Astronomers have made significant strides in understanding how the most prevalent type of planet in our galaxy, known as super-Earths, forms. A new study highlights that these planets begin life as “bloated” infants, rapidly shedding much of their thick atmospheres.

This groundbreaking research, published in Nature, observed four youthful exoplanets within the V1298 Tau system. Remarkably, their host star is merely 20 million years old—akin to a 5-month-old baby in cosmic terms.

Currently, these planets boast radii 5 to 10 times larger than Earth, but their masses only range from 5 to 15 times that of Earth, resulting in a density similar to that of Styrofoam.

This “bloated” state occurs as the young star’s heat and light cause the planet’s atmosphere to expand dramatically. Consequently, these planets are losing significant amounts of gas into space, eventually leading to a reduction in size to somewhere between that of Earth and Neptune.

Super-Earths and sub-Neptunes, as they are often called, have been detected around numerous other stars, establishing them as the most ubiquitous type of planet known today. However, they remain absent in our solar system.

Lacking nearby examples to study these intermediate worlds in detail has made them a “missing link” in our understanding of planetary formation and evolution.

“V1298 Tau is a crucial link between star- and planet-forming nebulae visible across the sky and the mature planetary systems we are currently discovering in abundance,” stated Dr. Eric Pettigura from the University of California, who was involved in the research.

Astronomers had speculated about this growth pattern for infant planets based on their sizes, but this marks the first instance of direct observation of the phenomenon.

“These planets have already experienced rapid changes, significantly losing their original atmospheres and cooling more swiftly than traditional models predict,” noted James Owen, a co-author from Imperial College London.

“Their evolution is ongoing. Over the next few billion years, they will continue to shed atmospheres and reduce in size, eventually forming a compact system of super-Earths and sub-Neptunes widely observed throughout the galaxy.”

A Stroke of Luck

As with many astronomical breakthroughs, this discovery resulted from both serendipity and diligent effort.

The researchers analyzed the planets by monitoring their transits—temporary declines in a star’s brightness when a planet moves in front of it. The depth of this dip indicates the planet’s radius, while the timing offers insights into its orbit.

While scientists were aware that these planets were on the larger side, part of the transits for the two outer planets was missed, leaving uncertainties about their orbits.

“We used computer models and educated guesses to narrow down hundreds of possibilities,” Pettigura explained.

Fortunately, their predictions proved accurate. Upon searching for the planets again using ground-based telescopes, they successfully located them on the first attempt.

“I was thrilled,” Pettigura remarked. “Given the uncertain timing, I anticipated needing at least six attempts. It felt like hitting a hole-in-one in golf.”

Once the orbits were confirmed, the team conducted a detailed analysis to determine the planets’ masses.

As planets move past each other, their gravitational forces subtly alter their orbits, affecting transit timings. The greater a planet’s mass, the stronger its gravitational pull. This enabled the researchers to differentiate timing variations to ascertain the planets’ masses.

“The unexpectedly large radii of these young planets led to the hypothesis of very low densities, which had not been previously measured,” stated Trevor David of the Flatiron Institute, who was a co-author on the first discovery of this system.

“By measuring the masses of these planets for the first time, we have provided crucial observational evidence that validates their ‘bulgy’ characteristics, establishing a significant benchmark for planetary evolution theory.”

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