In decades of searching, NASA has visually confirmed 5,044 planets beyond our own solar system. Finding all these exoplanets has reshaped our understanding of the Milky Way galaxy. The more planets there are, the less unusual our own planet is—and the less likely it is that we are unique. More planets means more places where alien life might thrive now, or might have thrived in the past.
Those 5,000 “exoplanets” within a radius of 28,000 light-years from Earth—the farthest our telescopes can see planets with meaningful fidelity—might be just the tip of the cosmological iceberg. There are undoubtedly countless planets, potentially a hundred billion just in our galaxy, that are too far away for us to see with even our best telescopes.
But it’s possible there are also hundreds of planets hiding unseen in star systems that are close enough for us to directly observe. A team led by UCLA astronomer Thea Faridani has proposed one way of determining where these “hidden companion” exoplanets might be—without actually laying eyes on them. The technique involves calculating the possible effect of their gravity on the visible planets in their system.
ADVERTISEMENT
Our current planetary survey methods—either directly observing a planet through a telescope or looking for a planet’s silhouette as it crosses between us and its star—“can hide small, close-in planets or far-orbiting (big or small) companions,” Faridani and their coauthors explained in a new peer-reviewed study that’s been accepted for publication in The Astrophysical Journal.
The small, close-in planets are too small to register as a silhouette while passing in front of their star. Meanwhile, the far-orbiting planets are so far from their stars that they’re permanently cloaked in darkness. But “these planets can still exert dynamical influence on known planets,” Faridani’s team wrote.
A better understanding of this influence could help us determine which of the thousands of star systems that are within range of our telescopes are likeliest to hide unseen companion planets. “Both close-in planets and the farther-away companions could be candidate planets to find in follow-up observation campaigns, according to this method,” Lingfeng Wei, an astronomer at University of California, San Diego and one of Faridani’s co-authors, told The Daily Beast.
Any potential hidden planets in the water-friendly “habitable zone” of these systems—places that are close enough to be warmed by the star but not so close that the star bakes it dry—would be “good candidates for SETI,” the search for extraterrestrial intelligence, Wei said.
To be clear, this approach isn’t new. For centuries, astronomers have taken gravity into account as they’ve mapped the cosmos, populating their charts with both objects they could and couldn’t see, but assumed were present. They’ve posited the presence of unseen planets by gauging these planets’ effects on the orbits of planets that are visible. An unseen planet can stretch a visible planet’s orbit or even steady it, contributing to the long-term stability of a star system.
The new study combines and refines existing models into a useful guide focused on a particular set of potential planets. The effects of gravity are more obvious with close-in planets that orbit around their stars (their “period”) in a day or less, compared to the 365 days it takes the Earth to orbit the Sun. That’s because gravitational effects are stronger between objects that are closer together.
So the closer hidden companions are a good place to start as we try to get a better count of possible exoplanets. “Our paper aims to motivate the search for ultra-short-period planets,” Faridani and co-author Smadar Naoz, a UCLA astronomer, told The Daily Beast in a joint statement.
The math in Faridani’s study is… complicated. It takes into account angular momentum exchange—or the tendency of one spinning body to speed up as a neighboring body slows down. It also factors in orbital “eccentricity.” This is the way a planet’s circular orbit elongates and becomes more oval as another planet’s gravity tugs on it.
The result is a set of criteria astronomers can apply to a faraway star system to make an educated guess whether the system includes any unseen planets.
“This seems like an interesting contribution,” Étienne Artigau, a Université de Montréal astrophysicist who isn’t involved in the study, told The Daily Beast. “The authors basically put together a number of concepts that were already known, and determined more general criteria that can be used by teams that discover planets.”
To test their criteria, Faridani and their coauthors picked a handful of relatively nearby systems and checked their math. They analyzed HD 15337, a system just 146 light-years from Earth that has an orange dwarf star and two planets, both of which are around eight times more massive than our own planet. Faridani and company then added a hypothetical, Earth-size planet close to the HD 15337 star—and concluded that the system’s orbital dynamics made sense with the extra planet.
That doesn’t mean HD 15337 definitely has a hidden-companion planet. But it does mean HD 15337 could have a hidden companion—and should be at the top of the list for a closer look once our telescopes are better. “To confirm the existence of suspected exoplanets we do need higher-resolution observations,” Wei said.
NASA’s new James Webb Space Telescope and other super-sensitive orbital telescopes that are in development are “a good start,” Naoz and Faridani said. It might also be possible to “stack” observations, they added. That is, compile images from a bunch of different telescopes and other instruments. One telescope might be precise where another is imprecise. Lumping the images together allows instruments to compensate for each other.
With access to better and better survey technology and a handy guidebook to the star systems that are likeliest to hide planets, astronomers are poised to add potentially hundreds or even thousands of exoplanets to the current tally, without looking any farther away. And every new planet we confirm is a new planet we could scrutinize for signs of life.
