What is Earth’s future? A planet 4000 light years away offers a glimpse

Astronomers have made a startling discovery of an Earth-like planet 4,000 light years away using a novel method known as gravitational microlensing. They believe this discovery hints at Earth’s fate 600 crore years from now when the Sun turns into a white dwarf, forcing Earth to migrate beyond Mars’s orbit.

A team of astronomers at the University of California, Berkeley, utilised data from Hawaii’s Keck 10-metre telescope to discover KMT-2020-BLG-0414 b, an Earth-like planet. The study, published in the current edition of the journal Nature Astronomy, informs scientists about the evolution of sun-like stars from red giants to white dwarfs and how it impacts the planets that orbit them.

The discovery

This exoplanetary system, a host star with planets around it, was discovered in 2020 using a Korean microlensing telescope network.

Imagine you’re staring at a dim light emerging from a distant car’s headlamps. Imagine someone traverses by holding a giant magnifying glass, and for a brief instant, the magnifying glass slides precisely between you and the headlight. For that brief time, the beam from the car appears brighter and a bit bigger as it’s focused by the glass. Once the person has moved on, the intensity returns to normal.

In microlensing, the “magnifying glass” is a massive object, such as a star or planet, which passes in front of a distant star. The gravity of this object bends and focuses distant starlight towards Earth, temporarily enhancing the star’s brilliance. This transient brightening enables astronomers to discover things that would otherwise be too tiny, dark, or distant to observe directly.

A faint star, around 25,000 light years away, became a thousand times brighter as the KMT-2020-BLG-0414 L passed in front of it. The Korea Microlensing Telescope Network detected the sudden brightening. A team led by Keming Zhang, a former doctoral student at the University of California, Berkeley, who is now an Eric and Wendy Schmidt AI in Science Postdoctoral fellow at UC San Diego, meticulously teased out the minute changes in the transient brightness of the starlight and concluded that two planets must be orbiting the host star.

“Our conclusions are based on ruling out the alternative scenarios, since a normal star would have been easily seen,” said study leader Keming Zhang. “Because the lens is both dark and low mass, we concluded that it can only be a white dwarf.”

“Microlensing has turned into a very interesting way of studying other star systems that can’t be observed and detected by the conventional means, i.e. the transit method or the radial velocity method,” UC Berkeley astronomer Joshua Bloom, one of the team members, said. “There is a whole set of worlds that are now opening up to us through the microlensing channel, and what’s exciting is that we’re on the precipice of finding exotic configurations like this.”

What’s in a name?

In “KMT-2020-BLG-0414 L”, as the host star is named, “KMT” stands for Korea Microlensing Telescope, while 2020 is the year of discovery. BLG-0414 is the “block 0414” region of the sky where the star is located. The L at the end stands for “lensing”.

By convention, planets are called using alphabets in small letters followed by the host star’s name in the order of discovery. Thus, KMT-2020-BLG-0414 b and KMT-2020-BLG-0414 c are the two planets discovered by subtle variations in light intensity during the micro-lensing event.

KMT-2020-BLG-0414L b is a terrestrial planet comparable to Earth, with a mass of around 0.96 times Earth. It orbits at a distance 1.26 times that of the Earth-Sun and completes one orbit around the planet in around 2.8 years.

The second, KMT-2020-BLG-0414L c, has 15.4 times Jupiter’s mass and revolves very close to the host star, at only 0.12 times the distance between Earth and the Sun. It completes one cycle in 29.8 days.

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Life of a star

Like humans, stars have a life cycle in which they are born, develop, and die as white dwarfs, neutron stars, or black holes, depending on their initial mass.

The Sun formed from a massive, spinning cloud of gas and dust known as the solar nebula, which collapsed under its own gravity some 460 crore years ago. Under the crushing pressure and scorching heat of the collapsing nebula’s centre, ionised hydrogen atoms fused to form helium, a process known as the thermonuclear reaction. This process releases enormous energy in the form of radiation.

At this stage, the outward push of the radiation balances the inward pull of gravity, and the star is relatively stable. The star is said to be in the “main sequence”. The Sun is mid-life and will continue in its main sequence stage for another 500 crore years.

