Birth of a planet
Ken Tapping, October 30, 2018
Although we have a pretty good idea how stars and planets form, we have only now got our first image of a newborn planet. This image was obtained by astronomers using the Very Large Telescope, in Chile. It shows a star surrounded by a disc of gas and dust, with a gap in the disc, and sitting in that gap is a planet. It is a big one, 5 to 14 times the mass of Jupiter, the largest planet in our Solar System. The large size of this planet is probably why it could be detected. Getting this image was a tremendous challenge.
When we point a telescope or binoculars at a star in the sky, we see something twinkling, flashing different colours and dancing around. What we are seeing is beautiful but has little to do with the star. The huge distances stars are from us mean that even through our biggest telescopes they would appear as mere points of light; at least they would if there were no atmosphere. Putting telescopes in space bypasses the atmosphere problem, but at the moment the only telescopes we can deploy in space are in the small to moderate size range, with mirrors maybe up to three or four metres. The Very Large Telescope consists of four telescopes, each with a mirror 8.2 metres in diameter. In order to minimize the atmosphere problem, it is located on a high plateau. This helps, but is not a total cure.
To image a star and its planets requires an ability to resolve extremely fine detail, and to see something extremely dim – a planet – very close in the sky to something extremely bright – a star. Even on that high plateau, the shimmering in the image due to the atmosphere is still enough to wipe out fine details in what we try to observe. If this were the end of the story, then there would have been no point in making this telescope, because the atmosphere would prevent the instrument ever reaching its true imaging potential. What made the project worthwhile is a technique known as adaptive optics. This is easy to visualize but technically extremely challenging to actually make happen.
If we are looking at a star in the sky, we can predict what its image should look like through our telescope. However, the turbulence in the atmosphere makes it look like something else altogether. So we add a very flexible mirror to the telescope, which has a lot of computer‑controlled actuators on the back of it. The computer then rapidly adjusts the actuators to correct that star image to make it look the way it should, and in the process, the rest of the image is corrected too. If there is no suitable reference star, we shine a laser into the Earth's upper atmosphere to emulate one. In order to make the planet visible despite being close to its "sun", a blocking disc was used to block out the starlight, rather like using our hands to block out the Sun's glare on a sunny day. The image shows a planet that has swept out a clearing in the disc of material around the path of its orbit. From how far it has got in doing this suggests the planet is no more than about 5 million years old. Our Earth is about 4.5 billion years old.
This very young system supports an idea that astronomers have been discussing for a while, namely that when giant planets form, they slow down and ultimately limit the growth of their star. They do this by taking up material that would have become part of the star, and then gravitationally interfering with the spiralling in of material the star would otherwise have captured.
That young planet is already much bigger than Jupiter and is still growing. An interesting question is whether it will grab enough more material for it to get promoted from giant planet to red dwarf star. However, we won't find out for at least a million years or so.
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