Two Planets Lighter Than Cotton Wool Discovered in Our Galaxy

Astronomers have discovered two ultra-low-density exoplanets—one with a density so light it resembles cotton candy—challenging existing models of planetary formation and atmospheric composition. The findings, published June 27, reveal planets with densities as low as 0.09 grams per cubic centimeter, far lighter than water or even Earth’s atmosphere.

According to SWI swissinfo.ch, the planets were initially flagged by their unusual transit signals—deep but brief dips in starlight that suggested massive but diffuse objects. "These planets are like nothing we’ve seen before," said a researcher. "Their atmospheres are so puffed up that they’re barely held together by gravity."

The lighter of the two, designated TOI-3757 b, has a density comparable to that of cotton candy—about 0.09 g/cm³—while its companion, TOI-3757 c, measures 0.12 g/cm³. For context, Earth’s average density is 5.51 g/cm³, and even Saturn, the solar system’s least dense planet, sits at 0.69 g/cm³. The findings were cross-verified by Spiegel and Euronews, which noted that the planets’ atmospheres likely contain a high fraction of hydrogen and helium, with traces of heavier elements like water vapor or even metallic compounds.

Why it matters

The discovery forces a reevaluation of how gas giants form and evolve. Traditional models assume that planets with such low densities would either lose their atmospheres over time or collapse under their own gravity. Yet TOI-3757 b and c defy this expectation, suggesting they may have formed far from their current orbits or experienced unusual heating mechanisms. “This could imply a new class of planets we haven’t accounted for,” said a researcher. FOCUS online highlighted that the planets’ proximity to their star means their surfaces could reach temperatures exceeding 1,000°C, further inflating their atmospheres.

How they compare to known exoplanets

The two planets stand out even among the most extreme exoplanets cataloged. A comparison of key figures:

• Density: TOI-3757 b (0.09 g/cm³) vs. TOI-3757 c (0.12 g/cm³)
• Radius: Both exceed Jupiter’s (1.5x Jupiter’s radius for TOI-3757 b)
• Star type: Both orbit a K-type main-sequence star, cooler than the Sun

t3n noted that the planets’ discovery could also impact the search for habitable worlds. “If such low-density planets can exist, it raises questions about how common they are and whether we’ve missed similar systems in our own galaxy,” said a researcher. The team plans to use the James Webb Space Telescope (JWST) to analyze the planets’ atmospheric compositions in greater detail, particularly searching for signs of water or organic molecules.

What comes next

Follow-up observations will focus on two key questions: how these planets retained their inflated atmospheres despite their proximity to their star, and whether their compositions differ significantly from other gas giants. “JWST is our best tool to answer these questions,” said a researcher. SWI swissinfo.ch reported that the team has already secured observation time for late 2026, with preliminary spectra expected by early 2027.

Beyond individual discoveries, the findings may prompt a broader reassessment of planetary science. If low-density planets are more common than assumed, it could reshape models of planetary migration, atmospheric escape, and even the potential for life in extreme environments. For now, TOI-3757 b and c remain the cosmic equivalent of a mystery wrapped in an enigma—light as air, yet defying every rule astronomers thought they knew.