When it comes to planetarium shows, “questions and answers” is our absolutely favourite time. But last week one primary school visitors took Q&A to the next level. They came to their Wonderdome Astronomy lesson with questions already prepared and written down! More questions came up during the show, so some of the prepared questions we didn’t have time to go through. There was only one thing we could do!
Have there always been eight planets and what were they called?
Until 2006 (when astronomers voted to reclassify Pluto as a dwarf planet) there were 9 planets in the Solar System. Now, it’s only 8: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
On several occasions throughout history scientists raised the alarm and claimed they found evidence of a new planet.
For example, in the 19th century it was noted that Mercury moved around the Sun in a rather peculiar way. Many astronomers concluded that there was a yet unseen planet between the Sun and Mercury that pulled on Mercury affecting its orbit. Scientists searched for this mysterious planet (they called it planet Vulcan after the Roman god of fire) for many years but never found it! It took Albert Einstein and his theory of general relativity to explain why Mercury moves the way it does!
Similarly, in 2016 astronomers discovered the strange behavior of several small bodies on the outskirts of the Solar System. They decided that this behaviour can be best explained by the presence of a large and yet undetected planet. Many professional and amateur astronomers search for this proposed “planet 9” but so far, we cannot find it! You will find more information about planet 9, why scientists think it is real, what it might be like and where to look for it in Wonderdome space blog post The search for planet 9 goes on.
Why is Venus the brightest planet?
Indeed, Venus is the brightest planet and one of the brightest objects in the sky after the Sun and the Moon. Sometimes the International Space Station outshines Venus, thanks to the new bright solar panels, but the ISS is only visible for several minutes and not even every night. Depending on your location, you might be able to spot the station anywhere from a few times a week to once a month.
Venus looks bright because it’s close to the Sun and because the thick clouds that cover the planet (they are made of sulfuric acid droplets) reflect 75% of the incoming sunlight! Astronomers talk about how bright space things are in terms of their ALBEDO. The word albedo comes from Latin and means whiteness. When you shine light on any object, some of the light is absorbed by the object’s surface, and some is reflected back. In the same way when the Sun shines on a planet (or moon or a space rock), a portion of the sunlight is absorbed by the surface and a portion is reflected. ALBEDO is how much of the light that shines on an object is reflected back, i.e. how bright this object appears to be. Albedo can be between 0 (meaning all light is absorbed, the body is totally black) and 1 (meaning all light is reflected). Venus has an albedo of 0.75! That means it reflects 75% of the sunlight that falls on it. To compare, the Moon has an albedo of 0.11. It is about as reflective as a worn-out asphalt!
If the Moon is not a planet or a star, than what is it?
The easy way to remember what celestial bodies are is by how they move.
For example, stars orbit around the center of a galaxy. Planets orbit around stars. Natural satellites (or moons with a small m) orbit around planets.
So our Moon is the Earth’s natural satellite. Or the Earth’s moon.
This is a simplified picture, because there are other objects in Space, like dwarf planets, asteroids and comets. They also orbit around their star and some of them also have moons.
According to the International Astronomical Union (that’s the organization responsible for naming astronomical objects) “A natural satellite, a solid object in orbit around a planet, a dwarf planet, a minor planet, or a transneptunian object is sometimes referred to as a moon (with a small m) in public usage”.
How old is Space? How old are the planets?
The Universe is about 13.8 billion years old. But scientists might be wrong!
How do we know it? Astronomers estimate the lower limit of how old the whole Universe is by calculating the age of the oldest space objects, such as the tightly packed groups of stars called globular clusters. After all, it cannot be younger than the stars in it, right?
