Phantom worlds
Finding planets in unlikely places Exoplanets are scattered by the billions across the universe - and an unlikely percentage of them often find themselves in dire circumstances
Exoplanets that find themselves in dire circumstances
Roughly 4.6 billion years ago the Sun was born from the collapse of a nebula – a giant cloud of gas and dust. Since then planets, moons, asteroids, comets and other debris have formed to give rise to the Solar System. In another 5 billion years the Sun will begin its transition into a red giant star, swelling up due to the lack of internal fuel to power nuclear fusion.
Just by looking at the sky you can see that our Solar System has the ideal conditions for planet formation, but what about more extreme conditions? What about the stars that aren’t a G2V-type, Sun-like star? What about the stars that haven’t been able to make it, or have evolved to the concluding phases of their lives? These questions are a hotly studied
topic as astronomers around the world lock in on extrasolar planet – or exoplanet – research. These astronomers are not only looking for a ‘new
Earth’, they are trying to figure out what kind of environments can accommodate planets and how many exoplanets are hidden away from us in the universe.
The search for an unusual planetary system did not take long at all – for many years before astronomers were convinced they were out there. The first confirmed exoplanets were found to orbit the millisecond pulsar named PSR B1257+12 some 2,300 light years away. The first-ever ‘pulsar planets’, doomed worlds that orbit a dead star, were Poltergeist (PSR B1257+12 c) and Phobetor (PSR B1257+12 d), announced by astronomers Aleksander Wolszczan and Dale Frail on 9 January 1992. These two planets orbit the pulsar at roughly the same distance that Mercury orbits the Sun.
However, there are not many other known pulsar planets, as Dr Dimitri Veras, an assistant professor at the University of Warwick, UK, tells All About
Space: “A type of dead star which did experience a supernova is called a pulsar, and a total of four planets are known to orbit pulsars.” It is not much of a surprise that such a small number of these planets exist due to the incredibly hostile nature of their environment. A pulsar is the last stage of a massive star’s life, as long as the star originally had four- to eight-times the mass of the Sun. After billions of years, but a shorter time period than our Sun, the star will explode as a supernova and leave behind its dense, powerful core, known as a neutron star. A neutron star can rotate rapidly, sending out jets of material and radiation from its poles. If Earth is in their line of sight astronomers pick up these rapid flashes of light, known as pulsars.
These stars are tremendously dense; if you were to somehow get a teaspoon of the stuff it would weigh a billion tons. Because these stars are so dense they are very hot and release a lot of radiation in gamma rays, X-rays, ultraviolet rays and so on. This is not healthy for a planet. If the Earth was constantly bombarded by these levels of radiation there is no way life would survive in any form.
A study released by Dr Alessandro Patruno from Leiden University, The Netherlands, and Dr Mihkel Kama of the University of Cambridge, UK, stated that planets could be habitable around pulsars. In order to make this claim these astronomers deduced the theoretical ‘habitable zone’ around a pulsar – the distance a planet would have to be from the pulsar in order for water to exist as a liquid. These results highlighted the specific criteria a planet must meet in order to survive here: a maximum distance from the pulsar than the Earth is to the Sun, the planet must be a super-Earth – a mass between one- and ten-times our Earth – and the atmosphere must be as thick as the conditions on the deepest ocean floors. If the atmosphere was relatively thin then the pulsar winds would strip it away within a thousand years – which is no time for life to evolve.
The origins of these planets are unknown, but there are two likely options. The first is that the planet was formed before the supernova from primordial debris around the same time as the birth of the star, and was far away enough to survive the supernova. The other side of the coin suggests that these planets formed from debris surrounding the pulsar, much like a second-generation planet.
In 2006 a team of researchers used NASA’s Spitzer Space Telescope to uncover new evidence that suggests second-generation planets were formed around a pulsar called 4U 0142+61, 13,000 light years away. "We're amazed that the planetformation process seems to be so universal," says Deepto Chakrabarty of the Massachusetts Institute of Technology, principal investigator of the research. "Pulsars emit a tremendous amount of high-energy radiation, yet within this harsh environment we have a disc that looks a lot like those around young stars where planets are [being born].”
Now what if a planet was to escape before a pulsar could pepper it with harmful radiation?
This is hypothetically possible, as a planet around a red supergiant could be expelled in the following supernova and cast away as a ‘rogue planet’. In fact, Veras even claims that “there could be up to one giant rogue planet for every star”.
“This possibility has not been considered in depth in the astronomical literature, probably because supernovae occur rarely – about once per century in our galaxy,” says Veras. This is unfortunate as this would be quite the revelation in terms of planets existing in unlikely situations. To think that there could potentially be thousands or even millions and billions of planets hidden in the cold, dark emptiness of space that are undetectable without a star’s light is a intriguing thought.
