*THE SOUTHWORTH PLANETARIUM 207-780-4249 <207-780-4249> www.usm.maine.edu/planet <http://www.usm.maine.edu/planet> 70 Falmouth Street Portland, Maine 04103 43.6667° N, 70.2667° W Founded January 1970* * "Putting the 'hyacinth' back in 'surreal.'"* *THE DAILY ASTRONOMER* *Tuesday. June 7. 2016* *Renegade Planets * We're rummaging though Pandora this week to answer the questions that have been languishing for longer than we intended. We love questions to the point of unhealthy obsession, so please send as many as you want. Only pesky time constraints cause us to neglect them. We are striving to empty Pandora's Jar so completely that only the residue of hope remains within it. This aim we will likely never achieve, much to our delight. Today, we address a subscriber's questions pertaining to renegade planets and their possible relation to asteroids. *If a renegade planet is hurtling at such a speed through a galaxy that has many stars and other planets, the likelihood of a renegade planet colliding with something has enough chance to bear merit right ? And if it collided into something, the collision could/would take away matter and mass, slowing it down and potentially sending it in a different direction than it was originally on a path for. Is it possible that some asteroids used to be renegade planets ? -Alyssa H, Standish, ME.* Greetings! Astronomers estimate that the Milky Way Galaxy might contain billions of renegade (also called 'rogue,' 'nomadic,' or 'starless') planets that are in orbit around the galactic nucleus and not gravitationally bound to a star. These planets might have once belonged to a larger system, but were propelled out of the system due to close planetary interactions. Also, some of these 'planets' might have formed from nebulae and are therefore labeled as "sub-brown dwarfs."* Unfortunately, most of these planets would be very difficult to detect as they are not reflecting starlight and, considering that they've been wandering around interstellar space for a long time, quite cold and dark. However, based on current estimates, let's assume that our galaxy might harbor about 200 billion rogue planets. This number is indeed substantial. Is a rogue planet collision probable? To answer this question, we must now calculate the density of rogue planets: the number of rogue planets within a given volume. A very rough calculation: Our galaxy's volume is about 20,000,000,000,000 cubic light years! By simple division, we can conclude that we could expect one rogue planet within 100 cubic light years of space! This equal to a sphere with a 2.9 light year radius. That is an immense volume for a single rogue planet to occupy! As an analogy, the distance separating rogue planets (and, for that matter stars) is equal to the distance separating five bees if they were scattered in different places along all of North America. The probabilities of a rogue planet collision, while not zero -hardly anything is- is still vanishingly small. This improbability notwithstanding, could the asteroids have been remnants of colliding rogue planets? Again, highly unlikely. In the first place, the combined mass of all the known asteroids is less than that of any designated planet. Although we tend to envision asteroid belts as being dense swarms of rocks, the asteroid belt in our solar system is quite diffuse. We cannot discount any possibility. And, it is certainly possible that colliding rogue planets would create smaller fragments, Although the energy involved in such collisions would likely vaporize most of the material, which would then form another world. This having all been said, your question was quite thoughtful and could describe what might happen to proximate rogue planets in other parts of the Universe. *The closest such object, named WISE 0855−0714, is only seven light years from Earth. **This rapid calculation does not take into account the variable stellar densities within the galaxy. More stars within the arms than in regions separating them, for instance.