It turns out our Solar System is somewhat of an unusual case. While we observe rocky planets close to the Sun and gas giants farther out, the majority of stars in the Milky Way house something entirely different. We’re talking about worlds ranging in size between Earth and Neptune, orbiting their stars closer than Mercury orbits ours. These “super-Earths” and “sub-Neptunes” represent the most ubiquitous planet types in the Galaxy, found around nearly every sun-like star ever studied—until now, that is.
An international team of astronomers has pinpointed what they describe as a crucial missing link: a nascent planetary system captured precisely during the formation of the Galaxy’s most common planet type. Much like the renowned ‘Lucy’ fossil helped bridge the gap between humans and apes, this system offers a direct view into exactly how the Universe constructs its favored class of worlds.
The key lies with the star V1298 Tau, a relatively young sun, only 20 million years old—a mere infant compared to our 4.5-billion-year-old Sun. Orbiting this star are four massive planets, each scaled from Neptune to Jupiter in size, which researchers suggest are currently undergoing a turbulent phase of rapid evolution. Over the span of the next few billion years, these bloated worlds are expected to shrink dramatically, morphing into the compact super-Earths and Neptune-sized planets abundant throughout our Galaxy.
The researchers dedicated a decade to meticulously measuring the timing of each planet’s passage in front of its star, an event known as a transit. Yet, their focus wasn’t just the transits themselves, but the tiny deviations observed within them. The mutual gravitational pull between the planets causes minute shifts in their orbital patterns, speeding up or slowing down their passage by mere minutes. These variations in transit timing enabled the investigators to determine the planets’ masses for the first time, bypassing the usual methods which prove ineffective with young, temperamental stars.
The findings even surprised the research team. Despite having radii 5 to 10 times that of Earth, these planets possess masses of only 5 to 15 Earth masses. This gives them an incredibly low density, making them resemble cotton-candy-sized planets more than what we typically consider substantial worlds.
This puffiness resolves a long-standing puzzle. Conventional planet formation models predict newly formed worlds should be far more compact. The analysis indicates these planets underwent a dramatic early transformation, rapidly shedding much of their initial atmosphere as the gas-rich disk surrounding their young star dissipated. However, their evolution is ongoing; across billions of years, they will continue to lose atmosphere and substantially contract.
“We are essentially witnessing the construction site of the most successful planetary architecture in the Universe,” commented lead author John Livingston from the Tokyo Center for Astrobiology.
This discovery might also account for the Solar System’s lack of the Galaxy’s most common planets; perhaps our evolutionary path diverged, choosing a less traveled cosmic route.
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