When to see four planets in the night sky at once

View of planet Mercury from space. Image elements furnished by NASA.

A view of the planet Mercury from space. Image elements furnished by NASA. - Credit: Getty Images/iStockphoto

What’s in the sky this month?

On January 7 you’ll have an opportunity to see several planets at once. Mercury will be at its greatest elongation in its orbit from the Sun.

Mercury is a difficult planet to view, as its location closest to the Sun means it is only visible at sunset or sunrise for a brief period of time. At its greatest elongation, Mercury is at its maximum distance from the Sun, giving us a better view the planet. It will still be hard to catch, with the planet appearing only 11 degrees above the horizon right after sunset at 16:10 GMT.

If you happen to have a clear, flat view of the horizon, though, you may also be able to spot Venus just barely above the horizon to the southwest. Mercury will be just following Venus, with Saturn just behind Mercury along the ecliptic.

Trailing these planets will be Jupiter and the Moon to the south. Quite a crowded sky! This might be a great opportunity to try out your hand at astrophotography. Even if you don’t have a telescope or binoculars to zoom in, a view of all the planets line up is quite spectacular!

Astronomy at the University of Hertfordshire

Did you know the University of Hertfordshire has its own observatory? Bayfordbury Observatory was built in 1969 at the sight of Bayfordbury Mansion by Hatfield Polytechnic, before it became the University of Hertfordshire. When it first opened, the observatory only had one 16-inch Newtonian/Cassegrain telescope. Now, the observatory is home to seven optical telescopes and four radio telescopes. The observatory conducts a range of studies, from observing distant exoplanets to monitoring our own atmosphere for dust kicked up by the Sahara.

If you’d like to visit the observatory, group visits are currently available for booking. Groups of 15-40 people can visit the observatory, hear a talk from an astronomer, view a planetarium show, and take a peek through one of the observatory telescopes if the sky is clear. Open nights for the general public can also occur throughout the year and have something for all ages to enjoy. They are currently on hold, but keep checking the Bayfordbury website for updates on when open nights will resume!

Astronomy around the world

We have come a long way since 1992 when we first recognised the discovery of a planet beyond our solar system. We now know of thousands of other planets outside our solar system. Finding planets around other stars is not easy, and most techniques require indirect observations.

However, an old technique used to study the sun is allowing astronomers to directly image exoplanets. Coronagraphs are used to block out the light from the sun, allowing us to study its much fainter atmosphere. Similarly, coronagraphs can be used to block out the light of the central star, allowing us to see the glow from objects which would normally be drowned out by the starlight.

This technique has allowed astronomers to uncover the planet b Centauri b. Found orbiting the binary star system b Centauri,  the unusually named b Centauri b is the first planet discovered in the system. What makes this planet so exciting when we know of thousands of other exoplanets? For one, its orbit around two stars would mean the planet formed under conditions thought to normally prevent planetary formation.

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Yet the planet is massive, 11 times more massive than Jupiter. This places the planet’s mass just shy of a brown dwarf, a type of failed star. The existence of this system means astronomers are expanding their understanding of how exoplanets form.

A common theory is that a central star forms and the remaining orbiting material comes together to form planets along an accretion disk, a scenario where the large binary stars would likely halt planet formation. In this case, astronomers think the planet was massive enough to have its own disk of material, separate from the two binary stars.

Discoveries like b Centauri b continue to expand our knowledge of how star and planets systems form.