Scientists catch for the first time how a star engulfs a planet: The same fate awaits Earth

Astronomers at the Massachusetts Institute of Technology (MIT) have reportedly observed a sun-like star devouring a planet, providing new insight into the fate of the Earth in about four billion years.

The new study focused on stars that have almost exhausted their primary source of fuel, which leads their cores to contract and their outer shells to expand and cool. During this phase, known as a "red giant," stars can grow anywhere from 100 to 1,000 times their original diameter, causing them to swallow any planets orbiting closely, Space.com reports.

The study is the first known instance of a star swallowing a planet, and the event was discovered by analyzing a burst of radiation known as ZTF SLRN-2020, which occurred in the Milky Way's disk around 12,000 light-years away from Earth. During the event, a star brightened by a factor of 100 over the course of a week.

The initial discovery was made by analyzing data collected by the Zwicky Transient Facility, which scans the sky for stars that rapidly change in brightness. Upon further analysis, the astronomers discovered that the star had swallowed a planet. They based their conclusion on the spectrum of light from the outburst, which showed that the source was primarily surrounded by cool gas, rather than hot gas, which is typical of novas. Cool gas from such bursts often results from merging stars.

When the researchers followed up by looking at data from the same star collected by the Keck Observatory in Hawaii, they also found molecules that can only exist at very cold temperatures. Cold gas can condense to form dust over time.

About a year after the initial discovery, the team analyzed data from the same star using an infrared camera at the Palomar Observatory. Infrared data can yield signals of colder material, in contrast to bright visible light signals that often come from novas and other powerful events. The scientists found that the brief outburst of visible light from the star was accompanied by extraordinarily bright near-infrared light signals that slowly faded over the course of six months. This confirmed the suspicion that the source had indeed formed a lot of dust.

The final piece of the puzzle came when the researchers examined data collected by NASA's infrared space telescope, NEOWISE, which suggested the total amount of energy the star released since its initial outburst was surprisingly small — about a thousandth the magnitude of any stellar merger observed in the past. "That means that whatever merged with the star has to be 1,000 times smaller than any other star we've seen," said study lead author Kishalay De. "And it's a happy coincidence that the mass of Jupiter is about one-thousandth the mass of the sun. That's when we realized: This was a planet, crashing into its star."

The astronomers estimated that the event released hydrogen equal to about 33 times the Earth's mass, as well as about 0.33 Earth-masses of dust. From this, they suggest the progenitor star was about 0.8 to 1.5 times the mass of our sun and the engulfed planet was about 1 to 10 times the mass of Jupiter. Based on the nature of the outburst, they believe the planet crashed into the star, but it is unclear if the planet survived the plunge or was annihilated into the stellar material.

De said there are many questions the discovery raises. "Did the planet come into contact with the stellar surface because of the star's natural expansion, or did something give it an ever-so-slight push to go close to the star? All these questions will become clear as we get more data on this object and find more events in the future."

The study sheds light on the fate that will befall Earth in about four billion years when our sun swells to to engulf our world.