“Hot Jupiter” on a spiral course


The Exoplanet WASP-12b approaches its star in a spiral orbit. (Image: NASA / JPL-Caltec)

A hot gas giant orbits its star in extreme proximity at a distance of around 600 light years – WASP-12b is a so-called hot Jupiter. Now astronomers have found that this exoplanet is on a deadly spiral course. Because measurements show that the orbital times of this planet have shortened measurably over the past few years. Current observations now confirm that this shortening is caused by shrinking the orbit of WASP-12b. Accordingly, the gravity of the nearby star pulls this planet ever closer to itself and forces it on a spiral course towards the star. In a few million years from now, this will likely end with the destruction of the planet, as the researchers report.

Among the thousands of extrasolar planets discovered to date are many large gas planets orbiting their stars in relatively narrow orbits. These planets, also known as “hot Jupiter” because of their size and high temperature, move in a zone of their systems that is characterized by strong tidal forces, strong radiation and the enormous gravity of their star. “Since the discovery of the first ‘hot Jupiter’ in 1995, we have been wondering how long such planets can last,” said Princeton University co-author Joshua Winn. “But we were pretty sure that the gravity interactions between star and planet would have to spiral it inwards and ultimately destroy it. But nobody was able to estimate how long this process would take. ”So far, astronomers have never been able to clearly prove this“ death spiral ”in a hot Jupiter – until now.

A hot gas giant puzzles

The planet WASP-12b is one of the most promising candidates for a hot Jupiter at the end of his life. This gas giant is about 600 light years away from us in the constellation Auriga and is about twice as large and one and a half times as heavy as Jupiter. Because the planet only needs 26 hours to orbit its star, it has to orbit in a very narrow orbit. According to measurements, the two are only about 3.4 million kilometers apart – that’s about a 44th of the Earth-Sun distance. As early as 2013, astronomers discovered with the help of the Hubble space telescope that this close proximity to the stars is affecting the planet. Because according to spectral analysis, the star had already started to suck gas from its planetary companion. The atmosphere of WASP-12b is also inflated like a balloon on the outside. In 2017, repeated observations of transits of the planet in front of its star also provided the first indications that the orbit of WASP-12b is also changing: since its discovery, the time it takes for the planet to pass in front of its star has increased by 29 Shortened milliseconds.

The problem, however, is that such a reduction in transit times does not have to be due to a spiral narrowing of the planet’s orbit; instead, a slow “egg” of the eccentric orbit can also cause this effect – a so-called precession. A team led by Winn and lead author Samuel Yee from Princeton University has now examined in more detail which scenario is actually behind the changes in WASP-12b. To do this, they observe several other transit events of this system and additionally use NASA’s Spitzer space telescope and a telescope at the Palomar Observatory in California to track five occultations – the phases in which the planet passes us behind its star and therefore from is covered up. “If this is the case of a narrowing orbit, the occultation intervals would have to decrease by the same amount as the transit intervals,” the researchers explain. If, on the other hand, there is a precession of the orbit, the occultation times should be longer.

Clear evidence of a spiral course

As the astronomers found, the transit and occlusion times are reduced with WASP-12b. “These new data confirm the orbital degradation scenario,” says Yee. “Based on this, we can now reliably say that this planet is actually moving in a spiral towards its star. Although this phenomenon has long been predicted for near-star gas giants like WASP-12b, it is the first time that we see this process in action. ”Researchers estimate that the planet could reach the point in about three million years from now tearing him apart from the star’s tidal forces. At first, the gas envelope of the gas giant could be almost completely lost, leaving only the solid core of the planet. After further rapprochement, the planet core would also be destroyed.

As Yee and his team explain, the discovery of this gas giant trapped in a “spiral of death” also provides general insights into the lifetimes of such hot Jupiter and the processes involved in their destruction. “If we can find more planets like WASP-12b, it will tell us more about the development and fate of exoplanet systems,” says Yee.

Source: Samuel Yee (Princeton University) et al., Astrophysical Journal Letters, doi: 10.3847 / 2041-8213 / ab5c16

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