Once ejected these rogue planets don’t have many options except to wonder through space alone. They bring a whole different meaning to the term ‘phantom’ as they ghost through the universe until they find a new home. “Simulations have shown that planetary systems can capture rogue planets, although the process is rare and depends on several factors,” says Veras. This could mean that planets around dead stars could be imposters sent from another system, but then again there are so many unknown factors to take into account in this scenario, with such little data it would be outlandish to make any definitive claims.
When looking at dead stars the mind can’t help but relate back home, bringing thoughts of what will happen to our own Sun at the end of its life. When the Sun runs low on hydrogen to burn its outer layers will swell. “The Sun will engulf Mercury and Venus. Mars, Jupiter, Saturn, Uranus and Neptune will all survive, and the fate of the
“Pulsars emit a tremendous amount of high-energy radiation, yet within this environment we have a disc”
Deepto Chakrabarty
“A type of dead star which did experience a supernova is called a pulsar, and a total of four planets are known to orbit pulsars”
Dr Dimitri Veras
Earth is unclear,” explains Veras. “The asteroid belt will likely be destroyed by rotational spin-up due to the Sun's increased luminosity on its giant branch stellar evolutionary phase.”
Afterwards the outer layer will be expelled as a beautiful planetary nebula. What’s left is a white dwarf star that is 54 per cent of its original mass. This dense leftover core could also become the host star of a planetary system. Unfortunately there has been no definitive system confirmed yet, but there has been extremely compelling evidence found in attempting to form one.
For instance, in June 2015 a group of astronomers used Spitzer to observe a white dwarf system and found there to be excessive infrared radiation. It is known that white dwarfs have a circumstellar disc – which can also be referred to as ‘polluted’ white dwarfs – that could well be scattering the stars' light and providing this infrared boost. However, in the case of white dwarf PG 0010+280, there is the strong possibility that a planet formed from the pollution and gives out the fresh infrared glow that new planets emit. “So far we have detected about 40 white dwarfs with infrared excesses, and the consensus is that the infrared excess comes from dust discs surrounding the white dwarfs. In all those cases
the white dwarf would show signatures in its spectra from accreting material from the circumstellar disc,” Dr Siyi Xu, an assistant astronomer at the Gemini Observatory, Hawaii, and a member of this research team, tells All About
Space. “PG 0010+280 is the only outlier with a strong infrared excess but no pollution. That is why we speculated in our 2015 paper that another possibility is that the infrared excess comes from a rejuvenated planet – a planet that accreted a lot of material during the red giant stage of the star and it is hot and young again.
“Since 2015 there have been a few more discoveries of white dwarfs with infrared excesses, but they are mostly polluted. There have not been new discoveries of PG 0010+280-like objects.” Similar to pulsar planets, there is the possibility that a second generation of phantom worlds could form from the death of their host star.
There are also the stars that don’t quite make it. There are stars that don’t envelop enough material during formation to reach the heights of the Sun, and these are called red dwarf stars. These stars are roughly between 7.5 and 50 per cent the mass of our Sun and only burn to temperatures of around 3,500 degrees Celsius (6,380 degrees Fahrenheit), which is only roughly 60 per cent of the Sun’s effective temperature.
These cooler stars are much more abundant in Earth’s vicinity, with scientists predicting 20 out of 30 stars nearest to Earth are red dwarfs.
As it happens, the nearest star to Earth, Proxima Centauri, located just four light years away, is a M-type red dwarf star with its own Earth-like planet
held within its habitable zone. As tantalising as it seems to have an Earth-like planet so close to Earth itself, however, Proxima b’s habitability is sadly deemed ‘unlikely’, as astronomers witnessed a super flare erupt from Proxima that would have doused the planet in huge amounts of ultraviolet radiation.
Red dwarf targets are not as doomed as the pulsars, white dwarfs or even Proxima b, however, as these stars could hold some of the most exciting prospects for life beyond our Solar System. A popular example is TRAPPIST-1, the ultra-cool red dwarf star, and its seven planets. This star is about 12 per cent the size of our Sun and burns at a measly 2,220 degrees Celsius (4,000 degrees Fahrenheit), yet it holds planets that astronomers believe could hold something special.
Possibilities are endless when it comes to the universe, and no one can deny that one of these phantom wolds around seemingly dead, insignificant stars could prove to be some of the most stimulating prospects around. These systems could even hold clues as to the fate of the Solar System and what is to happen to Earth.