10-07-2025

Fastest-growing planet ever seen is expanding at a rate that's difficult to comprehend

ByEric Ralls

Earth.com staff writer

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Astronomers have caught a starless rogue planet, named Cha 1107-7626, growing faster than any planet ever seen or imagined to be possible.

The free-floating world is swallowing gas and dust at roughly six billion tons each second, the fastest planetary growth ever recorded.

The rogue planet is collecting material from a small surrounding disk through accretion, a process where gravity drags nearby gas and dust inward.

Understanding rogue planets

Rogue planets are worlds that drift through space without orbiting a star. Unlike Earth or Jupiter, which are bound to the Sun’s gravity, rogue planets wander freely across the galaxy.

Scientists think many of them formed around stars but were later kicked out of their home systems by powerful gravitational interactions.

Others may have formed directly from collapsing clouds of gas, much like stars do, but without gathering enough mass to ignite.

These planets don’t get light or warmth from a star, so their surfaces are likely extremely cold and dark.

Astronomers detect rogue planets like like Cha 1107-7626 by spotting how their gravity briefly bends the light of distant stars, a method called gravitational microlensing.

Cha 1107-7626 has no home

The study of Cha 1107-7626 was led by Víctor Almendros-Abad at the Astronomical Observatory of Palermo, National Institute for Astrophysics in Italy (I NAF).

“People may think of planets as quiet and stable worlds, but with this discovery, we see that planetary-mass objects freely floating in space can be exciting places,” said Almendros-Abad.

By August 2025, the inflow had jumped to about eight times the rate measured only a few months earlier. The team tracked the surge as it continued for at least two months.

A spike this strong challenges simple stories of how planets grow. It also bridges ideas about planets and young stars, because bursts like this are well known in the star-forming world.

“This is the strongest accretion episode ever recorded for a planetary-mass object,” said Almendros-Abad.

This particular rogue planet sits in a sweet spot for testing formation theories. If it grew like a tiny star, it should show the same fingerprints that mark stellar feeding events.

Measuring Cha 1107-7626’s growth

The team used the X-shooter instrument on the Very Large Telescope and instruments on the James Webb Space Telescope (JWST) to record spectra across optical and infrared wavelengths of Cha 1107-7626.

X-shooter is a spectrograph, a tool that spreads light into its component colors and reveals the physics of hot gas falling in.

To turn emission line strengths into mass inflow, the researchers applied empirical relations that connect line luminosities to accretion power, a method widely used for young stars.

Independent tracers agreed on the same story, showing a rise of roughly 0.8 to 0.9 orders of magnitude between spring and late summer.

Optical continuum light brightened by about 1.5 to 2 magnitudes during the burst. Mid-infrared emission rose by 10 to 20 percent, a pattern expected when extra energy heats the inner disk.

Rogue planet mimics star growth

The most striking clue lies in H-alpha’s (Hα) line. It became broad and double-peaked, with a dip shifted to slightly longer wavelengths – a classic sign that cold gas is falling along magnetic funnels toward a hot impact region.

This profile is a hallmark of magnetospheric accretion in young stars, as shown by models and observations that link redshifted absorption to infalling columns, and the team sees the same physics at work here.

These magnetically guided flows have been tied to similar Hα behavior in T Tauri stars in past models.

That match hints at a shared mechanism across a vast mass range. A planetary mass body appears to tap the same magnetic plumbing used by stars.

Heat changes planet chemistry

The mid-infrared spectrum changed in step with the burst. Hydrocarbon features shifted in shape, and a new emission bump near 6.6 microns appeared during the high state.

That bump coincides with warm water emission seen in young disks, which brightens when extra energy heats the inner disk surface.

The detection during the surge, and its absence before, tie the chemistry directly to the accretion event.

Such on and off water signatures have been documented in forming stars during episodes of strong inflow. Seeing the same behavior next to a drifting rogue planet pushes that chemistry into a new regime.

Remarkable accretion rate

The accretion rate of Cha 1107-7626 reached roughly 10 to the minus 7 Jupiter masses per year during the peak by the team’s baseline calibration.

That level exceeds measurements in embedded protoplanets and matches or beats rare high rates seen in a handful of isolated objects.

One such object, OTS 44, showed strong and variable Paschen beta and Hα emission consistent with active accretion, though not at the sustained level seen here. The comparison places this burst at the top of the planetary mass accretion scale.

Bursts of this flavor also resemble events in the EXor class of young stars, which rise quickly and last weeks to months before fading.

A comprehensive review describes these episodes and their ability to rearrange disk material, a theme echoed by the chemical switches seen around this planet.

More questions about Cha 1107-7626

Could the brightening of this rogue planet come from patchy dust clearing instead of feeding?

The multi-line rise in accretion tracers argues against that, and the redshifted absorption points to inflow rather than changing extinction.

Might the Hα profile be wind-dominated? Wind signatures usually carve blue-shifted notches, while the observed feature is redshifted and paired with stronger inflow tracers that favor accretion.

Free-floating planets are faint, so catching one mid-feast takes both sensitivity and good timing.

The Extremely Large Telescope (ELT) coming online later this decade should expand the sample of lonely worlds and reveal whether bursts like this are common.

Better cadence will pin down how long these surges last and how often they recur. If these bursts repeat over the years, they could dominate how a drifting world grows in its first million years.