|Brown dwarfs (the small green dot in the middle) are detectable by NASA [CC – NASAblueshift]|
Rutland, United Kingdom – Our galaxy, the Milky Way, is composed of billions of stars and planets, dust and gas. According to school textbooks, everything works like clockwork; stars are born from clouds of gas (known as nebulae) and the disk of gas and dust surrounding newborn stars agglomerate to build the planets.
But, like any scientific field, our understanding of the galaxy gets reshaped and modified whenever we send a new space telescope into orbit or attach new optics to a monster observatory. And last month, the well-established idea that a planet needs a star to exist was turned upside-down. What’s more, by removing the necessity for a star, we may have stumbled on an interesting solution to interstellar travel.
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In February, a fascinating paper was published in the Monthly Notices of the Royal Astronomical Society detailing calculations on how many “nomad planets” the Milky Way must contain after estimating our galaxy’s mass from how much gravity it exerts on surrounding space. Scientists from the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) had uncovered something surprising – there are likely many more planets in the Milky Way than stars. In fact, this may not sound surprising at all; NASA’s Kepler space telescope has uncovered many multi-planetary systems not too dislike our Solar System – logic dictates that if most stars have planets orbiting them, many will have multiple planets orbiting them. But Louis Strigari and his Kavli team calculated that there must be 100,000 planets for every star in the Milky Way. That’s a lot of planets!
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But how can this be? Every star can’t have tens of thousands of planets ranging from Pluto-sized to Jupiter-sized. This planetary “excess” actually suggests the existence of planets that were born without a star – nomad planets. These planetary vagabonds somehow went through the planet-forming process in interstellar space, not in the dusty proto-planetary disk surrounding a young star.
This astonishing number was calculated by extrapolating a dozen “microlensing” events of nomad worlds passing in front of distant stars. When these nomad planets drifted in front of distant stars, they briefly focused the starlight with their gravity, causing the star to brighten. This brightening was captured by astronomers and the microlensing events could be analysed to reveal the characteristics of the nomad planets.
But that’s not all that lurks in interstellar space. The nomad planet’s massive cousin, the brown dwarf, is also out there. And fortunately, they’re easier to detect.
Not a star, not a planet
Brown dwarfs underwent star formation processes but didn’t get massive enough to allow nuclear fusion to be sustained in their cores. Nuclear fusion is maintained by the constant inward gravitational force of a star’s mass, crushing elements like hydrogen and helium together. The atoms fuse, generating vast amounts of energy, making a star (like our Sun) shine. However, in the case of a brown dwarf enough mass to form a massive ball of churning gas was accrued, with some of the characteristics of a star, but it didn’t get massive enough to allow sustained fusion in its core. It is therefore stuck in a stellar hinterland – it can’t be described as a star, and it can’t be described as a planet. It’s therefore a “failed star”. (Or an “overachieving planet”, depending on which way you look at it.)
To astronomers, brown dwarfs are a fascinating piece in the stellar evolution puzzle. At what point does a planet, like Jupiter, need to grow before it triggers fusion in its core to become a star? As it turns out, Jupiter would need 13 times more mass before its gravity can be strong enough to start a small amount of deuterium fusion (deuterium is a “heavy” isotope of hydrogen), at which point it can be considered to be a small brown dwarf. The largest brown dwarfs are thought to weigh-in at around 80 Jupiter masses – the point at which sustained nuclear fusion in the core will ignite the birth of a small star.
Although brown dwarfs are stellar failures and do not shine brightly in huge quantities of radiation like our Sun, they do generate heat and therefore produce infrared radiation. These glowing objects can be spotted by powerful space telescopes like NASA’s Wide-Field Infrared Survey Explorer (WISE) that is currently surveying the skies for infrared objects. As it turns out, there are many brown dwarfs floating around in interstellar space.
Interstellar stepping stones?
So far, the closest brown dwarfs orbiting other stars are 12 light-years from Earth. But in 2011, two more “close” brown dwarfs were spotted floating in interstellar space, approximately 15 and 18 light-years away. These discoveries have sparked some interesting ideas for interstellar travel. As the discoveries of cooler brown dwarfs increase, it is looking increasingly likely that “ultracool” brown dwarfs could be scattered between our Sun and the next-nearest star, Proxima Centauri, some 4 light-years from Earth.
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As discussed in a previous Al Jazeera English article, we need all the help we can get if we are to venture to another star, so these ultracool brown dwarfs could become much-needed “stepping stones” for future starships to refuel on their light-years of journey time. There may be the possibility that these sub-stellar objects may even become more desirable targets for interstellar travellers. After all, there may be dozens of these invisible objects between here and Proxima just waiting to be uncovered by the sophisticated infrared telescopes of the future; they’d certainly make for more accessible scientific curiosities.
What of the huge excess of interstellar nomad planets? As they are a lot smaller than brown dwarfs, they will be harder targets for infrared telescopes to detect, but there are likely more planets between here and Proxima than ultracool brown dwarfs. So one of the first tasks of NASA’s Wide-Field Infrared Survey Telescope (WFIRST) – due for launch in the mid-2020s – will likely be to take a headcount of these nomad worlds.
Perhaps in the future, we’ll discover that our galaxy doesn’t just contain stars with planets orbiting them. Perhaps the majority of planets actually roam through interstellar space with no star to call “home” and brown dwarfs, far from being failed stars, could become starship rest stops.
Even more profound than that, perhaps nomad planets hold the key to finding out how diverse life can be. As hypothesised by Louis Strigari: “If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist.”
Follow him on Twitter: @astroengine