Not all Planets orbit stars. Some planets have been kicked out of their star system and now roam interstellar space without a stellar companion. Recent research suggests that these Rogue Planets may actually outnumber stars in our galaxy by a factor of 20. The upcoming Nancy Grace Roman Telescope is expected to greatly contribute to our understanding of these intriguing worlds, with astronomers predicting that it will discover around 400 Earth-mass rogue planets.

According to David Bennett, a senior research scientist at NASA’s Goddard Space Flight Center, our galaxy could be home to trillions of rogue planets, vastly outnumbering the stars. This groundbreaking estimate is the first measurement of the number of rogue planets that is sensitive to planets smaller than Earth.

Astronomers have determined that low-mass rocky worlds might be the most common type of rogue planet. Due to their smaller size, they are more easily expelled from their star systems. So far, only one of these terrestrial rogue planets has been observed. However, a nine-year survey has provided two significant findings: the second-ever observation of a terrestrial rogue planet and an estimate of how many similar planets the Roman Telescope will be able to detect once it becomes operational.

Finding rogue planets is a challenging task because they do not emit light and are thus difficult to detect. The transit method, which relies on the planet passing in front of its star and causing a small dip in brightness, is the usual approach for planet discovery. However, astronomers must rely on a fortuitous alignment to detect rogue planets. If a rogue planet passes in front of a star from our line of sight, it will cause a slight increase in the star’s brightness, a phenomenon known as microlensing.

The discovery of rogue planets through microlensing is a one-off event since we can only study them when they pass in front of stars. Despite this limitation, the transit duration can last from a few hours to a day, allowing astronomers a brief yet valuable opportunity to study these distant worlds. Professor Takahiro Sumi, the lead author of the research estimating the number of rogue planets in our galaxy, stated that microlensing is essential for finding low-mass free-floating planets and other elusive objects, such as primordial black holes.

The Roman Telescope will focus its search for rogue planets towards the center of our galaxy, where there are numerous stars, providing more opportunities for a rogue planet to pass in front of one of them. If a rogue planet is detected, ground-based instruments like Japan’s PRIME (Prime-focus Infrared Microlensing Experiment) telescope will follow up with further observations.

The Roman Telescope, being a space-based instrument, will have the advantage of being able to detect even lower-mass rogue planets. Its wide field of view and superior resolution will enable detailed studies of these objects, which would not be possible with ground-based telescopes alone. This prospect is truly exciting for Naoki Koshimoto, the leader of the team that announced the discovery of a second candidate terrestrial-mass rogue planet.

The findings of these two studies will be published in upcoming issues of The Astronomical Journal. With the advancements in technology and the launch of the Roman Telescope expected in 2027, our understanding of rogue planets is set to expand significantly, shedding light on the mysteries of these wandering worlds in our galaxy.

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