Level 1 - Absolute Beginner

Space has many planets outside our solar system. We call them exoplanets. Scientists discover new exoplanets using special space telescopes.

Scientists found two new exoplanets. They orbit a star that is far away, about 1,113 light-years from Earth. The star is similar to our Sun.

These planets are very light and big at the same time. They are called super-puffs or cotton candy planets. They are almost as big as Jupiter, but much lighter.

A space telescope called TESS found these planets. Scientists want to study them more using the James Webb Space Telescope. That is one of the most powerful telescopes ever made.

exoplanet

a planet that orbits a star other than our Sun

orbit

to travel in a circular path around a star or planet

light-year

the distance that light travels in one year, used to measure very large distances in space

Jupiter

the largest planet in our solar system, a gas giant

telescope

an instrument that makes faraway objects appear larger and closer so scientists can study them

density

how much matter is packed into a given space; a low-density object is light for its size

super-puff

a nickname for exoplanets that are very large but have extremely low density, like a ball of cotton candy

solar system

the Sun and all the objects, including planets, that orbit it

Level 2 - Beginner

Astronomers have discovered two extraordinary exoplanets orbiting TOI-791, a Sun-like star located 1,113 light-years from Earth. The planets, named TOI-791 b and TOI-791 c, belong to a rare class called super-puffs because of their extremely low densities.

TOI-791 b has a mass of about 3% of Jupiter's mass but is almost as large as Jupiter in diameter. TOI-791 c is slightly heavier at 5.9% of Jupiter's mass but similarly large. This makes them the lightest gas giants ever discovered.

Scientists first spotted the planets using TESS, NASA's Transiting Exoplanet Survey Satellite, which detects planets by watching for dips in starlight as a planet passes in front of its star. The discovery was published in a scientific journal on June 25, 2026.

Researchers are puzzled by how planets this large can have such little mass. One idea is that they have very expanded hydrogen and helium atmospheres inflated by heat from their star. The James Webb Space Telescope will analyse the planets' atmospheres to test these theories.

astronomer

a scientist who studies stars, planets, and other objects in space

diameter

the distance across the widest part of a circular object

transiting

passing across the face of a star as seen from Earth, causing a small dip in the star's brightness

dip in starlight

a brief reduction in the brightness of a star caused by a planet passing in front of it

hydrogen and helium

the two lightest and most abundant chemical elements in the universe, forming the bulk of gas giant planets

atmosphere

the layer of gases surrounding a planet

inflated

expanded to a much larger size than expected, often by heat

theory

a well-reasoned explanation for observed facts, tested through experiments or further observations

Level 3 - Intermediate

A team of astronomers announced on June 25, 2026 the discovery of two ultra-low-density exoplanets, TOI-791 b and TOI-791 c, in orbit around a G-type star 1,113 light-years away. The planets set new records for the least massive gas giants ever confirmed, with TOI-791 b at approximately 3% of Jupiter's mass and TOI-791 c at 5.9%, while both span close to one Jupiter radius in diameter.

The planets were identified in photometric data from NASA's TESS satellite, which flagged periodic dips in the light curve of TOI-791 indicating two bodies transiting the star on different orbital periods. Ground-based radial velocity measurements then confirmed the planetary masses by detecting the tiny gravitational wobble each planet imparts to its host star.

Their bulk densities, calculated from mass and radius, are lower than any previously catalogued gas giants, placing them firmly in the 'super-puff' category alongside a handful of comparably fluffy worlds found over the past decade. Proposed explanations include intense stellar irradiation inflating the upper atmosphere, ongoing mass loss through photoevaporation, and the possibility that an unusually high proportion of the planet's volume is occupied by an extended haze layer of high-altitude aerosols.

Scheduled JWST transmission spectroscopy observations aim to measure the molecular composition of both atmospheres. If the planets are losing mass to photoevaporation at the predicted rate, JWST may detect spectral signatures of escaping hydrogen and helium, providing the first direct observational evidence of planetary erosion on worlds with near-Jupiter radii.

G-type star

a yellow main-sequence star similar to our Sun, with a surface temperature between roughly 5,200 and 6,000 Kelvin

light curve

a graph showing how the brightness of a star changes over time, used to detect transiting planets

radial velocity

the component of a star's motion toward or away from Earth, used to detect the gravitational pull of orbiting planets

bulk density

the average density of a planet calculated from its total mass divided by its total volume

photoevaporation

the process by which high-energy radiation from a star strips gas from a planet's atmosphere

aerosols

tiny solid or liquid particles suspended in a gas, capable of forming high-altitude haze layers on planets

transmission spectroscopy

a technique that analyses starlight filtered through a planet's atmosphere during transit to identify its chemical composition

planetary erosion

the gradual loss of a planet's atmosphere or surface material, often caused by stellar radiation

Level 4 - Advanced

The announcement of TOI-791 b and TOI-791 c on June 25, 2026 pushed the observational frontier of low-density gas giants into territory that challenges conventional models of planetary formation and atmospheric evolution. With measured masses of approximately 0.031 and 0.059 Jupiter masses respectively, and radii approaching one Jupiter radius, the two planets yield mean densities on the order of 0.02 to 0.06 grams per cubic centimetre, a factor of 10 to 25 below Saturn, itself the least dense planet in our solar system.

TESS photometry flagged both planets via periodic transit signals in the TOI-791 light curve. High-precision radial velocity follow-up with HARPS and ESPRESSO confirmed the planetary nature and constrained the masses, with the mass uncertainties still large enough to admit a broad range of interior structure solutions. Bayesian atmospheric modelling suggests both planets are hydrogen-helium-dominated, with mean molecular weights that rule out any significant water or rock enrichment of the envelope.

Three competing hypotheses have emerged to account for the extraordinary puffiness. The stellar irradiation hypothesis proposes that intense ultraviolet and X-ray flux from TOI-791 deposits energy deep enough in the convective envelope to sustain radius inflation over gigayear timescales. The photoevaporative mass loss hypothesis argues the planets are currently losing several Earth masses of atmosphere per billion years, making their present configuration a transient snapshot of progressive deflation. A third explanation invokes a global high-altitude aerosol haze, perhaps composed of photochemically produced tholins, which increases the effective photospheric radius as observed in transit without requiring unusually low bulk densities in the deeper envelope.

Scheduled Cycle 4 JWST observations combining NIRSpec transit spectroscopy with MIRI mid-infrared thermal emission measurements are designed to discriminate between these scenarios. Detection of an anomalously flat transmission spectrum would support the aerosol haze model; a helium 10830-Angstrom outflow feature would confirm active photoevaporation; and an unexpectedly deep thermal emission signal would point toward an irradiation-inflated but compositionally standard gas giant. TOI-791 b and c thus arrive as a natural laboratory for atmospheric physics at the edge of what planetary science thought possible.

mean molecular weight

the average mass of molecules in a gas mixture, used to infer atmospheric composition on exoplanets

Bayesian atmospheric modelling

a statistical technique that quantifies the probability of different atmospheric compositions given observed data

convective envelope

the outer layer of a gas giant where heat is transported upward by convection currents

tholins

complex organic molecules formed by ultraviolet irradiation of nitrogen and methane, creating reddish or brownish hazes