How did the lava-covered hell planet get so close to its star?

Of the more than 5,000 known planets outside our solar system, one of the most dramatic is 55 Cancri e. Affectionately known as the “planet from hell,” the star orbits so closely that its temperature reaches 3,600 degrees Fahrenheit, and its surface is thought to be covered with an ocean of lava. Located 40 light-years away, the planet has been a source of fascination for its extreme conditions, and recently researchers shared a new theory on how the planet got so hot.

The planet orbits its star 55 Cancri A at a distance of 1.5 million miles, meaning that a year on Earth takes less than a day. “While Earth completes one orbit around our sun in 365 days, the planet studied here orbits its host star 55 Cnc, orbiting it every 17.5 hours,” study author Debra Fischer of Yale University said in a statement.

An artist’s impression of the planet Janssen (orange circle), an ocean of lava that orbits its star so closely that its entire surface reaches nearly 2,000 degrees Celsius. Lucy Reading-Ikkanda/Simons Foundation

The researchers used an instrument called the EXtreme PREcision Spectrometer (EXPRES) on the Lowell Discovery Telescope of the Lowell Observatory in Arizona to look at the light from the host star, focusing on how the light changes as the planet moves between the star and Earth. This told them that the planet, unlike the other planets in the system, revolves around the equator of the star. There are five exoplanets in the system, planets with a double star at the center and rotating at different degrees with respect to their orbital plane.

This is different from our solar system, where all the planets basically sit on the same flat plane. In our case, this is probably because all planets are made up of the same disk of dust and gas. In other words, the different orbits seen in the 55 Cancri system suggest that these planets may have formed in different ways.

Planet 55 Cancri e is thought to form in a cooler orbit before approaching the star. That’s why it was so hot. “Astronomers think this planet formed much further away and then returned to its current orbit,” Fischer said. “This trip could have thrown the planet out of the star’s equatorial plane, but this result shows the planet being held tight.”

The research was published in the journal Nature Astronomy.

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