Hot, hot planet sets sizzling new record | Science News for Students

Hot, hot planet sets sizzling new record

This odd exoplanet is hotter than many stars, astronomers say
Jun 21, 2017 — 7:10 am EST
Hot exoplanet

Planet KELT 9b, the giant illustrated here facing its sun, is the hottest planet astronomers have discovered to date.


A newly discovered planet is shattering records for being hot. This big ball of gas is actually hotter than many stars, astronomers say.

Called KELT 9b, it's located some 650 light-years from Earth. A bit like Jupiter, it’s a gas giant. It orbits its host star every 36 hours. But only one side of the planet faces its sun, just as one side of our moon always faces Earth.

It’s “pretty much something out of a science fiction novel,” says Scott Gaudi. An astronomer at Ohio State University in Columbus, he helped discover this unusual exoplanet.

Temperatures on KELT 9b’s dayside, which faces its star, hover at about 4,300Celsius (7,772o Fahrenheit). Jupiter, by comparison, is a chilly -145 oC (-234 oF). That sizzling temp is more than 700 degrees more blistering than the next-hottest exoplanet known. KELT 9b is so hot, in fact, that atoms in its atmosphere cannot bind together to form molecules!

“It’s like a star-planet hybrid,” concludes Drake Deming. He is a planetary scientist at the University of Maryland in College Park and was not involved in KELT 9b’s discovery. He says this is “a kind of object we’ve never seen before.”

One of its oddities: KELT 9b doesn’t orbit around the equator of its sun. Instead, it swoops around its star’s poles. Because the planet orbits so close, it receives an intense blast of radiation from that star. Indeed, that "stellar wind" is so intense that it blows the planet’s atmosphere out like a comet tail. One day, it may eventually strip the planet’s atmosphere away completely.

The planet is so bizarre that it took scientists nearly three years to convince themselves it was real, Gaudi says. And, adds Deming, the bizarre planet is probably not alone out there. He suspects KELT 9b is “the tip of the iceberg” for an undiscovered population of scalding-hot gas giants.

Gaudi’s team described the planet online June 5 in Nature.

Power Words

(more about Power Words)

atmosphere     The envelope of gases surrounding Earth or another planet.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

comet     A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”

equator     An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.

exoplanet     A planet that orbits a star outside the solar system. Also called an extrasolar planet.

gas giant     A giant planet that is made mostly of the gases helium and hydrogen. Jupiter and Saturn are gas giants.

hybrid     An organism produced by interbreeding of two animals or plants of different species or of genetically distinct populations within a species. Such offspring often possess genes passed on by each parent, yielding a combination of traits not known in previous generations. The term is also used in reference to any object that is a mix of two or more things.

Jupiter     (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium.

light-year     The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

moon     The natural satellite of any planet.

orbit     The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, the object must be big enough to have pulled neighboring objects into the planet itself or to have slung them around the planet and off into outer space.

radiation     (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

science fiction     A field of literary or filmed stories that take place against a backdrop of fantasy, usually based on speculations about how science and engineering will direct developments in the distant future. The plots in many of these stories focus on space travel, exaggerated changes attributed to evolution or life in (or on) alien worlds.

solar wind     A flow of charged particles (including atomic nuclei) that have been ejected from the surface of the star, such as our sun. It can permeate the solar system. This is called a stellar wind, when it radiates out from a star other than our sun.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.


  • MS-ESS1-3
  • HS-ESS1-2
  • HS-ESS1-4


Journal:​ ​ S. Gaudi et al. A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star hostNature. Published online June 5, 2017. doi: 10.1038/nature22392.