusion in their cores, they lose control between the outward push from core reactions and the inward pull of external gravity. The result is a swelling to a massive size. This is the red giant phase.
Eventually, the star will completely expel its outer material, and the core will collapse under gravitational forces to form a dense object. Its brilliant light is not generated by nuclear fusion reactions but is the residual heat remaining after the collapse. That hot core is the white dwarf, which will take trillions of years to cool down to the point of complete darkness.
Many incredible things happen during the red giant phase. The outer atmosphere of the star can expand hundreds of times its original size. From this, predictions can be made about the Sun’s future in about 5 billion years. During its transformation into a red giant, it is predicted that the Sun could swell to the orbit of Mars, potentially engulfing Mercury, Venus, and Earth in the process.
We do not know what implications this discovery about the aforementioned exoplanet will have for our planet. Its destruction is possible. However, this new discovery of an Earth-like exoplanet orbiting a white dwarf suggests that there is still a chance of survival through the inflation phase of the host star.
We and Earth May Escape
Zhang states: “The current orbit of the planet is 2.1 astronomical units (1 astronomical unit is the distance from Earth to the Sun), exactly where you would expect to find an Earth-like planet after the Sun turns into a white dwarf.”
Current models are inconsistent regarding whether Earth can avoid being engulfed by the swelling Sun because we do not know the exact rate at which the Sun will lose mass as it becomes a red giant. Therefore, our discovery suggests that some previous predictions that Earth could not survive may be overly pessimistic. On the day the Sun dies, Earth could narrowly escape being engulfed, much like the system around the white dwarf they just discovered.”
Life on Earth at the time the Sun transitions to the red giant phase may have long since vanished or could be vastly different. The Sun is gradually becoming hotter and brighter over time, although it is not enough for us to notice in our lifetimes. However, in about a billion years, the Sun will be hot enough for all water on Earth to evaporate. The remaining world on Earth will be extremely harsh for life.
But perhaps by then, we will have figured out how to create life elsewhere. Zhang suggests: “When the Sun becomes a red giant, the habitable zone will move to the orbits around Jupiter and Saturn, and many of the moons of these two planets will become more hospitable to life. I believe, in that case, humanity may migrate there.”
