Hot Jupiters are a mould-breaking type of planet; huge Jupiter-sized gas giants that orbit shockingly close to their star. The recent detection of HIP 67522 b could provide answers to the most pressing question about these goliaths. How did they form? We have no examples in our own solar system, so exoplanets like this one are helping us further understand how planet formation works.
What are Hot Jupiters?
Hot Jupiters are so named to exemplify their size. They are the biggest planets we know of, likened to our own solar system’s largest planet, Jupiter. But they are also incredibly hot: a by-product of their proximity to their stars. Some of these planets orbit close enough to orbit their star in 18 hours.
The very first exoplanet to be discovered in 1995 was a hot Jupiter. Scientists have found hundreds of them since, as they are the easiest kind of exoplanet to detect. This is because of how the most used method, the transit method, works. It measures periodic dips in the brightness of stars, indicating that a planet has moved in front of it. Since hot Jupiters are very large and very close to their stars, they block a lot of the light during a transit, making the dip easier to detect.
What are the current theories?
There are 3 main theories about the formation of hot Jupiters. They could simply form where they are, but this seems unlikely. In the region that close to a star, the intense heat and stellar explosions would make it very difficult for material to stay bound together. The other 2 theories address this, by assuming that the planets’ form further out from the star, moving inwards over time.
The next question is: what causes them to move to closer orbits? One theory speculates that hot Jupiters are formed in the system’s early days, when the disk of gas and dust that formed it is still surrounding the star. This disk contains the material that first forms the star, then any planets and other bodies which form around it. If hot Jupiters form early on, the disk’s gravity could act on the planet, pulling it in towards the star.
Similarly, the hot Jupiters could move inwards later on, amongst other, older planets whose gravity acts on them. This theory, however, is not supported by the recent discovery of HIP 67522 b. Read on to find out why!
A New Hot Jupiter
So what has scientists so excited about HIP 67522 b? The reason is that this planet is one of the youngest hot Jupiters ever discovered, and so could provide valuable clues as to how they form. The star it orbits is very well studied, and at about 17 million years old, it means that this hot Jupiter is likely a few million years younger. This might sound like a lot to us, but most hot Jupiters are found to be well over a billion years old.
HIP 67522 b was first brought to scientists’ attention by NASA’s Transiting Exoplanet Survey Satellite (TESS), which uses the transit method to search for exoplanets. Stars have one feature that can produce false readings though: starspots. Also known as sunspots, these dark patches can dip light in a similar way to a transit. To combat this, scientists used data from the recently retired Spitzer Space Telescope to confirm the measurements. This new planet orbits its star in just 7 days, and is located only 490 light years away from Earth.
We can already dismiss one of the formation theories for this new hot Jupiter, as its age means that if it did migrate towards its star, it had to have done this fairly early in its life. But one example is not enough to accept one theory for them all. It’s quite possible that all the theories are true, with different planets forming in different ways. For now, we just don’t know, but astronomers are determined to find out!