Our solar system is just one of many star systems in the Galaxy. It's the only one officially called a 'solar system', but astronomers have already discovered more than 2,500 other stars with planets orbiting around them in the milky way, and there are about 200 billion stars estimated in the Milky Way. Those are just the ones we already found — there are many more planetary systems out there waiting to be discovered!

Even though Pluto revolves around the Sun and is the right shape, it's not a planet and doesn't have a 'pure orbit'. That is why the International Astronomical Union demoted Pluto from 'planet' to 'dwarf planet'.
What does 'pure orbit' mean? Imagine if you add up the mass of all other objects in the orbit of Pluto, then as a result of calculations, the mass of Pluto would be only a tiny part of the total mass. In fact, Pluto would be 0.07 times more massive than anything else. For comparison, if you did the same thing with all other objects in Earth orbit, our planet would be 1.5 million times more massive.

Despite decades of intensive efforts, even the best available estimates of the mass of the Milky Way differ greatly. Only recently, by combining the new data from the ESA 'Gaia' mission with observations made using the Hubble space telescope, astronomers calculated that our Galaxy 'weighs' about 1.5 trillion solar masses within a radius of 129,000 light-years from the center.

An exoplanet or extrasolar planet is a planet outside our solar system. Finding planets near other stars is tricky, because they're so hard to see directly through telescopes. Exoplanets are hidden in the bright glare of the stars they orbit. So, astronomers had to figure out other ways to detect and study these distant planets — they look at the shadows these planets leave when they orbit their stars, and can calculate the planets' size and speed from those. Astronomers have understood that other stars have planets for a long time, but they only detected the first exoplanets in the late 1980s.

A comet is a small celestial body orbiting the Sun in a very elongated orbit in the form of a conic section. Approaching the Sun, the comet forms a comma and, sometimes, a spectacular tail of gas and dust. Probably the most famous one is Halley's Comet, which passes Earth only once in every 75 years — don't miss it if you get the chance!

White dwarfs are stars with large mass and small radius, which is less than Chandrasekhar’s limit for the selected mass, and is the product of the red giants’ evolution. Their process of production of thermonuclear energy is terminated, which leads to the special properties of these stars. According to various estimates, in our galaxy their number is from 3 to 10% of the total stellar population.