1. Ask Wonderdome!

2. SO…
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.

3. 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!

4. 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”.

5. 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.

6. 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.

7. 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!

8. 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.

9. 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?

10. 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!

11. 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).

12. 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!

13. 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?
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