Still searching the solar system

Nichola Fenton, of Balclutha, asked:

Are there any planets we haven’t heard of yet?

Alan Gilmore, an astronomer at the University of Canterbury’s Mt John Observatory, responded:

That’s a broad question. If we are talking about planets around other stars, then there are many (billions?) in our galaxy that we don’t know about. With a few exceptions, we are likely to discover planets only around relatively nearby stars, those within a few hundred lightyears of us. (One lightyear is nearly 10 million million [trillion] km.)

In our own solar system, we keep discovering small planets called asteroids. Some of the distant ones are Pluto’s size. But Pluto isn’t big. Our moon is five times the mass or weight of Pluto. It was the discovery of Plutosized asteroids further out than Pluto that led the International Astronomical Union to define what the word ‘‘planet’’ meant. That caused Pluto to be reclassified as a ‘‘dwarf planet’’, along with several other big asteroids.

The orbits of some of the distant dwarf planets have a similar orientation in space.

Some astronomers think this is caused by the gravity of a big planet far from the sun. They have even called it ‘‘planet nine’’.

That planet, if it exists, could be five times heavier than Earth. Other astronomers think the similar orbits of the dwarf planets have simpler explanations. Even so, large telescopes are being used to look for the possible planet. Though big, ‘‘planet nine’’ would be very faint, being so far from the sun.