Science Interviews

Interview

Fri, 20th Nov 2015

Birth of planet observed for first time

Professor David Rothery, Open University

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So far, scientists have logged more than 1,900 planets orbiting other stars planetoutside our solar system. But no one has yet spotted any that are still in the process of formation. Until now that is, because researchers in the US have finally spotted just such a planet. It's in orbit around a young 2-million-year-old star, which is 450 light years away from Earth and catchily called LkCa 15. This star is actually quite similar to our own Sun. Planetary scientist David Rothery, who wasnít involved in the study, explained the significance of the discovery to Connie Orbach...

David - Whatís exciting is weíre seeing a planetary system in the act of a formation.  There are three planets orbiting this star; the nearest oneís about ten times Earth's distance away from the star, so thatís twice Jupiterís distance away from our sun, and the furthest one is about 20-25 astronomical units, so thatís a little bit less than Neptuneís distance from our Sun.  So three giant planets, and the innermost one has still got gas falling into it, which has been measured at this very high temperature of 10,000 degrees; its losing potential energy as it falls into the planetís gravity whirl and it gets really hot.  So weíre seeing giant planets forming.

Connie - How does a planet form in the first place?

David - What happens is, as a star forms, it forms within a cloud of gas and dust which contracts gravitationally, and it contracts. To conserve angular momentum it must start spinning faster, so the gas and dust gets shaped into a disc rather than a three dimensional cloud, and within this disc you get concentrations of material which collapse onto each other to form planets.  Now the temperature is changing all the time this is going on, so you get the materials that can condense at high temperatures forming first; thatís the metals and the rock, and later on you get ices forming, and then with a body thatís iron and rock surrounded by ice, thatís got enough gravity to start sucking the gas out of the cloud as well and probably weíre seeing that stage occurring round LkCa 15.  Youíve got the kernel of a giant planet which is now scavenging all the gas that it can, and itís the in-fall of the gas which is shining so brightly and enabling the process to be imaged.

Connie - Why hasnĎt this been seen before?

David - Well, you need very sophisticated instrumentation to see this. The disc of gas and dust was seen round this star fifteen years ago. The innermost planet, LkCa 15b was imaged five years ago. Now they are using adaptive optics on the the large binocular telescope, and infrared filters to isolate different wavelengths of light that are being emitted as the gas falls inward and acretes onto the planet. So, itís a combination of big telescopes; very high resolution because youíve got big lenses, and adaptive optics to sharpen the focus, and the right kind of spectral data to show the glow of the gas.

Connie - Oh wow!  So this is a really good example then of why weíre wanting to get bigger and bigger telescopes, more and more technology. You know, they may cost a lot but theyíre showing us so much more.

David - Yes.  Iíve seen quotes suggesting that what weíre seeing here is better than Hubble space telescope could ever achieve for example. Itís not a mountain in Arizona, itís above a lot of the Earthís atmosphere which helps, itís got adaptive optics to increase the sharpness because the atmosphere is shimmering all the time, even from a cold mountain top in Arizona you need to correct for the atmospheric effects, and they are doing that very successfully now.

Connie - What can we learn from this sort of planetary formation?

David -   I think weíre seeing an evolution in knowledge here.  This isnít going to revolutionise anybodyís ideas, but weíre able to demonstrate how early in the life of a star planets form, and maybe we can get some handle on the rate at which planets grow, because actually we donít really know that.  Some people suggest you can go from gas and dust to full size planets in a million years, some people say tens or hundreds of million years, and it would be good to gate a rate of processes.  Thatís what Iím looking forward to seeing from this but, of course, itís only one example, and it may not be typical of all planetary systems, but at least itís a start.

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