Outer Planet Surprise

Unexpected Results

In the predawn hours of August 23rd, just out of curiosity, I decided to point Seestar at Neptune. Expectation was low because Seestar isn't designed to produce spectacular detailed planetary images. The 2-minute exposure I took at 4:02am EDT showed an overexposed blob. Upon closer inspection, however, I noticed a definite spherical bump on one side of the blob. Could this be one of Neptune's moons? The situation is shown in the next picture, a cropped image enlarged to three times normal size.

The spherical bump was, indeed, the Neptunian moon, Triton! At the time this image was captured Triton was only 16.3 arc seconds from Neptune - far enough away to escape the overexposed Neptunian blob. A better optical system and exposure might have produced something like the following idealized picture from planetarium program Sky Safari 6.

You can see how Triton in my Seestar image is in the correct orientation relative to Neptune. Amazing! I actually detected a moon of Neptune! The angular diameter of Neptune this day was 2.4 arc seconds, so the approximate 16.3 arc second radius of the overexposed blob was much bigger than Neptune's true size. Triton shines at magnitude 13.7. Another Neptunian moon, Nereid, at magnitude 19.3 was too dim to detect. 

Almost three weeks later at 10:18pm EDT on the evening of September 12th I took another 2-minute Neptune exposure. This time Neptune's overexposed blob seemed slightly less spread out, as you can see in the next cropped image enlarged three times.

By September 12th Triton's orbital motion had clearly brought it to a new position relative to Neptune. This was confirmed by the idealized Sky Safari 6 image below.

After capturing Neptune before sunrise on August 23rd I also imaged Uranus at 5:13am EDT with a 15-minute exposure. Once again, I was amazed to see two possible Uranian moons near the overexposed Uranus blob. The next picture, enlarged two times, clearly shows possible moons to the right of Uranus.

I wondered if these objects were really moons. Perhaps they were two background stars that happened to be near Uranus. So, I checked Sky Safari which reproduced the following arrangement of moons near Uranus at 5:13am on August 23rd. Titania and Oberon on the right seemed to have the correct orientation, but Ariel and Umbriel also had the correct orientation.

To resolve the ambiguity, I checked angular separations from Uranus. Titania was 30.2 arc seconds from the planet and Oberon was 41 arc seconds away. Ariel was only 13.2 arc seconds away and Umbriel was 18.1 arc seconds away. It seems like Titania was almost at the outer edge of the overexposed planetary blob because Umbriel at magnitude 15 would be too dim to see. Therefore, I think the situation is accurately represented by the following picture.

The area within the red circle (including the wide red boundary) is the overexposed planetary blob. Uranus' actual angular diameter this night was 3.6 arc seconds, and the blob's radius was approximately 27 arc seconds. Three Uranian moons are apparently hidden within the blob. Therefore, Titania and Oberon are the moons captured in my Seestar image. 

Almost a month later on September 19th I took a 4-minute exposure of Uranus at 1:11am EDT. Once again, two moons appear to the planet's right, as you can see in the next cropped image enlarged twice. The arrangement is slightly different from the situation on August 23rd.

Sky Safari reveals that Titania now appears below Oberon as shown in the next picture. (The other three moons, as before, are invisible within the overexposed blob.) Titania has an 8.71-day orbital period and Oberon has a 13.46-day orbital period. Between August 23rd and September 19th slower Oberon had gone around Uranus twice, while faster Titania had gone around slightly more than three times appearing now below Oberon. Titania and Oberon both shine at near 14.0 magnitude. Between August 23rd and September 19th Titania's separation from Uranus went from 30.2 to 31.5 arc seconds., and Oberon's separation went from 41 to 41.8 arc seconds.

Honestly, I'm rather astounded that little Seestar could reveal these distant planetary moons!