Scientists have discovered the hottest planet in the universe.

Scientists have found one of the craziest exoplanets yet. It's a huge gas giant not unlike our Solar System's own Jupiter, but it travels around its star in just 1.5 days, with a surface hotter than most stars.

The planet orbits a blazing star called KELT-9, which is some 650 light-years away. It's the first time astronomers have detected a planet near a star this hot, and it's different from anything they've seen before.

For one, KELT-9b is crazy hot, as its surface reaches around 4,600 Kelvin on the day side. For comparison, the surface of our Sun is around 5,800 Kelvin, and even its closest planet Mercury only reaches a measly 700 Kelvin on the surface.

The reason for this insane heat is the intimate proximity the gas giant has to its host star, which is itself among some of the hottest stars we know, reaching temperatures of roughly 10,170 Kelvin.

"It would be fair to say that this planet is hotter than at least 80 percent of all known stars, which is just mind-blowing," astronomer Jonti Horner from the University of Southern Queensland, who wasn't involved in the research, told Science Alert.

"It's essentially skimming the surface of its star, and the star itself is much more luminous and much hotter than our Sun. [It makes] this the hottest planet we've found to date by quite a long margin, by more than a thousand degrees."

Why is Venus the hottest planet even though Mercury is closer to the sun?

Mercury is closest to the sun, this is true. However, because of its proximity to the sun it lacks an atmosphere. Mercury's gaseous molecules reached escape velocity long ago and the atmosphere was 'burned off' leaving a vacant wasteland of rock with a similar surface to the earth's moon. There is nothing to hold in solar radiation, thus is flies out into space. Venus is a different story. Its atmosphere is primarily composed of carbon dioxide - the primary green house gas -, which acts like a one-way door. The solar heat enters, but it cannot leave. Thus, an oven is created. This is why Venus has no water. Pictures suggest that Venus once had an aqueous environment, but because of climatic changes and intense heat.... it evaporated. Also, because of an overactive core, Venus seems to recreate its continents every X years. So, we do not expect to find fossils or many bacteria. 

Terrestrial planets

The inner four worlds are called “terrestrial planets,” because, like Earth, their surfaces are all rocky. Pluto, too, has a solid surface (and a very frozen one) but has never been grouped with the four terrestrials.

Jovian planets

The four large outer worlds — Jupiter, Saturn, Uranus, and Neptune — are known as the “Jovian planets” (meaning “Jupiter-like”) because they are all huge compared to the terrestrial planets, and because they are gaseous in nature rather than having rocky surfaces (though some or all of them may have solid cores, astronomers say). According to NASA, "two of the outer planets beyond the orbit of Mars — Jupiter and Saturn — are known as gas giants; the more distant Uranus and Neptune are called ice giants." This is because, while the first two are dominated by gas, while the last two have more ice. All four contain mostly hydrogen and helium.

Dwarf planets

The IAU definition of a full-fledged planet goes like this: A body that circles the sun without being some other object's satellite, is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star) and has "cleared its neighborhood" of most other orbiting bodies. Yeah, that’s a mouthful.

The problem for Pluto, besides its small size and offbeat orbit, is that it shares its space with lots of other objects in the Kuiper Belt, beyond Neptune. Still, the demotion of Pluto remains controversial.

The IAU planet definition puts other small, round worlds in the dwarf planet category, including the Kuiper Belt objects Eris, Haumea, and Makemake.

Also now a dwarf planet is Ceres, a round object in the Asteroid Belt between Mars and Jupiter. Ceres was actually considered a planet when discovered in 1801 and then later deemed to be an asteroid. Some astronomers like to consider Ceres as a 10th planet (not to be confused with Nibiru or Planet X), but that line of thinking opens up the possibility of there being 13 planets, with more bound to be discovered.

Mercury

The closest planet to the sun, Mercury is only a bit larger than Earth's moon. Its day side is scorched by the sun and can reach 840 degrees Fahrenheit (450 Celsius), but on the night side, temperatures drop to hundreds of degrees below freezing. Mercury has virtually no atmosphere to absorb meteor impacts, so its surface is pockmarked with craters, just like the moon. Over its four-year mission, NASA's MESSENGER spacecraft has revealed views of the planet that have challenged astronomers' expectations.

Earth

The third planet from the sun, Earth is a waterworld, with two-thirds of the planet covered by ocean. It’s the only world known to harbor life. Earth’s atmosphere is rich in life-sustaining nitrogen and oxygen. Earth's surface rotates about its axis at 1,532 feet per second (467 meters per second) — slightly more than 1,000 mph (1,600 kph) — at the equator. The planet zips around the sun at more than 18 miles per second (29 km per second).

• Diameter: 7,926 miles (12,760 km)
• Orbit: 365.24 days
• Day: 23 hours, 56 minutes