Up until only a few decades ago, every planet we knew existed in the Universe as well as all asteroids and comets, orbited the Sun.

Since the first planet outside the Solar System orbiting another main-sequence star was made in 1995 – or Exoplanet  – was confirmed , there been some 5,470 exoplanets found.  Exoplanets have been found in a bewildering array of sizes, host star types and star system configurations.

(If you’re interested on keep track of the numbers of Exoplanets, the current count can be found at the NASA Exoplanet site here.)

These have been discovered using a wide variety of methods, both here on the ground or using space-based instruments. Exoplanets are generally discovered by indirect means, IE the effect they have on their host star. Some were found via the light dip of the host star as the Exoplanet transits in front of it, others by the wobble the orbiting planets caused the star, variations Exoplanets cause to variable stars and other techniques.

Even the arrangement of Exoplanets varies greatly. The original theories of rocky planets being close to the star and gas giants being on the edges of a star system quickly went out the windows as large, gas giants in orbits close to their host star were discovered in great numbers. It’s looking like the configuration of the Solar System – rocky planets in near the host star and all gas giants in the outer regions – might be the least common of all (Read more here.)

(One quick point though. I often get asked if you can see Exoplanets in a telescope. The short is answer is NO.)

But all Exoplanet discoveries until recently had one thing in common – they were in orbit around a host star. Could there be planets in the Milky Way that aren’t in orbit around a host star? Could there be Rogue planets in the Milky Way? Even if the answer is “yes”, they might be tricky to detect as Exoplanets are identified by the effects on their host star, how could planets without a star be found?

Why would a planet go Rogue?

Astronomers have long proposed the idea of a Rogue Planet. This is a planet that is travelling through the Milky Way on its own and not in orbit around a host star.  There’s a number of ways a planet could end up like this. It might achieve escape velocity in the star system where it was formed by gravitational interactions with other planets in the same system. In other words, it could get “flung”” put of its star system.  Planets form out of the interstellar materials that are attracted by gravity to form stars. The same process could occur with enough rocky materials bound together by gravity away from star formation. (There’s some conjecture as to whether extremely low mass Rogue planets could even form on their own in this way.)

How could a Rogue Planet be found?

With no host star, Rogue planets cannot be observed using the indirect detection methods mentioned above. They’re also too far away from any star to reflect light from it. (This is how was see Jupiter and Saturn in our own Solar System. We see the light of the Sun reflected off them.) There’s no light coming from Rogue Planets. There’s also nothing nearby to effect so we can’t observe their influence on a nearby body.

Rather, astronomers look for events called gravitational microlensing. The minute effect the gravity of an object in the foreground has on the space around it, bending space and effecting the path of the light of a star behind it. This bending of light around objects was proposed by Albert Einstein at the start of the 20th century.  Astronomers observe a tiny warping in the light from stars. By analysing these results, it’s possible to detect whether a body such as a Rogue planet has passed in front of the star, possibly even at vast distances from it.

Only a handful of Rogue planets have been confirmed compared to the number of Exoplanets, including a group of 70 discovered by ESO which was announced last year.

Artist’s impression shows an example of a rogue planet with the Rho Ophiuchi cloud 


Much like Exoplanets which are though to exist around most stars, it’s estimated Rogue planets are travelling through the Solar System in enormous numbers.

“We estimate that our galaxy is home to 20 times more rogue planets than stars – trillions of worlds wandering alone,” said David Bennett, a senior research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

This means that the number of Rogue planets vastly the outnumber the planets that orbit stars. This points to planetary systems forming around stars more likely to eject planets than keeping them in orbit around them.

The upcoming NASA’s Nancy Grace Roman Space Telescope, due for launch in 2027, might even be locate approx. 400 Earth-sized Rogue planets, with two candidates already discovered.

“We found that Earth-size rogues are more common than more massive ones,” Sumi said. “The difference in star-bound and free-floating planets’ average masses holds a key to understanding planetary formation mechanisms.” You can read more about what the new space telescope might find here.

Do we have direct observation of bodies are travelling through the Solar System that aren’t attached to a star?

We sure do as three interstellar bodies have been detected in the Solar System. Discovered in 2017, “Oumuamua” cruised through the inner Solar System and is now on the way back out towards deep-space. Interstellar comet “Borisov” disintegrated as it went around the Sun in 2020 and an interstellar meteor with the funky name of CNEOS 2014-01-08 hit the Earth in 2014. (There’s a controversial effort underway to recover fragments of this meteor from the ocean floor off the coast of New Guinea.)

As always, there’s so much to discover in our Universe!


Earl White


July 20th July 2023



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