The hydrogen in the core will be depleted gradually and steadily, leaving a helium-rich state. The thermonuclear process will stop when the core hydrogen is virtually exhausted. The core will buckle inward and shrink without necessary radiation pressure to counteract gravity’s crushing force. As a crushed wet sponge expels water, the shrinking core will emit intense radiation, pushing away the outer layer and causing the star to expand like an inflated balloon. The star is now a giant, but the surface temperature is comparatively cold, about 2,200 to 3,200 degrees Celsius, which is a little more than half as hot as the Sun, giving it a reddish-orangish hue.

A star’s death

For sun-like stars, this stage, known as a red giant, marks the beginning of the end. As the core shrinks to one-tenth to one-hundredth of its original size, the pressure and temperature inside the red giant star rise, causing the helium nucleus to fuse and manufacture carbon. While the main sequence stage lasts about 1000 crore years, the Sun will only be in its red giant phase for 50 to 100 crore years, a mere blip in the cosmic scale.

When the helium in the core is depleted, the thermonuclear process in sun-like stars is permanently shut down. The star eventually becomes primarily carbon-based, falling under its own gravity and gradually releasing the accumulated energy. For a sun-like star, this is the terminal stage, white dwarf. At this stage, it is one-hundredth of its previous size, a dwarf compared to the sun-like star. However, because all the energy is emitted across the limited surface area, it is “white” hot between 8000 C and 40,000 C.

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Fate of Earth

In another 500 crore years, the Sun will reach the red giant stage, expanding its radius around 200 times, swallowing Mercury and probably Venus. Until now, scientists believed that the Earth might escape through whiskers but would be charred in the extreme heat of the expanding Sun.

However, this stellar system suggests that an Earth-like planet may relocate from its initial orbit to a new one when the central star metamorphoses into a red giant. When a sun-like star evolves into a white dwarf, its mass shrinks significantly. As sun-like stars lose mass, their gravitational attraction decreases, leading the planets to drift outwards, giving Earth a narrow chance of surviving the expanding Sun.

If Earth survives, it will most likely be in an orbit double its current size around the Sun, which will have lost half of its mass and reduced to a size approximately equal to Earth’s.

“Given the planetary orbital expansion during the final evolutionary stages of the host star, this Earth-mass planet may have existed in an initial orbit close to the Earth-Sun distance, thereby offering a glimpse into the possible survival of planet Earth in the distant future,” says Keming Zhang.

The discovery of the faraway planet KMT-2020-BLG-0414L b provides a slim hope for the long-term survival of our planet.

Fate of humanity

Not all planets orbiting a host star are habitable. The habitable zone surrounding the central star, often known as the Goldilocks condition, must have the appropriate temperature and radiation, not too cold or hot, but just right.

At roughly twice the Earth-Sun distance, KMT-2020-BLG-0414L b is outside the small, faint host star’s habitable zone. Even if it escapes the claws of the expanding red giant Sun, the Earth will become uninhabitable long before that stage. The scorching heat of the enormous Sun, now considerably closer to the planet, will have frizzled and scorched the Earth. “In any case, planet Earth will only be habitable for around another billion years, at which point Earth’s oceans would be vaporised by runaway greenhouse effect — long before the risk of getting swallowed by the red giant,” Zhang said.

Zhang suggests that humanity could find sanctuary in the outer solar system. Several moons of Jupiter and Saturn, including Europa, Callisto, Ganymede, and Enceladus, have frozen seas of water that may defrost when the Sun expands, providing refuge for earthlings.

“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. I think, in that case, humanity could migrate out there,” Zhang offered.

“Whether life can survive on Earth through that (red giant) period is unknown. But certainly, the most important thing is that Earth isn’t swallowed by the Sun when it becomes a red giant,” said Jessica Lu, associate professor and chair of astronomy at UC Berkeley, another team member involved in this study.

The other members of the research team were Weicheng Zang and Shude Mao of Tsinghua University in Beijing, China, Kareem El-Badry, former UC Berkeley doctoral student and now an assistant professor at the California Institute of Technology in Pasadena, Eric Agol of the University of Washington in Seattle, B Scott Gaudi of The Ohio State University in Columbus, Quinn Konopacky of UC San Diego, Natalie LeBaron of UC Berkeley, and Sean Terry of the University of Maryland in College Park.