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What do we know about alien worlds?

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Discover the five strangest exoplanets ever found

To boldly go – in Star Trek the starship Enterprise would visit new planets every week from the Sixties onwards, but until 1995 we didn’t even know whether planets around normal stars existed. Then astronomers found the first hot Jupiter, called 51 Pegasi b, which is a gas giant like our Jupiter, but extremely close to its star. That discovery opened the floodgates and today we know of over 1800 confirmed planets of all types – large and small, hot and cold, gas and ice.

 

To differentiate them from the planets of our familiar Solar System home, astronomers call these alien worlds extra-solar planets, or exoplanets for short. Of all these planets, less than two dozen have actually been photographed (and in the pictures they are just points of light). The others are detected through several methods, the two dominant ones called the radial velocity technique and the transit method. The former makes use of the gravitational interaction between a star and a planet – the star orbits the centre of mass between the star and planet, and so to us it appears to wobble, sometimes by just a few centimetres, but this causes its light to be Doppler shifted. The size of the Doppler shift and the period of the wobble tells us about the mass of the orbiting exoplanet and the size of its orbit.

 

The first planet around a Sun-like star, 51 Pegasi, was discovered in 1995

 

Transits happen when a planet passes in front of its star. Our telescopes are not powerful enough to resolve the silhouette of the planet in front of its star, but we can detect the tiny dip in the star’s light. The size of the dip, and the regularity with which the transits happen tell us the diameter of the planet and how far from its star it is. If astronomers are able to see a transit and measure the radial velocity, they can then measure both the mass and diameter of the planet, and calculate its density and work out whether it is rocky, gaseous or some mixture of the two.

 

Exoplanets are discovered with both ground-based telescopes and space-based telescopes, like the Kepler planet-finding satellite, and with a new wave of planet-finding space missions being built, as well as giant ground-based telescopes, we can expect to discover thousands more planets, and perhaps even the holy grail of a planet just like Earth.

 

The exoplanet most like Earth
One of the great quests is to find a planet that is like Earth and could support life. Astronomers tend to categorise these planets as being found in the habitable zone, where temperatures are just right for liquid water on the surface. The best candidate so far is GJ 667Cc, which orbits a red dwarf in a triple star system. It is a super-earth, nearly four times the mass, and would be slightly hotter than Earth. It is unknown whether there is alien life.

 

This artist’s impression shows a sunset seen from the super-Earth Gliese 667 Cc

 

The planet-sized hurricane
The strongest winds ever measured on Earth was 408km/h, but this was just a breeze compared to the winds measured on HD 80606b, which reach 17380km/h! The reason for these winds is the planet’s egg-shaped orbit, which at times brings it just 4.5 million km from the Sun. This causes the atmosphere to heat up rapidly each time it comes close. This heat drives a superstorm in its atmosphere.

 

Artist’s impression of the extrasolar planet HD 189733b

 

The planet from hell
What happens though when a rocky planet finds itself in a similar situation to a hot Jupiter? CoRoT-7b is a molten world with a temperature between 1800 and 2600 degrees Celsius on its sun-facing side. It is tidally locked, so it always shows the same face to its star like the Moon does to Earth. The dayside’s surface will be an ocean of lava, while the gravity from the nearby star will flex the planet’s interior, causing the farside to be covered in giant volcanoes.

 

The exoplanet Corot-7b is so close to its Sun-like host star that it must experience extreme conditions

 

Big daddy of the planets
Besides the powerful winds, the heat that hot Jupiters receive warms their atmospheres so much that their atmospheres expand, increasing their diameters. When it was discovered, WASP-12b was the hottest planet known, with a temperature of 2250 degrees Celsius. Its expanding atmosphere, which increases the planet’s diameter to 419,000km, is vulnerable to being torn away by the gravity of its sun at a rate of 189 quadrillion tonnes per year, which forms a large tail of gas, a bit like a comet. The gravitational tidal forces also distort the planet into an egg-shape.
 This is one very messed-up planet.

 

Artist’s impression of WASP-12b. Its expanded atmosphere is being torn away into a long tail that forms a disk around its star

 

A diamond in the rough
Astronomers tend to focus on the surfaces, or cloud tops of planets, but sometimes what lies beneath is even more interesting. The planet known as 55 Cancri e is a huge ‘Super-Earth’. It is dry, with no chemical signature of water, and it is rich in carbon, amounting to a third of the planet’s mass. In its core, all this carbon will be compressed under high pressures, to the point that deep within 55 Cancri e there is quite possibly a giant core of diamond.

 

This artist’s concept contrasts our familiar Earth with the exceptionally strange planet known as 55 Cancri e

 

What are giant Super-Earth’s?
Super-Earths are rocky planets like Earth or Mars, but much, much bigger. They can be up to ten times the mass of our planet! These worlds will not have crushing gravity, however – surface gravity depends on the radius of the planet, and the further the surface from the core, where most of the mass is contained, the less the gravity is. Most Super-Earths will have gravity between 1 and 1.5 times Earth’s gravity. Our Solar System does not have a Super-Earth, meaning they are truly alien planets.

 

This article was first published in How It Works issue 69, written by the How It Works team

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