To get a more precise answer, scientists use the fact that the Universe is expanding and therefore galaxies get further and further away from each other. Astronomers run computer simulations and “replay” the expansion back to the “beginning”, to the moment when the Universe was born, to see how long it takes. Of course, to do that you need to know the rate at which the Universe expands, i.e. how fast the galaxies are running away from each other. The expansion rate is known as the Hubble constant, after Edwin Hubble, the astronomer who first notices that the Universe is expanding. Measuring the Hubble’s constant is a very tricky task. Scientists use different methods and, confusingly, get different results. Last month, in September 2019, German astronomers used yet another method to determine the expansion rate. They concluded that the Universe might actually be not 13.8 billion years old, but a couple of billion years younger! You will find the original publication here.
So, how old is the Universe EXACTLY? We still don’t know!
Now, what about the age of the planets? We assume you are asking about the planets in our own Solar System! The Solar System and everything in it is about 4.6 billion years old. We know this from studying the Solar System “fossils”, the space rocks left over from the formation of the planets. Unlike the Earth rocks that go through rock cycle, i.e. they change over time, the space rocks remain unaltered! So every meteorite (a space rock found on Earth) is a time capsule! Scientists use a method called radiometric dating to figure the age of these space visitors.
What is the biggest volcano in the Solar System?
Olympus Mons on the planet Mars is the tallest volcano in the Solar System. It’s height “base-to top” is 13.6 miles (or almost 16 miles above the “mean surface”) and the diameter is 374 miles. Because it is so wide, this mountain is not at all steep and not difficult to climb. In fact you will barely notice you are standing on a mountain at all!
To compare, the tallest mountain on Earth, Mount Everest, towers 5.5 miles above the sea level. That’s less than half the size of the Olympus Mons! But Everest is so steep that out of almost a 1000 people who attempt to climb it every year less than half reaches the summit!
Olympus Mons is not only the biggest Solar System volcano, but also one of the tallest mountains! It used to be THE TALLEST, but in 2011 astronomers discovered that the central peak of the crater Rheasilvia on steroid 4 Vesta* rises to the height of 14.2 miles.
*Asteroids are space rocks left over from the formation of the Solar System. They reside in the Asteroid Belt between Mars and Jupiter.
Scientists think that Olympus Mons formed via a process called hot-spot volcanism, it’s when molten material gets to the surface through a weak spot in the crust. The same mechanism here on Earth created Hawaiian Islands!
How big is the Sun?
The diameter of the Sun is 864,938 miles! It is so big that 1.3 million Earths could fit inside our star. Although that sounds pretty impressive, when it comes to star sizes, the Sun is rather on the small side. The biggest (and the rarest) of all stars are hypergiants. The largest of them, the star called UV Scuti in the constellation Scutum, the shield, is 1700 times the size of the Sun. Can you tell how many Earth will fit inside that one? If we could place UV Scuti in the centre of the Solar System instead of the Sun, it would take up the space all the way to Saturn!
How does the Moon look the same size as the Sun from the Earth?
Now, that is fantastic observation! The Moon is, of course, much smaller than the Sun, about 400 times smaller. But then the Sun is 400 times further away. Therefore when we look at them from the Earth, they appear to be the same size! And because their “apparent sizes” are the same, something very unusual happens sometimes. When the Earth, the Moon and the Sun line up, the Moon can cover the entire solar disc! That’s a solar eclipse. Back in 2017 Wonderdome traveled to the US to see the Great American Eclipse. The next total solar eclipse will happen on Dec 14, 2020. It will be visible in Chile, Argentina and South Africa.
Why can’t you live on Mars?
Let me ask you: what makes our Earth so special? It’s our air, liquid water, the right amount of heat and gravity to name a few, right? Let’s look at Mars.
Average temperature: -50 degrees.
Air: very thin and made of Carbon Dioxide. In other words, unsuitable for humans to breath and not providing enough pressure.
Water: frozen in the polar caps and deep underground.
Gravity: 38% of the gravity on Earth
In addition to all that, Mars does not have a global magnetic field to block harmful radiation from the Sun. Those rays can make humans very very sick!
SO, Mars is not perfect, but it is nevertheless the most Earth-like planet in the Solar System! We might still be able to visit and even live on Mars in the future! But visitors to Mars will have to build special habitats, figure out how to grow food on the red planet and wear pressurized spacesuits when out and about. Would you like to visit Mars one day?
