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SYFY WIRE Bad Astronomy

A Chance to See Moons Around the Exoplanet Beta Pic B in 2017

By Phil Plait

I don’t usually write about observations that haven’t been done yet, but this is so interesting I wanted to tell y’all about it. Next year, due to a quirk of geometry, it’ll be possible for astronomers to search for moons or other debris around the exoplanet Beta Pictoris b. They won’t be able to see the planet itself, but if there’s stuff around, that might be detectable.

The star in question is Beta Pictoris, which is about 60 light-years away (that’s relatively close). It’s more massive, hotter, and brighter than the Sun. It’s also considerably younger: It’s about 25 million years old, while our star is more than 4.5 billion years in age.

In 1983 astronomers found that Beta Pic was putting out more infrared light than expected, and detailed observations revealed it was surrounded by a disk of warm dust and debris, the material left over from the formation of the star (I remember doing a homework problem in grad school about the excess IR light from the star, using it to calculate the mass of the disk). As it happens, we see this disk nearly edge-on.

Then things got even more exciting: In 2003, observations using the Very Large Telescope directly imaged an exoplanet orbiting Beta Pic!

Called Beta Pictoris b, it has about seven times the mass of Jupiter, and orbits the star roughly every 20 years. In fact, its motion around the star can be seen; observations made in 2009 and 2010 show it on the other side of the star from where it was in 2003!

Getting the exact orbit of the planet is difficult because it’s hard to get accurate enough observations to really pin it down. However, using new techniques, a team of astronomers looked at data from the massive Gemini telescope to get amazingly accurate positional data from the planet. What they found was good news and bad news.

The bad news is that the planet’s orbit is almost certainly not edge-on as seen from Earth (that would need a tilt of 90°, but the tilt they found is 88.8°—close but no cigar). That means the planet doesn’t actually transit its star, passing directly across the star’s face as seen from Earth. That would be nice, because that sort of observation can be used to get the planet’s size.

But not all is lost. The good news is that the planet gets very close to transiting the star, close enough that its Hill sphere will actually pass in front of it.

What’s a Hill sphere? Why, I’m glad I asked.

Gravity depends in part on how far you are from a massive object. As you move farther away, it weakens. If that object is a planet, and it orbits a star, at some distance from the planet the star’s gravity has more influence on an object in space than the planet’s gravity. If you’re closer to the planet the planet’s gravity dominates, and it can hold on to orbiting objects. That defines a spherical region around the planet, called the Hill sphere.

The size of the sphere depends on the mass of the planet, the mass of the star, and the distance between them. For example, the Earth’s Hill sphere reaches out to about 1.5 million kilometers. The Moon, orbiting 380,000 km away, is well inside that, so its motion is mostly influenced by the Earth (some people like to say the Moon orbits the Sun more than it does the Earth, but those people are wrong). Weirdly, Pluto’s Hill sphere is much larger than Earth’s, but that’s because it’s so far from the Sun that an object can orbit Pluto from farther away and still be heavily influenced by it.

The Hill sphere for Beta Pic b is more than 160 million kilometers in radius, which is pretty large. That means it can hold on to moons or some of that leftover debris out to that distance. Although the planet itself won’t transit the star, it turns out that this region will, starting in April 2017 and ending in January 2018. The planet’s closest approach to the star will be in August, which is when the transit could allow astronomers to probe material closest to the planet.

I won’t mince words: This is really cool! The astronomers suggest a campaign to observe the star during these times, though they note that the first half of the Hill sphere transit won’t be easily visible to ground-based telescopes because it will be near the Sun in the sky (though space-based ‘scopes should be able to take a peek). But the second half of the transit should be visible to observatories in the Southern Hemisphere.

Hopefully constant monitoring will be done. There’s no guarantee anything will be seen, but if a large moon does happen to pass in front of the star, this would be the first such exomoon ever seen (assuming none is found between now and then orbiting some other exoplanet). That’s a very big deal indeed. Moons orbiting planets give us a chance to determine the mass of the planet, for one thing.

And for another, we’ll be detecting a moon orbiting a planet that in turn is orbiting an alien star!

That is just simply cool. But it’s also premature. We’ll have to wait and see, and keep our astronomical eyes open. This is a chance that won’t come back until Beta Pic b circles its star once again … in just another 20 years or so.

Tip o’ the dew shield to Jason Wang on the Exoplanet Imaging Group on Facebook.

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