Getting To Know Rogue Planets

In our Earthling minds, planets exist in solar systems with a Sun in the middle and objects large and small orbiting around it.   This is hardly surprising since planets are pretty much exclusively illustrated in solar systems and, until the onset of the 21st century, no other kind of planet had been identified.

That changed in the last two decades with the discovery of “rogue planets” very large and now quite small — all apparently isolated object speeding through interstellar space and unattached to any Sun or solar system.

The earliest rogue planets identified were large Jupiter-type planets or even larger brown dwarfs, which have masses between that of a large Jupiter and a small star.

But since then smaller planets have been discovered while the estimated population size of rogue planets has ballooned.

Now, new research from NASA and Japan’s Osaka University suggests that rogue planets untethered to a star far outnumber planets that orbit stars in the Milky way.

These results imply that NASA’s Nancy Grace Roman Space Telescope, set to launch by 2027 with a goal to identify rogue planets, could find hundreds of the Earth-mass variety. Indeed, this new study has already identified one such candidate.

“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, and a co-author of two papers describing the results.

“We found that Earth-size rogues are more common than more massive ones,” said Takahiro Sumi, a professor at Osaka University in Japan and lead author of the paper with a new estimate of our galaxy’s rogue planets.

The roughly Earth-mass rogue planet the team found marks the second discovery of its kind. The paper describing the finding will appear in a future issue of The Astronomical Journal. A second paper, which presents a demographic analysis that concludes that rogue planets are six times more abundant than worlds that orbit stars in our galaxy, will be published in the same journal.

Because of their location outside of solar systems and away from a warming Sun, rogue planets are considered quite unlikely to harbor life.  But with that diverse population of rogue planets, Sumi said,  they can play an important role in understanding planets generally.

“The difference in star-bound and free-floating planets’ average masses,” he said in a NASA release, “holds a key to understanding planetary formation mechanisms.”

Rogue planets, also known as interstellar, free-floating, nomad or orphan planets,  wander through space without any known orbit or gravitational relationship with other celestial objects.

Many originate in solar systems where they are formed and later ejected during the chaotic early times, when forming bodies gravitationally interacted as they settle into their orbits.

Smaller planets aren’t tethered as strongly to their star so some of these interactions end up flinging these worlds off into space.  They then travel through interstellar space at speeds estimated to be in the millions of miles per hour.

They can also form on their own, outside a planetary system. Some planetary-mass objects may have formed in a similar way to stars but were not massive enough to sustain nuclear fusion of ordinary hydrogen (H) into helium.  The International Astronomical Union has proposed that these objects be called sub-brown dwarfs.

Many of the rogue objected detected so far are these failed stars, but that has been a function of the limits of telescope observing power rather than the absence of small and rocky planets.

Typically, planets outside our solar system are detected by the effects they have on their host stars. For instance, an exoplanet can cause Earth-based viewers to witness a drop in its star’s light as the planet’s trajectory takes it between the star and our planet. Or, an exoplanet can affect such light through a wobble it creates in the orbit of its host star while gravitationally tugging on the glowing body.

But the fact that rogue planets, especially smaller ones,  are so far away from host stars makes them impossible to identify using these methods.

This is where the NASA’s Nancy Grace Roman Telescope comes in.  Observing rogue planets is considered one of the important goals of the Roman mission.

“Roman will be sensitive to even lower-mass rogue planets since it will observe from space,” said Naoki Koshimoto, lead author of the other study and an assistant professor at Osaka University, in a statement.

“The combination of Roman’s wide view and sharp vision will allow us to study the objects it finds in more detail than we can do using only ground-based telescopes, which is a thrilling prospect.”

Scientists had previously estimated that Roman would be capable of finding about 50 Earth-sized rogue planets.  The findings from the new studies have increased that estimate to 400.

The nearest rogue planet to our Earth is WISE-0855-0714. It was discovered in 2014 and is approximately 7.2 light-years away from us. The mass of this planet is around 10 times of Jupiter.

This animation illustrates the concept of gravitational microlensing. When one star in the sky appears to pass nearly in front of another, the light rays of the background source star become bent due to the warped space-time around the foreground star. This star is then a virtual magnifying glass, amplifying the brightness of the background source star, so we refer to the foreground star as the lens star. If the lens star harbors a planetary system, then those planets can also act as lenses, each one producing a short deviation in the brightness of the source. Thus we discover the presence of exoplanets, and measure its mass and separation from its star. (NASA’s Goddard Space Flight Center/CI Lab)

The team’s findings stem from a nine-year survey called MOA (Microlensing Observations in Astrophysics), conducted at the Mount John University Observatory in New Zealand.

Microlensing events occur when an object such as a star or planet comes into near-perfect alignment with an unrelated background star from our vantage point. Because anything with mass warps the fabric of space-time, light from the distant star bends around the nearer object as it passes close by.  This phenomenon was predicted a century ago by Einstein’s theory of general relativity.

The nearer object acts as a natural lens, creating a brief spike in the brightness of the background star’s light that gives astronomers clues about the intervening object that they can’t get any other way.

“Microlensing is the only way we can find objects like low-mass free-floating planets and even primordial black holes,” said Takahiro Sumi, a professor at Osaka University, and lead author of the paper with a new estimate of our galaxy’s rogue planets. “It’s very exciting to use gravity to discover objects we could never hope to see directly.”

The roughly Earth-mass rogue planet the team found marks the second discovery of its kind. The paper describing the finding will appear in a future issue of The Astronomical Journal. A second paper, which presents a demographic analysis that concludes that rogue planets are six times more abundant than worlds that orbit stars in our galaxy, will be published in the same journal.

As recounted in the NASA,  an early episodes of the original Star Trek serie sfeatured an encountered with one lone planet amid a ‘star desert.” The crew was surprised to find the fictional Gothos, the starless planet, habitable.

But the rogue planet team of scientists emphasized that the newly detected “rogue Earth”  does not share characteristics with our planet beyond a similar mass.

The Nancy Grace Roman Space Telescope is a NASA observatory designed to unravel the secrets of dark energy and dark matter, search for and image exoplanets, and explore many topics in infrared astrophysics.

The space observatory has a 2.4 meter telescope, the same size as Hubble’s but with a view 100 times greater than Hubble’s.  It is named after the woman known to many as the “Mother of Hubble” for her foundational role in the observatory’s planning and program structure.

Although the telescope is the same size as the iconic Hubble, its Wide Field Instrument will have a field of view that is 100 times greater than the Hubble infrared instrument, capturing more of the sky with less observing time.

As the primary instrument, the Wide Field Instrument will measure light from a billion galaxies over the course of the mission lifetime. It will perform a microlensing survey of the inner Milky Way to find about 2,600 exoplanets. The Coronagraph Instrument technology demonstration will perform high contrast imaging and spectroscopy of individual nearby exoplanets.

Each microlensing event is a one-time occurrence, meaning astronomers can’t go back and repeat the observations once they’re over. But they’re not instantaneous.

Koshimoto said in a NASA release that “a microlensing signal from a rogue planet can take from a few hours up to about a day, so astronomers will have a chance to do simultaneous observations with Roman and PRIME,” the Japanese (Prime-focus Infrared Microlensing Experiment) telescope, located at the South African Astronomical Observatory.

Seeing them from both Earth and Roman’s location a million miles away in space will help scientists measure the masses of rogue planets much more accurately than ever before.