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Hot jupiters bad for
earth-like planets

Posted: 13 April

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A windfall of nine new planets, including some that orbit backwards, may turn theories of how ‘hot jupiters’ come to be on their head. These planets, which were announced today at the Royal Astronomical Society’s National Astronomy Meeting at the University of Glasgow, would seem to rule out the possibility of hot jupiters and Earth-like planets existing together.

The planets have been discovered by the SuperWASP (Wide Angle Search for Planets) project, run by a group of British universities and which uses off the shelf CCD cameras and wide angle lenses at sites in La Palma and South Africa. SuperWASP searches for transiting planets that are moving in front of their star, blocking some of its light. The amount of light blocked indicates the diameter of the planet. The masses of the planets were then confirmed by observations using various spectrographs, including the HARPS spectrograph on the 3.6-metre telescope at the European Southern Observatory (ESO) in Chile, which measures the amount of wobble in the a star caused by the mass of an orbiting planet. All nine new planets are gas giants close to their star, but in one way or another all nine are a little weird.

An artist’s impression of WASP-8 transiting in front of its star. Image: ESO/L. Calçada.

Take planets WASP-21 and WASP-29 for example. “The interesting thing about them is that WASP-21 is extremely bloated, and WASP-29 is extremely compact, which suggests that WASP-29 has a massive core and a relatively low mass gaseous envelope, whilst WASP-21 is probably almost entirely gas, which is consistent with the fact that it is orbiting a very old star,” says Professor Andrew Collier-Cameron of the University of St Andrews, one of the leading researchers on SuperWASP. This difference is strange given that both have masses similar to Saturn, 0.3 and 0.25 times the mass of Jupiter respectively, so clearly the diameter of a planet is not dependent on its mass. The SuperWASP results, when combined with previous exoplanet surveys, suggest that the more heavy elements there are in a star (and hence in the protoplanetary disc that formed the planets), the smaller the planet is at a given mass. This is exemplified by the case of WASP-21, which is orbiting one of the older and more heavy-element poor stars in the Milky Way that formed over eight billion years ago. “At last,” says Cameron, “we have a predictor for the diameter of a planet if we know what proportion of heavy elements a star has.”

Another strange case from this batch of nine is WASP-33. The star around which this hot jupiter orbits (a hot jupiter is a gas giant like Jupiter, but one that is very close to its star, closer than Mercury is to the Sun) is the hottest star yet found to have a planet orbiting so close, but this planet is also orbiting its star in completely the opposite direction to which the star is rotating.

The protoplanetary disc of gas and dust from which planets form spins in the same direction as the star at its centre, so logic would dictate that all planets would also orbit in this direction. This conventional view of planet-formation makes it difficult to explain planets with orbits that are tilted, eccentric like comets, or simply backwards. However, given the sheer numbers of backwards or eccentric planets that astronomers are now coming across, perhaps it is time to rethink what we consider to be conventional, say Cameron and Didier Queloz of Geneva Observatory, who participated in the radial velocity measurements of the SuperWASP planets at ESO.

One planet in particular – WASP-8 – may hold the key to explaining why some planets have bizarre orbits. Wasp-8 not only has a backwards orbit, it also has an eccentric orbit that is tilted by 114 degrees to the plane of its planetary system. Crucially, observations point to a brown dwarf or maybe even a red dwarf companion lurking nearby. According to a phenomenon known as the Kozai effect, the primary star and its low mass companion embark in a tug of war over the unfortunate planet stuck in the middle.

“The planet’s orbit swiftly goes off centre, flailing around wildly, and every time it goes close to the [primary] star it raises a tidal bulge on the star, losing a little bit of angular momentum in the process,” says Cameron. “This causes its orbit to shrink, and over billions of years in a game of ‘cosmic pinball’ the end product is a hot jupiter.”

This is in stark contrast to the conventional model of gas giants migrating inwards towards their star during their first few million years. Whilst Cameron and Queloz don’t rule out migration entirely – there is still some evidence that the gas giants in our own Solar System migrated to a lesser degree – they argue that the Kozai effect is the missing piece of the puzzle, because migration cannot explain eccentric or backwards orbiting planets. And this is bad news for the search for Earth-like planets.

Of the 400-plus exoplanets discovered so far, the vast majority are hot jupiters, and they appear to be extremely common in the Galaxy. In the migrationary scenario raw material for building terrestrial planets could remain in the protoplanetary disc further out than the hot jupiter. In the Kozai effect, the game of cosmic pinball would blow apart the protoplanetary disc, and any Earth-sized worlds that had formed would be flung into the depths of deep space. “Hot jupiters and Earth-like planets don’t mix,” says Cameron.

There are still some puzzles. To our knowledge not all stars with hot jupiters have low mass brown or red dwarf companions. Cameron admits that their origin is still a little sketchy, but if their orbits were disturbed by a passing star in a hit and run, the evidence may have long since departed the scene. Overall, however, the Kozai effect has the potential to revolutionise our understanding of exoplanets, and really drum home how out of the ordinary our own Solar System may turn out to be. According to Queloz, who was part of the team that discovered the first hot jupiter exoplanet around 51 Pegasi in 1995, the new parameter of looking at the angles of exoplanet orbits has opened up a new dimension. “Fifteen years ago we discovered the Rosetta stone, but only now am I starting to read it and understand it,” he says. “This is a really big day!”

For Andrew Cameron, it is justification for the SuperWASP project, which has uncovered a total of 27 new exoplanets since 2006, with more to come. “SuperWASP was designed to find planets in sufficiently large numbers and really put the theories to the test,” he says, and with these backward planets SuperWASP is really helping to move exoplanet research forward.