Enormous planets orbit infant star

18 October 2018

Four enormous gas planets have been found orbiting a very young star known as CI Tau, a record for a star so cosmically immature. Not only that, but this star has the most extreme range of orbits ever seen, with its outer planet orbiting 1000 times farther out than the innermost...

CI Tau is only 2 million years old and is still surrounded by its protoplanetary disk, a ring of dust and gas from which planets and asteroids form. And by using a network of large radio telescopes called ALMA [Atacama Large Millimeter/submillimeter Array], in the Chilean mountains, researchers at the University of Cambridge took very high resolution images of this disk and the central star to reveal 4 distinct, dark rings carved into the brighter dust. These show the paths of the four large planets orbiting CI Tau.

The team selected CI Tau for further study owing to a record it held already as the youngest star known to have a close-in orbiting planet, known as a “hot Jupiter”.

“Hot Jupiters were among the first exoplanets to be discovered, they are remarkable objects,” explains the study's lead author, Cathie Clarke. “You’ve got a massive planet, which is orbiting very close to its parent star, actually inward of the orbit of Mercury, if it had been in our own system.”

The planets were spotted using a phenomenon known as the Doppler shift. The close proximity of the planet means that its gravity periodically warps the light from the host star, producing characterstic changes in the wavelength of the starlight as it orbits.

According to Clarke, it is unlikely though that this and similar planets start life this close to the star.

“There’s a lot of simulation now to be done in figuring out what histories can give rise to what we see. We’re wondering, when we see a system like this, what role have these siblings played in promoting or demoting this hot jupiter so it ends up almost being swallowed by its star?”

The hot Jupiter and its planetary siblings are all gaseous planets, ranging from objects the size of Saturn to ten times the size of Jupiter. Rocky planets, on the other hand, are too small to carve visible tracks through the dust disk, so they cannot be spotted this way. But this doesn’t mean that they’re not there, and there is more than enough material in the protoplanetary disk to form further planets around CI Tau.

Perhaps, when the more powerful Extremely Large Telescope (ELT), which is under construction as part of the European Southern Observatory in Chile, goes live in 2024 scientists will be able to see these planets directly. Until then, watch this space...

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