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This illustration shows HD 189733b, a huge gas giant that orbits very close to its host star HD 189733. The planet’s atmosphere is scorching with a temperature of over 1000 degrees Celsius, and it rains glass, sideways, in howling 7000 kilometre-per-hour winds. At a distance of 63 light-years from us, this turbulent alien world is one of the nearest exoplanets to Earth that can be seen crossing the face of its star. By observing this planet before, during, and after it disappeared behind its host star during orbit, astronomers were able to deduce that HD 189733b is a deep, azure blue — reminiscent of Earth’s colour as seen from space. Image and words: NASA

 

Not many years ago we wondered whether there were any other possible Earth-like planets out there. “Were we alone?” was the great question of the astronomical age.

And it remains the same today in many ways – except for one extraordinary change in our assessment. The Earth is in fact one of many such planets in the Milky Way Galaxy, the swarm of some 200 billion stars that revolve around a central core of densely-packed stars and ultra-dense black-holes.

Over the last ten or so years we had come to realise that our own Sun is a fairly non-descript sort of star in the big scheme of things; as we observed other stars, we could see that they also had planets. We could tell this by the way in which the light from those stars was diminished very slightly as a planet passed in front of it as we watched it. Soon we had confirmation that hundreds, and then thousands, of stars were surrounded by their own “solar-systems “of orbiting planets.

Soon, the thousands turned into millions. Today, with the latest satellites providing extraordinary levels of detail, we are fairly certain that there are in fact between 500 billion and a trillion planets in our galaxy. What is more, not all are attached to a star!

Recent findings show that there are many “unbound”planets that have been cast off their parent-star at some point. This can happen for many reasons: some stars explode, others pass by another star too closely and their planets are “stolen” – but do not achieve orbit with the new start: and so on. So how many of these wandering planets are out there? About 50 billion!

The estimate is based on observation of the proportion of stars that might form these free-floating planets. Computer simulations indicate that that between 25 and 75% of solar system do so. It is also the result of studies of a Deep Space Object called the Orion Nebula, or M42. This is a massive cloud of dust amongst which new stars are being formed in what is called a Nursery.

 

Image of the Orion Nebula, an region of dust out of which stars are formed. At night, this bright spot may be seen as the middle point of light in Orion’s sword below the belt. The bright light is in fact the centre of the star-forming area, the light from which is reflected on the surrounding clouds of dust and gas. It is a favourite target for amateur astro-imagers; a reasonable, modern telescope and camera can produce images comparable to this view taken from Hubble, though Hubble can of course “zoom-in” to provide astonishing levels of detail.

 

Scientists observed some 1500 stars in the area of the Nebula and then simulated the way in which the solar systems form. They then simulated some 2500 planets orbiting 500 stars and concluded that about 350 will break out of orbit and become free-floating or unbound. 80% will then go on to leave the cluster around Orion altogether, doomed to wander the cold vastness of space between the stars for billions of years.

It is hard to see any of these planets supporting life as they will be so far away from the warmth and light of a host star such as our own Sun.  They might eventually become sucked into another system – but in the process of doing so could cause chaos in a previously stable group of planets.

There is some speculation that our own Solar System is harbouring one of these planets, as there are signs that there is a planet “out there” somewhere in a hugely  elliptical orbit of the Sun, coming into the centre of the system every three or four thousand years. As it does so, it is almost bound to cause a wobble or two to all other planets in the system. Some think it might have been the cause of catastrophes in ancient times, as is recorded in religious mythology down through the ages.

Sadly – or otherwise – none of us alive today is going to see the “Ninth Planet”. The timescales for all such events are so vast that the next time it appears (if it is out there), might be in a thousand or two years.

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