What are some skywatching highlights in September 2020? Spot the Moon together with Mars and Venus, along with the flickering star Fomalhaut, which had itself a planet...until it didn't!


What's Up for September? The Moon with Mars and Venus, and a star with a planet...that wasn't.

On September 6th, you'll find the Moon extremely close to Mars in the predawn sky. Now, they were even closer back on August 9th, but still a really pretty spectacle this month. If you're up early and can step outside for a look, they'll be only a couple of degrees apart, meaning they'll appear in the same field of view if you take a look with most binoculars.

On September 13th and 14th, look in the east before dawn to see the slim crescent Moon slip past brilliant Venus. On the 13th you'll find the Moon hanging above Venus with about 20 percent of its surface illuminated. By the next morning, the Moon has moved here, to the left of Venus, and has only about 10 percent of its sunlit surface visible.

Looking toward the south in September, there's really only one relatively bright star for most of us who live near cities. That star is called Fomalhaut, and it's got a pretty interesting story. The star is about 25 light years away, meaning it's relatively close by. It's also fairly young, at just a few hundred million years, and it's still surrounded by a disk of debris, which is a common feature for stars during their planet-forming phase. Now we've discovered thousands of exoplanets - planets outside our solar system - but Fomalhaut appeared to be the first star to have a planet detected by direct imaging with a telescope (that being the Hubble Space Telescope). Astronomers announced the find back in 2008.

So Fomalhaut had itself a planet! But this is where it gets interesting, as the "planet" had a funky orbit, wasn't giving off excess heat like a young planet should, and proceeded to grow fainter over the several years that followed, disappearing by 2014. In April 2020, another team of astronomers using Hubble announced their finding that Fomalhaut's "planet" wasn't a planet after all. In fact, their study showed what Hubble detected was likely a giant, expanding cloud of debris resulting from a huge collision of two small bodies made of dust and ice, similar to worlds you might find in our own Kuiper Belt.

The scientists calculate collisions like this happen around Fomalhaut only every couple hundred thousand years, so Hubble just happened to be looking at the right time, not long after the collision took place.

So we may have lost a planet, but we gained a cool insight into how planetary systems form and evolve. You can find Fomalhaut low in the south a couple of hours after sunset, to the left of the bright pair of Saturn and Jupiter. Since it's bright and low in the sky, it sometimes appears to flicker from atmospheric turbulence. That can cause some skywatchers to wonder just what the heck it is. Now you know: it's Fomalhaut, the nearby star where it appears we witnessed a dramatic planetary collision.

Here are the phases of the Moon for September. You can catch up on all of NASA's missions to explore the solar system and beyond at nasa.gov. I'm Preston Dyches from NASA's Jet Propulsion Laboratory, and that's What's Up for this month.

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