Look through the list of PEST discoveries and ground-based planet searches generally, and you will find plenty of so-called hot Jupiters. Strangely we now know that hot Jupiters are rare. Yes, the universe seems pretty keen to produce planets, with some recent research estimating that on average every star hosts at least one planet. But only 0.5% to 1% of stars have hot Jupiters.
So why have we found so many more of them than earth sized planets? After all the earths are much more interesting if we are looking for life. To understand why let’s first take a look at our Jupiter.
Jupiter is the largest planet in our solar system. It sits in an outer suburb, far from the warmth of our sun – 778 mln km, or 5.2 times the mean distance of Earth from the Sun (Astronomical Unit, AU). As a result its mean temperature is a chilly -145 °C. Its year is 12 Earth years long.
In contrast hot Jupiters are gas planets that are seriously close to their stars and consequently zip around with a period (‘year’) that can be as short as an Earth day.
Despite their relative scarcity, we tend to find more of them because;
- Transiting planets are found by observing many stars for a some time (say 2-3 yrs) and looking for minute dips in the light curve that repeat at exactly the same intervals when the planet passes in front of the host star (as seen from Earth). Short period planets may show hundreds of transits, building up a convincing signal. Our Jupiter goes around the sun once in 12 years, so is not likely to transit at all during the observation period, even if the orientation of the orbit with respect to an observer is favourable – see point 2 below;
- A planet will only be seen to be transiting if its orbit is pretty much edge-on to us. The closer-in the planet the more the orbit can be tilted away from perfectly edge-on and still show a transit. Think of a transparent dinner plate with a cherry at its centre and an ant somewhere on the plate. If the ant is close to the circumference of the plate, you cannot tilt the plate very much away from edge-on and still be able to see the ant against the cherry.
- A larger planet blocks more of the host star’s light, so a transit is easier to detect. For example Jupiter’s radius is 11 times that of Earth. If we could view it transiting our sun, it would block 112 = 121 times the light that Earth would.
- The other major planet hunting technique is to measure the wobble (radial velocity) of a host star as its planet(s) orbits. The closer-in the star, and the more massive, the larger the gravitational pull between planet and star, and the larger the wobble.
Given that hot Jupiters are not where you would look for life, and that we have found so many of them already, do we need any more? That’s a good question – and a good topic for a future post.