What's Up for September? Jupiter, Saturn, the Moon, and a star with a planet...that wasn't.
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.
From Friday evening into early Saturday morning, the waxing gibbous Moon will have shifted to the east, appearing about 4 degrees to the lower left of the planet Saturn, with the bright planet Jupiter appearing farther to the right. For the Washington, D.C. area, the Moon will appear 26 degrees above the horizon in the south-southeast as evening twilight ends at 7:57 p.m. EDT, the Moon will be at its highest for the night (27 degrees above the horizon in the south) at 8:46 p.m., Jupiter will set first in the west-southwest on Saturday morning at 12:48 a.m., Saturn next at 1:25, a.m., and the Moon last at 1:35 a.m.
Thursday, Oct. 1, 2020, will be when the planet Mercury reaches its greatest angular separation from the Sun as seen from Earth for this apparition (called greatest elongation), appearing half-lit through a large enough telescope. Because the angle of the line between the Sun and Mercury and the horizon changes with the seasons, the date when Mercury and the Sun appear farthest apart as seen from Earth is not the same as when Mercury will appear highest above the horizon in the west-southwest 30 minutes after sunset (an approximation of when it might be dark enough to see Mercury), which occurred earlier in September.
Sometime in late September or early October 2020 (2020-Oct-01 14:59 UTC with 6 days, 17 hours, 18 minutes uncertainty), a near-Earth object (2001 GP2), between 36 to 81 feet (11 and 25 meters) across, will pass the Earth at between 0.4 and 14.3 lunar distances (nominally 6.1), traveling at 4,950 miles per hour (2.21 kilometers per second).
The next full Moon will be on Thursday evening, Oct. 1, 2020. This will be the Harvest Moon, the Chinese Mid-Autumn Festival, and one of the smallest full Moons of the year (the opposite of a supermoon, sometimes called a micro full Moon).