What are black holes? Do black holes ever end?
Take a very heavy star and squeeze it into a tiny ball. The object you will end up with will be so compact and heavy, that its gravity will be extremely strong. By extremely we mean that nothing, not even light, will be able to escape it. That’ a black hole.
This squeezing and collapsing happens to stars naturally at the end of their lives. But not all stars turn into black holes, only a very massive ones.
Stars shine brightly as they burn fuel in their cores. Once they run out of stuff to burn, they become unstable and “die”. Now, stars “die” in different ways depending on their mass.
Small stars, like our Sun, swell, then cool down and contract. They end up as white dwarfs.
But when massive stars run out of fuel, their cores collapse while their outer layers explode as supernovae. The remnant of the explosion is a neutron star, a strange very hot, extremely dense, fast rotating objects that we can observe as pulsar, or, for extremely heavy stars, a black hole.
You will find more information about various kinds of black holes (yes, there are different kinds!) in Wonderdome space blog Black Holes: the size matters!
SO if nothing comes out of a black holes, will they “be there” forever? Astronomer Stephen Hawking calculated that black holes eventually evaporate via a process called Harking radiation. So at the end even black holes disappear!
What are meteoroids? What causes meteor showers?
Meteoroids are small chunks of rock or metal (or both) that fly through space. They can be as small as a grain of sand or as big as a 1 meter boulder, though most meteoroids are the size of a pebble.
If a meteoroid (or asteroid or comet) crosses the Earth’s path and enters the Earth’s atmosphere, we see its burning trail as a “shooting star”, or a meteor. If a meteor survives the fall and lands on Earth, we call it a meteorite. Millions of meteors fly through our atmosphere every day, but most of them burn up before they reach the ground.
Meteoroids are rocky crumbs that form when asteroids collide with each other or with objects like Mars or the Moon. But some of these crumbs come from comets. Comets are “dirty snowballs” that orbit the Sun. Every time they get close to the Sun in their orbits, they start to evaporate and shed some of the (previously frozen-in) rocky material. These unfrozen rocky bits spread out along the comet’s orbit. When the Earth crosses the path of the comet’s trail, the rocky particles enter the atmosphere causing meteor showers.
There are two October meteor showers: Draconids (will peak on October 9th) and Orionids (will peak on October 21st).
Is there any other planet with life? Are there aliens?
No planet that we know of, but we keep searching! It is quite possible that life exists on other planets. Hundreds of scientists are looking for aliens and alien-friendly planets right now. But if there are aliens, they are probably very different from how we imagine them (with green skin, scales, tentacles and googly eyes). Scientists think that the life we are most likely to find out there is microscopic.
What is the coldest planet in the Solar System?
The coldest planet in the Solar System is Uranus! It is hard to believe, but Uranus can get even colder than the farthest planet from the Sun Neptune! The surface temperatures of the two “ice giants” are only slightly different, which is strange given that they are over 1 million miles apart. Uranus, unlike Neptune, produces very little internal heat to warm itself up from the inside!
Why is the Sun white?
Stars come in different colour. The colour of a star depends on its surface temperature as well as atmospheric chemistry.
Astronomers put stars into different groups called spectral classes.
The classes are (from hottest to coldest): O-B-A-F-G-K-M
O stars (blue-white stars with temperature of the “surface” reaching up to 50.000 degrees Celsius)
B stars (blue-white stars, their surface temperatures range from 10 000 to 28 000 degrees Celsius)
A stars (white stars, their surface temperature is 7 500- 10 000 C)
F stars (yelow-whitish stars, their surface surface temperatures range from 6000 to 7 500 C)
G stars (yellow stars with surface temperatures between 4 900 and 6000 degrees Celsius)
K stars (cool orange stars with temperatures 3500-4900 degrees Celsius)
M stars (red stars)
Can you guess what spectral class our Sun belongs to? Class G! Or, G2, to be precise. So what colour is the Sun?