Randy McDonald (rfmcdpei) wrote,
Randy McDonald

[BRIEF NOTE] On Kappa Andromedae b

News that an exoplanet has been discovered orbiting the star Kappa Andromedae, a relatively massive and bright star some 170 light years away, stands out because of the method of Kappa Andromedae b's discovery: it was directly imaged.

Kappa Andromeda b discovery image

The basic elements of this discovery were described by Universe Today's Nancy Atkinson.

Kappa Andromedae (k And) is a very young star, with an estimated age of 30 million years (in comparison our Sun is around 5 billion years old). The planet, called k And b (“Kappa Andromedae b), is about 10% larger than Jupiter, but it is a heavy world — it has a mass of about 13 times that of Jupiter.

This means that it could very well be either a planet or a very lightweight brown dwarf, an object that is intermediate between planets and stars. However, the astronomers are leaning towards the circumstantial evidence which indicates that it is likely to be a planet.

Since stars are much brighter than their planets –typically by a factor of a billion or more – exoplanets are usually lost in the star’s glare when using traditional observational techniques. The Subaru team used a different technique called angular differential imaging, which combines a time-series of individual images in a manner that allows for the otherwise overwhelming glare of the host star to be removed.

In the infrared image, above, the tiny point of light that is the planet Kappa And b. Since the planet orbits the star at some distance, the SEEDS observing team was able to distinguish the object’s faint light by effectively covering up the light of the star.

The large mass of both the host star and gas giant provide a sharp contrast with our own solar system. Observers and theorists have argued recently that large stars like Kappa Andromedae are likely to have large planets, perhaps following a simple scaled-up model of our own solar system. But experts predict that there is a limit to such extrapolations; if a star is too massive, its powerful radiation may disrupt the normal planet formation process that would otherwise occur. The discovery of the super-Jupiter around Kappa Andromedae demonstrates that stars as large as 2.5 solar masses are still fully capable of producing planets within their primordial circumstellar disks. This is key information for researchers working on models of planet formation.

Bad Astronomy's Phil Plait reacted to the interesting question of whether Kappa Andromedae b is actually a planet or a brown dwarf.

The planet κ And b pops right out in the Subaru telescope picture, which is pretty amazing; typically a star is a billion times brighter than any planets orbiting it, so being able to see the planet so clearly is very nice. It helps that the planet orbits the star over 8 billion kilometers (5 billion miles) out, reducing the glare significantly. It also promises that more observations in the future will reveal a lot more information about it.

The current analysis indicates the planet is about 13 times the mass of Jupiter, and that made me raise my eyebrows. We don’t really have a good definition of what a “planet” is, but a star is something so massive it can fuse hydrogen into helium in its core—the pressure and temperature are so high in the star’s heart that it acts something like a controlled thermonuclear bomb.

But there exists a type of object called a brown dwarf that is midway between a planet and a star. It’s massive enough that for a time in its core it can fuse a particular flavor of hydrogen called deuterium, but after a while that turns off. Anything below that mass doesn’t have the oomph to get fusion started, and we call those objects planets. The upper mass limit before deuterium fusion starts? Thirteen times that of Jupiter, right where κ And b is.

So it’s possible that, by strict definition, κ And b isn’t a planet. However, the mass estimate for it depends on a lot of things: the age of the system, how bright it is, its temperature, what colors it has (literally, comparing how much light it gives off in the blue end of the spectrum versus red, and so on; that can tell an astronomer a lot about the object). These numbers are fed into computer models that then calculate the mass.

The age is most important, because while an old planet (like Earth) shines by reflecting light from its star, a young planet is still hot from its formation, and glows on its own, fading with time. It’s like a hot coal that shines white, then fades and turns redder with time. The more massive a young planet is, the brighter (and bluer) it is. For κ And b, one model gives a mass of 13 Jupiters, another gives it a lower mass of closer to 12. If the latter is correct, we have a true planet on our hands. Even if it’s more massive than that, other factors can come into play keeping it a planet by definition.

But either way this is pretty cool. For one thing, it tests our models to their limits, which is always good. We want better models! Finding test cases near the limits is a good way to figure out how to refine our understanding of the physics. And honestly, our definitions are a little arbitrary anyway. Don’t let some hard-and-fast interpretation keep you away from the sheer wonder of this discovery!

The discovery paper is online at arXiv, "Direct Imaging Discovery of a `Super-Jupiter' Around the late B-Type Star Kappa And".
Tags: space science
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