Giant baby planet found orbiting an infant sun

The youngest ever exoplanet has been detected, orbiting a star a mere two million years old – the stellar equivalent of a week-old human.

An international team of astronomers peered at slight wobble of a nearby star and calculated it to have a “hot Jupiter” companion orbiting only a 20th the distance between the Earth and the sun.

The discovery, published in Nature, shows that such planets can spring up and move around very early in a solar system’s life, says lead author Jean-François Donati, an astronomer at the Institute for Research in Astrophysics and Planetology in Toulouse, France, and “likely play a central role in shaping the overall architecture of planetary systems”.

In our solar system, rocky planets such as Earth and Mars orbit the sun closer than the gas giants. Jupiter generally stays around 780 million kilometres from the sun. But in the 1990s, so-called hot Jupiters were discovered – gas giants that had crept closer to their star.

Why some gas giants orbit their star closely – far closer than our innermost planet, Mercury, which circles the sun from around 57 million kilometres – has had astronomers stumped. How did they get there? Are we the exception to the rule?

So seeing how other solar systems are constructed provides astronomers with clues. The problem is it’s difficult to directly “see” exoplanets – planets outside our solar system. Most are inferred from a star’s wobble, because orbiting planets exert a little gravitational pull on their star.

Using instruments mounted on three telescopes – the Canada-France-Hawaii and Gemini North telescopes in Hawaii and the Bernard Lyot telescope in France – Donati and his colleagues looked at the motion of V830 Tau, a two-million-year-old star in the Taurus stellar nursery 430 light-years away.

They saw the star oscillated every 4.9 days. The size and distance of an orbiting planet to generate such a wobble was, they calculated, a gas giant almost as big as Jupiter whizzing around the star at only 8.5 million kilometres away.

In cosmological terms, this is very close. Earth’s distance to the sun, for instance, is around 150 million kilometres. 

The astronomers suggest the hot Jupiter formed further out and migrated inwards. This could have happened in a multitude of ways – some may be knocked in by other planets or gravitationally dragged by neighbouring stars. Others probably gently drifted inwards. 

So why hasn’t the hot Jupiter been completely dragged into V830 Tau? The exoplanet has its star’s magnetic field to thank, the researchers write, which formed a boundary halting its progress.

Next, they intend to look at the V830 Tau system and other young stars with beefed-up telescope instruments scheduled for “first light” in 2017 and 2019, says study co-author Louise Yu. This will let them “explore the formation of new worlds with unprecedented sensitivity”.