The search for the hottest planet in the Milky Way galaxy leads far beyond the familiar bounds of our own solar system. While planets in our neighborhood reach significant temperatures, they pale in comparison to the extremes found orbiting other stars. The conditions that create these record-holding worlds involve astronomical forces far more intense than any we experience near the Sun. These distant exoplanets exist in environments where temperatures rival those of small stars, creating bizarre atmospheric phenomena.
Setting the Baseline: The Hottest Planet in Our Solar System
The hottest planet in our local system is not the one closest to the Sun, but Venus, with a scorching average surface temperature of approximately 464 degrees Celsius (867 degrees Fahrenheit). This heat is not simply a result of its proximity to the Sun, but rather a consequence of an extreme atmospheric condition. Venus’s atmosphere is composed of over 96 percent carbon dioxide, which is about 92 times denser than Earth’s atmosphere. This composition creates a dense, inescapable greenhouse effect that traps heat efficiently, maintaining temperatures hot enough to melt lead on the surface.
The Search for Extremes: Factors Driving Exoplanet Temperature
The primary factor determining a planet’s temperature is its proximity to its host star, but the star’s characteristics are also important. Stars much hotter and larger than our Sun, such as those classified as O, B, or A-type, emit far more ultraviolet and high-energy radiation. A planet orbiting such a massive star will absorb significantly more energy than an equivalent planet orbiting a cooler, smaller star. The tight, short orbits of a class of worlds known as “Hot Jupiters” are a common feature among the hottest exoplanet candidates. These gas giants are comparable in size to Jupiter but orbit their stars in mere days, sometimes even hours.
Astronomers measure the extreme heat of these distant worlds using a method called thermal emission spectroscopy. This technique involves observing the planet during a secondary eclipse, when it passes behind its star. By comparing the total light from the star-plus-planet system to the light from just the star, scientists isolate the planet’s heat signature. This allows them to analyze the planet’s emitted radiation, which peaks in the infrared spectrum due to the high temperature. The resulting thermal spectrum reveals the planet’s dayside temperature and the thermal structure of its atmosphere.
Identifying the Record Holder: The Hottest Planet in the Milky Way
The current confirmed record holder for the hottest exoplanet is KELT-9b, an ultra-hot Jupiter located about 670 light-years from Earth. Its dayside temperature approaches a searing 4,600 Kelvin (approximately 4,327 degrees Celsius or 7,820 degrees Fahrenheit). This is hotter than the surface of many stars, including our own Sun’s much cooler orange dwarf neighbors. The planet orbits an exceptionally hot star named KELT-9, a B9.5-A0 type star with a surface temperature over 10,000 Kelvin.
The planet’s orbit is incredibly tight and fast, completing a single revolution in less than 1.5 Earth days. KELT-9b is tidally locked, meaning one side permanently faces its star, receiving the full, intense blast of radiation. This extreme irradiation has caused the planet’s atmosphere to puff up significantly, giving it a density less than half that of Jupiter. The intense stellar wind and radiation are causing the planet to lose its atmosphere rapidly, shedding a comet-like tail of evaporated material into space.
The Physics of Ultra-Hot Worlds: Planetary Dissociation
The intense thermal environment of ultra-hot worlds like KELT-9b results in unique atmospheric physics. Temperatures above 2,500 Kelvin cause molecular dissociation, where the bonds holding molecules together are ripped apart by heat. Common molecules like water vapor, carbon dioxide, and methane cannot exist in their molecular form on the dayside of these planets. Even molecular hydrogen, the most abundant component of gas giants, is broken down into its constituent atoms.
This process transforms the atmosphere into a sea of individual atoms and ions. Observers have detected the spectral signatures of exotic, gaseous heavy metals, including iron and titanium, in the atmosphere of KELT-9b. These metals exist as gas because the temperature is far above their condensation points. On the planet’s much cooler nightside, some atoms, such as hydrogen, may recombine briefly, only to be swept back to the dayside and dissociated again by fierce winds. The planet’s atmosphere is constantly in a state of high-energy destruction and reformation.