Multiple Earth-Mass Rogue Planets Have Been Discovered Drifting Through the Milky Way

Last year we reported on how the Roman Space Telescope’s backers hoped it would be able to detect rogue planets using a technique called “microlensing”.  Now, a team led by Iain McDonald, then at the University of Manchester, beat them to the punch by finding a few examples of Earth-sized rogue planets using data from an already aging space telescope – Kepler.

Both collecting and analyzing the data used in the study wasn’t easy though.  Kepler embarked on a two-month campaign in 2016 that had it looking at millions of stars located near the center of the Milky Way every 30 minutes.  Even with that much data, picking the signal from the noise was difficult.  

UT video explaining gravitational lensing, which microlensing is a smaller example of.

They are difficult because microlensing is exhibited by tiny fluctuations in the light of stars when an object passes in front of them.  According to Dr. McDonald, about every one in a million stars in the galaxy is subject to microlensing at any point in time.  So of the million of stars towards the center of the Milky Way, several could be undergoing microlensing right now.

Those events can last anywhere from minutes to days, as it depends on the difference between the foreground object and background stars, as well as the weight of the foreground object.  Of the many microlensing events that take place facing the galactic core, only approximately 1% of them are caused by rogue planets, and the signals from those events are much smaller when compared to microlenses caused by foreground stars.

Video showing what a microlensing event looks like from Earth.
Credit – David Specht / Eamonn Kerins / University of Manchester

Despite all the difficulties in collecting data with an old telescope, siphoning through all the additional data and background noise, and trying to differentiate between events caused by stars and those caused by planets, Dr. McDonald and his co-author, Eamonn Kerins were able to find 27 candidates for microlensing events. Of those, four could have potentially been caused by Earth-sized rogue planets.  

The durations of the events varied widely – from an hour and 10 days in length, and some had previously been detected using ground-based observation.  But the four that were indicative of Earth-sized exoplanets were completely novel.

UT Video discussing rogue planets.

Most likely, these new potential rogue planets will be the beginning of a wave of new discoveries.  Not only is Nancy Grace specifically designed to detect rogue planets using the technique in the paper (and which Kepler was very clearly not designed for), there is an ESA mission called Euclid, that will hopefully launch sometime next year, which is also tailored to search for microlensing events.

Speculation about rogue planets has existed for decades, and while we’re getting closer to definitively saying that they do exist, these microlensing events are only tentative proof. But if confirmed, they will have a dramatic impact on our understanding of both how planets form, but also our estimates of just how many might be lurking in the vast dark between stars.

Learn More:
RAS – Kepler telescope glimpses population of free-floating planets
SciTechDaily – Mysterious Population of Rogue Planets Spotted Near the Center of Our Galaxy
Gizmodo – Astronomers Spot a Batch of Rogue Planets Near the Galactic Core

Lead Image –
Artist’s conception of a rogue planet.
Credit – A. Stelter / Wikimedia Commons