Exoplanets survive their star's fiery death. Or were they born from it...?

Bad Astronomy
The entire universe in blog form
Oct. 22 2010 10:42 AM

Exoplanets survive their star's fiery death. Or were they born from it...?

nnserpentisIt's kind of amazing that with nearly 500 planets discovered orbiting other stars, we're still finding ones that are really weird. Massive planets orbiting so close to their stars they are practically plowing through the stellar atmosphere; hot spots on the planet not aligned with their stars; planets orbiting so far out it's a struggle to understand how they got there.

And now we can add the planets NN Serpentis c and d to that list.

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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Lying about 1500 light years from Earth, NN Ser is a binary star -- most stars in the sky are part of multiple systems, so that in itself isn't all that odd. But NN Ser is weird: it's a very dinky red dwarf orbiting very close to a white dwarf. And by very close, I mean really close: they're separated by only 600,000 km (360,000 miles), which isn't much farther apart than the Earth and the Moon!

The planets

I'll get back to the stars in a sec. The planets found (named c and d because the two stars are a and b, according to the naming conventions) are Jupiter-scale beasts, with masses of about 6 and 2 times Jupiter's, orbiting the binary stars at a distance of roughly 825 and 450 million km (500 million and 270 million miles).

Those numbers don't seem too odd; lots of planets have been found with similar characteristics. But when you take a closer look at the system...

The stars

Let's go back to the stars. They orbit each other very rapidly: a complete orbit only takes about 3 hours! So those stars are really hauling. As it happens, the orbit is seen almost edge-on from here on Earth, so we literally see the stars pass in front of each other twice per orbit. That's pretty cool, because it means that over a short time we can watch lots of eclipses and get really good statistics on how long the eclipses last, what the time period is between them, and so on. What the astronomers found is that the period of the eclipses is very slowly changing, and the best explanation is that of the two planets tugging on the stars as they orbit.

This turns out to be an incredibly difficult thing to measure; in fact a possible planet was reported just last year for this system, but the parameters for the planet didn't fit more recent data. These new results for two planets are based on more data and analysis, and look pretty solid to me.

The bizarre history of NN Ser

Where things get really weird is when you look at the stars, or more specifically, the white dwarf. These are dense balls of compressed material left over when a star like the Sun dies. As it ages, a star like this will turn into a red giant, swelling up to hundreds of times its current size, and blow off vast amounts of material. Over tens of thousands of years or more, it loses mass, shedding its outer layers. Eventually, all that's left is the core of the star, a hot compact object we call a white dwarf.

So that one star in NN Ser must have once been a star like the Sun which swelled up, blew off its material, then became a white dwarf. But wait a second... when it was a red giant, it was probably a hundred million kilometers across! But the other star in the system, the red dwarf, is only a few hundred thousand kilometers away. How does that work?

Almost certainly, the red dwarf used to be millions of kilometers or more away from the primary star. When the more massive star turned into a red giant, it would have literally engulfed the dwarf. Through friction, the red dwarf would've spiraled in, getting closer and closer to the core of the star. Eventually, when the bigger star blew off its outer bits, what was left was that white dwarf, and the red dwarf in its current, extremely tight orbit. It sounds incredible, but we've seen this happen before, and may be a relatively common occurrence in the Universe.

But hang on again! What about those two planets? How did this affect them?

Well, that's a bit of a poser. There are two scenarios. One is that they formed along with the stars long ago, and somehow survived this cataclysm. However, this strikes me as pretty unlikely. When the primary star went red giant and started blowing off matter, it was losing mass, and therefore its gravity got weaker. That means its hold on those planets would've gotten more tenuous, and they would have migrated outwards. This in turn means they would've been much closer to the star in the past. But we know the red dwarf is there, and while it's a dinky star, it's far more massive than a planet. It's very hard to see how planets could be in stable orbits so close to such a massive object. Models show they'd be ejected from the system relatively quickly.

That makes it unlikely they formed with the two stars. That leaves the second scenario: they formed after the primary star turned into a red giant!

That's pretty weird, too. But some models suggest that as the red dwarf spiraled into the core of the red giant, a massive disk of material would form around it. This disk could then be the raw material from which the two planets were formed. That seems fantastic to me as well, but look: we have two planets orbiting a very tight binary star where one is a white dwarf and the other is red dwarf. Something weird must've happened here! The real choice is to pick which is the least bizarre.

The view

Whatever happened, we've got what we've got: two planets orbiting this weird binary. Now imagine you're standing on one of those planets (well, since they're almost certainly gas giants, imagine you're standing on the surface of one of their moons). Look up. What would you see?

From the inner of the two planets, the two stars would be a bit less than a tenth of a degree apart; about 1/5th the width of the full Moon. You'd be able to see them as separate stars. The red dwarf would barely resolve itself as a disk; it wouldn't look like just a dot in the sky. The star is far less luminous than the Sun, but would still shine about 20 times brighter than the full Moon on Earth. In other words, if it were the only object in the sky you could read by it, and looking at it would make you squint a bit.

The white dwarf, on the other hand, is tiny: only about 30,000 km (roughly 20,000 miles) across. It would be a dot in the sky from that distance. However, it's so hot that it shines more brightly than the Sun does, and from that inner planet would be about half as bright as the Sun appears to us from the Earth. It would be an intense pinprick in the sky, a brilliant dot that would be very painful to look at. In fact, it would drown out the red dwarf completely, shining thousands of times more brightly.

What an incredible sight that would be! If alien life developed on a moon of one of those worlds, the only way they'd know of the existence of the red star would be due to the eclipses. Every 3 hours and 7 minutes, the primary star would suddenly disappear for a few minutes as the bigger but far less massive and bright star blocked it out. At that time, and pretty much only then, would the faint red star be visible at all.

Cultures all over the Earth worshiped the Sun for obvious reasons: bringer of light and heat, we depended and still depend on it. What sort of myths would have arisen had the Sun's light been completely cut off a half dozen times a day?

And I have to wonder what other strange things await us as we discover more planets orbiting other stars. We have a pretty good idea of how stars age and die, but there will always be systems on the edge, ones we'll have a hard time understanding. What new things will we uncover then? And what would the sky look like from those alien worlds?



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