I was asked recently by a reader to explain the hot Jupiter problem in astronomy, and while it is not my specialist subject I do believe that it is an interesting topic and as such I will explain it as best as I can.

For most of human history, philosophers and scientists have wondered whether there are other planets in the galaxy aside from our own solar system. Even the ancient cultures throughout the world knew about the inner planets like Venus and Mars, and the gas giants like Saturn and Jupiter, and then modern astronomers added more distant planets such as Neptune and Uranus. But they never found a planet that didn't orbit our own sun, leaving many to wonder whether such exoplanets exist.

Then in the 1990s then answer came in with a very definite affirmative. By measuring the recoil of distant stars caused by the orbit of massive planets, astronomers proved that indeed exoplanets do exist. And once the methods were proven to work, ground based and space based telescopes rapidly discovered hundreds and then thousands of such planets. Not only do exoplanets exist, but they are plentiful.

No one really doubted that other stars have planets. However the surprise was the nature of the first exoplanets to be discovered. 

Based on the amount of movement of stars similar to our own, astronomers knew immediately that these exoplanets had to be very massive. These planets are as heavy or heavier than Jupiter, the largest in our own solar system. However the period of the orbit, which can also be measured by studying the parent star, showed that these objects completed one complete year in just a few days. If you lived on the surface of on of these planets, seasons would pass as quickly as days do on Earth. The only way this could work is that the exoplanets are very close to the surface of the star.

That means that these exoplanets are like Jupiter, but have surface temperatures of thousands of degrees. Thus the name 'Hot Jupiters'. 

But there is a problem here. At those temperatures, a gas giant like Jupiter would either never form or would quickly be burned away. If they are rocky planets like Earth or Mars, then they need to have more heavy elements in them than are seen in the entire solar system and probably more than any ordinary star could produce.  They are simply too heavy to be rock, and too to be made of gas. So what are they?

The simple answer is that we are not really certain. The best theories right now are that they form outside of the 'frost line' of their solar systems at which temperatures are sufficiently cool to form gas giants. Some unknown mechanism then causes them to migrate into the inner solar system, where they live a violent life for some period of time before ultimately being burned up.

However as yet we can only speculate on this migration mechanism. And this leads to more serious questions, such as why or when will our own gas giants migrate in? Why do we observe far more hot Jupiters than cold Jupiters? Is it just that they are easier to observe or are they truly more prevalent? What will happen to the Earth if Jupiter takes a trip? 

These are all interesting questions which could have an impact on our own survival as a planet, and need to be studied in more detail. It is just one more example of Nature proving to be far more complex than the imagination of mankind.