In many ways, the Earth can seem pretty unique.
But take a glance at the stars, and you’re actually looking at trillions of Earth-like planets and alien worlds.
It’s suspected that most stars have orbiting planets, and some will have solar systems much like our own.
Finding these exoplanets can be difficult, but scientists are in agreement that they’re out there.
And for an exoplanet to count as an exoplanet, it must have a star. Otherwise, this wandering celestial body is a rogue planet.
To learn more about exoplanets and their stars, as well as planets without stars, check out this guide.
What Is An Exoplanet?
An exoplanet is a planet outside our solar system that orbits a star, brown dwarf, or stellar remnants.
Exoplanets have long been theorized, but it’s only in recent years we’ve seen real proof that they exist.
It wasn’t until 1992 that we had real proof of other solar systems.
But that hadn’t stopped generations of scientists and science fiction writers alike from theorizing about exactly what these planets could be like.
Do Exoplanets Have Stars?
All exoplanets have stars because an exoplanet is defined as a planet outside our solar system that orbits a star.
Excitingly, Earth-like planets have been found to exist in the habitable zone of their star. There are planets that don’t orbit stars.
These are known as rogue planets, and they’re sometimes categorized alongside exoplanets.
However, as they don’t orbit a star, brown dwarf, or stellar remnant, they aren’t technically exoplanets.
Exoplanets have also been shown to orbit more than one star. When two stars are gravitationally bound, they’re known as binary stars.
An exoplanet might orbit just one of these stars, or it might be a circumbinary planet.
In this case, the planet orbits both stars, and they all share the center of the system.
Triple star systems, and even quadruple star systems, have also been shown to have orbiting planets.
As well as having stars, some exoplanets have also been shown to have moons.
As you can imagine, these are incredibly difficult to find and confirm.
However, they’re largely considered to be common in other planetary systems.
Stars with exoplanets are known as planet-hosting stars.
One in every five Sun-like stars are thought to have a planet similar in size to Earth, orbiting in the habitable zone.
On average, there’s assumed to be at least one planet per star.
It’s thought that a star is much more likely to host a planet than not.
Do All Stars Have Exoplanets?
It’s thought that most stars do have at least one orbiting exoplanet.
However, confirming the existence of these exoplanets can be a long and difficult process.
Finding an exoplanet is tricky because of its size in comparison to the stars they orbit.
When we look for exoplanets, it’s not quite as simple as spotting the shape of them in the night sky.
The exoplanets themselves tend to be rendered almost invisible by the light coming from their star.
Because of this, exoplanet discoveries have to be made through indirect methods. Only a handful of exoplanets have been confirmed.
One common method of finding these distant planets is radial velocity.
This is monitoring the speed that the star moves in relation to Earth and is affected by the orbit of the exoplanet.
When we notice these shifts, we can determine the existence of an exoplanet.
The most common method is transit photometry. This measures the brightness of a star.
When an exoplanet moves in front of the star, the brightness drops; this can help us identify the planet and determine its radius.
Even our nearest exoplanets can be tough to find.
Proxima Centauri is the nearest star outside our solar system, and we’ve only very recently confirmed that it has exoplanets.
And Proxima Centauri is only four light-years away, which is nothing compared to the size of the galaxy.
But if scientists are right, and most stars do have their own solar system, then that opens up possibilities for life in outer space.
There are thought to be roughly 2 trillion galaxies in the observable universe and an estimated 100 million stars in every single galaxy.
If each of those stars has just one exoplanet in orbit, that’s a staggering number of potential planets on which to search for life.
And that number might even be an underestimate! The search for exoplanets will certainly continue.
What Stars Have Planets?
The stars most likely to host a planet or planetary system are those similar to our Sun.
These are main-sequence stars in the spectral categories F, G, or K.
However, it’s worth noting that this might be a result of a bias in planet-search programs.
Also, the older radial velocity detection method struggled to find the lower mass planets most likely orbiting smaller stars.
With the newer Kepler space telescope, detecting a range of sizes has become easier.
Larger stars, such as spectral category A, are also hard to detect exoplanets around.
That’s because they rotate so quickly that it’s almost impossible to measure the Doppler shifts, even if they’re orbited by giant planets.
These distant planets are likely to be very different to our Earth.
Exceptionally large stars in spectral category O are blisteringly hot, and this may prevent planetary formation.
Very small stars and sub-stellar objects, such as brown dwarfs and sub-brown dwarfs, have also been shown to host planets and protoplanetary disks.
Overall, almost every type of star has shown the potential to host planets.
But due to the methods used to find exoplanets, they’re much easier to spot orbiting Sun-like stars.
Are There Any Planets Without Stars?
Yes, there are planets that don’t orbit stars. These are known as rogue planets.
A rogue planet might have once been part of a planetary system and later rejected, or it may never have been gravitationally bound.
Like exoplanets, rogue planets are surprisingly common.
In the Milky Way alone, there’s thought to be billions of trillions of these rogue planets.
Studies have theorized that for every star in the galaxy, there could be two Jupiter-mass rogue planets.
Others put the estimate much higher, suggesting that for each star, there could be 100,000 rogue planets (but this number includes smaller rogue planets).
Rogue planets can be formed in the same way as stars: gas clouds collapsing inwards due to the pressure of gravity.
Or they might be formed in the protoplanetary disk encompassing a young star, as exoplanets do, and later rejected from the system.
The nearest rogue planet to Earth is thought to be WISE 0855-0714, which is roughly 7.1 light-years away.
But there’s still some debate whether it’s a rogue planet or a sub-brown dwarf.
Exoplanets do have stars, but it’s perhaps more accurate to say that stars have exoplanets.
It’s thought that there’s an average of at least one exoplanet for every star, which means there is an absolutely massive amount of potential planets in the universe.
Stars of all types can potentially be home to exoplanets, although they’re easiest to spot around Sun-like stars.
But even here, finding an exoplanet is very difficult.
Planets without stars are rogue planets! These may have once been part of a solar system, or they could have formed by themselves.
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