PLANET IMAGED AROUND A SUNLIKE STAR?!

PLANET IMAGED AROUND A SUNLIKE STAR?!

PLANET IMAGED AROUND A SUNLIKE STAR?!

Bad Astronomy
The entire universe in blog form
Sept. 15 2008 9:07 AM

PLANET IMAGED AROUND A SUNLIKE STAR?!

Is this it? Is this the very first image of a planet orbiting a star like the Sun?

Planet orbiting a sunlike star?

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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The image come from the monster 8 meter Gemini North telescope in Hawaii. The star is 1RSX J160929.1-210524 (for those taking notes at home) -- it's a K7 dwarf, a bit cooler and smaller than the Sun -- and the planet is the blip circled at the upper left. It has no real name as yet -- it hasn't been confirmed yet; more on that in a sec -- but if it's a planet orbiting the star, it has a mass of about 8 times that of Jupiter.

The image is in near-infrared, just outside the human range of vision. This is a good place to hunt for young planets, because for millions of years after they are formed, planets are hot and glow in the infrared, while stars like the Sun are faint in the IR. Well, relatively faint; they still pour out energy, but it's a lot less than in the visible part of the spectrum. So using IR detectors means you're looking where young planets put out most of their light, and stars put out the least.

The planet seen by Gemini appears to be about 5 million years old-- the parent star is part of a cluster of stars whose age is known. They lie about 500 light years from Earth.

Spetcra of the potential exoplanet. Click to enlarge.
Spectra of the star and planet. Click to embiggen.
The reason astronomers think this is a planet is because they took spectra: they broke the light up into a rainbow, if you will, and when you carefully examine the spectrum you can determine all sorts of things about the object emitting the light: how hot it is, what chemical composition it has, how old it is, even if it's spinning!

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The spectrum of the object matches that of an old, very low mass star. That might make you think it's a star, but wait! The planet is young, and still hot. The light it gives off depends on its temperature, so a young low mass object, like a planet, can look just like a more massive object like a star. Since we know this object is young, we know it has a lower mass than its spectrum naively suggests. When models of how planets cool are used, we get a pretty good match for one with 8 times Jupiter's mass if it's the age of the star cluster, 5 million years.

But this is not confirmed! For example, it could be a low mass star that happens to be near the other star along our line-of-sight -- in other words, it's in the background. The best way to see if that's true is to wait a year or two and take more images. If the object moves against the background stars along with the brighter star, then it must be physically associated with the star, and therefore it's a planet. This is how we confirmed the first image of an exoplanet back in 2005 -- but that was orbiting a brown dwarf, a star very different than the Sun.

If confirmed, this one is pretty important, because the parent star is much like our own Sun. The most interesting thing about this is the distance of the purported planet from the star: 50 billion kilometers! That's 11 times the distance Neptune is from the Sun. And that's a lower limit; it might be farther. That makes me very suspicious: we don't know of any way to form planets that far from their parent stars.

Stars and planets form from rotating disks of gas and dust. The stuff collects in the middle to form the star, and the stuff farther out forms the planets -- we have seen many examples of this in the sky. The disks we see around new stars are big, sure, but by the time you get 50 billion km out they are very thin, and there's just not enough material out there to form a planet, let alone one with 8 times the mass of Jupiter.

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In this case, the most likely explanations for this image are that 1) this isn't a planet, but a background star, or 2) it formed closer in and was ejected by an encounter with another planet, moving it way the heck out from the star. (1) is a bummer, and is unlikely just due to statistics; it's isn't high odds to see an object like this by coincidence so near another star. (2) seems unlikely to me as well; it's hard to toss around a planet that mass unless an even more massive planet was involved. I hope this star is a target for searches for more planets, just to get more information on this scenario.

To their credit, the astronomers involved are also clear about this in their paper announcing the discovery. This is a carefully done observations, and they are appropriately careful in their announcement.

But if it's true... WOW. This would be the second planet ever seen directly in an image, and the first to orbit a star like the Sun. The implications would profound. It would be direct evidence of planets orbiting other stars at great distances. It would mean there could be another planet in our own solar system (unlikely, but I've written about that before). And it would mean that it's possible to use this method of near-infrared mapping to actually get pictures of more planets! Seeing one might be a fluke, seeing two means there are more to find.

The next clear night, do yourself a favor: go outside. Look up. See all those stars? Whether or not this particular planet pans out, we still know that a large fraction of those stars -- 10%? 20? -- may have planets. And some fraction of those may have planets like Earth orbiting them. We really weren't sure about this even 15 years ago, and now we're able to not only start plugging numbers into the equations, but we can actually take pictures of some of these objects.

My oh my. How I do love science.