Do 'rogue planets' outnumber stars in the Milky Way? NASA's upcoming mission will provide answers

The Nancy Grace Roman Space Telescope will look for these solo and elusive worlds that do not orbit stars and are hard to detect

                            Do 'rogue planets' outnumber stars in the Milky Way? NASA's upcoming mission will provide answers
This illustration shows a rogue planet traveling through space. (NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)

There is a lot of a mystery surrounding "rogue planets" - solo and wandering worlds that do not orbit stars. Astronomers do not have a record on their numbers in the Milky Way galaxy. Neither do they have conclusive answers on their origins. 

However, there are some theories. One suggests that rogue planets could outnumber stars in the Milky Way galaxy. Another predicts these floating objects were once orbiting young stars before getting ousted. None of them are confirmed. "We don’t know if rogue planets outnumber stars in our galaxy," Dr. Scott Gaudi, a professor of astronomy from Ohio State University, told MEA WorldWide (MEAWW). "We know they could, but we do not have answers yet," he added.

Enter NASA's upcoming mission called Nancy Grace Roman Space Telescope. It aims to provide a census of rogue planets in the Milky Way. It is also capable of testing theories on the origins of the elusive wandering worlds, according to a new study. What is more, "rogue planets give us a unique window into studying planet formation and star formation,” Samson Johnson, an astronomy graduate student from Ohio State and lead author of the study, told MEAWW. “Imagine our little rocky planet just floating freely in space – that’s what this mission will help us find."

The mission is named after Nancy Grace Roman, a noted American astronomer considered the “mother” of NASA’s Hubble Space Telescope. “It is because of Nancy Grace Roman’s leadership and vision that NASA became a pioneer in astrophysics and launched Hubble, the world’s most powerful and productive space telescope,” NASA Administrator Jim Bridenstine, said in a statement. It is scheduled for launch in the next five years.

An artist's rendering of the Nancy Grace Roman Telescope. (NASA)

Questions about their origins

Planet formation is likely messy, which means there is a possibility of rogue planets getting separated from their parent stars. "So, it possible that most stars eject one or more planets during the planet formation process."  The resulting matter -- rogue planets -- may have masses similar to Mars or Earth, Johnson explained.

Explaining further, Johnson said, assuming that all stars form planets and if each stellar body loses one planet during formation, we are likely to have an equal number of rogue planets and stars. "This would be an extreme case and is just an example to say that it would not take a lot to get to a greater number of rogue planets than stars," he added. Alternatively, rogue planets could form on their own, like stars. "This formation process would likely produce objects with masses similar to Jupiter, roughly a few hundred times that of the Earth," Johnson explained.

Why are they elusive?

According to Dr. Gaudi, astronomers rely on light to view objects in space. But rogue planets are cold and very faint, making them elusive, the researchers explained. These bodies are unlikely to host life. Reports suggest that the Milky Way is home to a dozen of wandering planets. It is here that Roman finds applications as it doesn’t require an object to emit light. Additionally, the telescope will probe dark energy and exoplanets.

Gravitational microlensing can help detect rogue planets (NASA)

Microlensing will help shed light on rogue planets

The Roman telescope can use microlensing to study rogue planets. "The best method to detect isolated, low-mass objects such as rogue planets is gravitational microlensing," Johnson said. It will help astronomers identify faint and distant objects in the Milky Way. 

How does that work? Astronomers will be able to detect a rogue planet if it passes in front of a distant star. When that happens, light from the stellar body bends as it passes through the warped space-time around the target and reaches the telescope. In other words, the wandering planet acts as a cosmic magnifying glass, boosting the brightness of the background star. If the telescope detects many low-mass rogue planets, it could help experts develop a better understanding of how planets form. "We'll know that as stars form planets, they're probably ejecting a bunch of other stuff out into the galaxy," he said. 

The study is published in The Astronomical Journal.

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