How can I tell if something in a photograph is a star or a planet?

Question

A couple days ago, I zoomed in with my 30x optical zoom camera, and after some exposure adjustments, a bright star in the night sky turned into this:

Are those other planets or other stars? Or is that a lens effect? More info: The bright object in question was ~60 degrees above the horizon, and ESE of me (East-south-east). I took the picture on 3-25-16 from Madison, Wisconsin at 9 pm local time (CST)

Answer 1

There exists software that automatically detects stars and planets in your astrophotography pictures. The technique is called plate solving, and they use a scale and rotation invariant hashing algorithm.

Examples are:

• uniMap: a free/opensource astronomy software, [...] mainly developed to do the plate-solving (automated detection and sky catalog matching) for astrophotography [...] also a wide range of features like plotting the asteroids path, artificial Earth satellites passing through, integrated telescope and digital/video camera control, online database/interface to work as a collaboration tool for plate solutions
• Sequence Generator Pro: whose goal is to provide a best-in-class image capture suite for astrophotography

There are also online solvers where you can upload a picture and get an annotated picture back.

Examples are:

• astrometry.net: If you have astronomical imaging of the sky with celestial coordinates you do not know—or do not trust—then Astrometry.net is for you. Input an image and we'll give you back astrometric calibration meta-data, plus lists of known objects falling inside the field of view. (Upload seems to be broken though)
• AstroBin: is an image hosting website specifically targeted to astrophotographers: it's the first and the last place where you need to upload your astrophotography images. Made by an astrophotographer, for the astrophotographers.

Here is an example of a picture of the night sky I took in Kakadu National park, Australia, and below is the annotated picture I got back from AstroBin.

Answer 2

Are those other planets or other stars? Or is that a lens effect?

Looks to me like a planet and some moons. I don't know where you are, but Jupiter has been very bright in the night sky lately in my neighborhood, and with a long enough lens it's not hard to see some of its moons.

Seen through a sufficiently powerful telescope, a planet looks very different form a star. Stars are brilliant, like diamonds. Planets are round and less brilliant, what you'd expect from giant spherical rocks that aren't on fire. But most camera lenses don't offer enough magnification to let you tell the difference, and because planets are relatively close they're often the brightest things in the sky other than our own moon, so they're usually overexposed in your photos.

Perhaps the easiest way to figure out what you're looking at is to get an astronomy app for your smartphone or computer. I use Sky Guide on my iPhone, which uses the phone's compass, accelerometer, and GPS to show you the part of the sky that the phone is pointed at, which makes it very easy to find the objects that you're interested in. And you can change the time and date that it shows, so you can see what the sky would have looked like 3 days or 100 years ago or in the future. If you can remember the direction in which you pointed the camera and read the date from a photo's EXIF data, you should be able to figure out what the objects in your photo are. Well worth the $3 price.

In the past, I've used Starry Night on my laptop computer, and it offers similar capabilities as Sky Guide. There are probably some other good software options as well -- the main point here is that there's software out there that makes it easy to figure this stuff out.

Answer 3

I'm almost certain that this is Jupiter and two moons. I base this on:

• We see three objects, one very bright and two much less so. The lesser two are of comparable brightness. This is consistent with a planet and two moons.
• The center of the three objects lie in a straight line, consistent with a planetary system.
• The angle of the straight line is consistent with the ecliptic when viewed close to the East (or West) horizon. Jupiter's moons do orbit in the ecliptic plane.
• The objects appear as dots and have not refraction spikes. Generally, planets appear in photographs as dots without refraction spikes as they can be resolved to a size larger than a point source. Stars have refractions spikes as they are a point source of light as viewed from Earth.
• I look at Jupiter, Saturn, Mars, and Venus through 90mm binoculars often enough to recognize them!
• Jupiter is currently at close to its closest approach to Earth and appears very bright. It is a compelling target for photographers, binoculars, and naked-eye observing as well.

Answer 4

The rule-of-thumb is: When you think "That's a bright star!" it's a planet.

Answer 5

According to Sky Map on google play, I am very confident that this is Jupiter.