In decades of research, NASA has visually confirmed 5,044 planets beyond our own solar system. The discovery of 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 we are to be unique. More planets means more places where extraterrestrial life could thrive now, or might have thrived in the past.
Those 5,000 “exoplanets” within 28,000 light years of Earth – the furthest our telescopes can see planets with any significant fidelity – might just be the tip of the cosmological iceberg. There are undoubtedly countless planets, potentially a hundred billion in our galaxy alone, that are too distant for us to see even with our best telescopes.
But it is possible that there are also hundreds of planets hidden in star systems that are close enough for us to observe it directly. A team led by UCLA astronomer Thea Faridani has proposed a way to determine where these “hidden companion” exoplanets might be, without seeing them. The technique consists of calculating the possible effect of their gravity on the visible planets of their system.
Our current methods of planetary study – either by directly observing a planet through a telescope or by looking for a planet’s silhouette as it intersects between us and its star – “may hide nearby smaller planets or companions in distant orbit (large or small)”. Faridani and their coauthors explained in a new peer-reviewed study that has been accepted for publication in The Astrophysical Journal.
Nearby minor planets are too small to be recorded as a silhouette as they pass in front of their star. Meanwhile, planets in distant orbits are so far of their stars that they are permanently shrouded in darkness. But “these planets can still exert a dynamic 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 lie within reach of our telescopes are most likely to hide invisible companion planets. “Near planets and more distant companions could be candidate planets to find in follow-up observation campaigns, according to this method,” said Lingfeng Wei, an astronomer at the University of California, San Diego and one of the co-authors of Faridani. , told The Daily Beast.
Any potential hidden planets in the water-friendly “habitable zone” of these systems – places close enough to be warmed by the star but not then nearly the star bakes it dry — would be “good candidates for SETI,” the search for extraterrestrial intelligence, Wei said.
To be clear, this approach is not new. For centuries, astronomers have taken gravity into account when mapping the cosmos, filling their maps with objects they could and couldn’t see, but assumed were there. They postulated the presence of invisible planets by measuring the effects of these planets on the orbits of planets that are visible. An invisible planet can stretch the orbit of a visible planet or even stabilize it, thus 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 evident with nearby planets orbiting their stars (their “period”) in a day or less, compared to the 365 days it takes Earth to orbit the Sun. This is because gravitational effects are stronger between objects closer together.
Hidden Closest Companions is therefore a good starting point as we try to get as many exoplanets as possible. “Our paper aims to motivate the search for ultrashort-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 the exchange of angular momentum – or the tendency of a rotating body to speed up while a neighboring body slows down. It also takes into account the orbital “eccentricity”. This is how a planet’s circular orbit lengthens and becomes more oval as another planet’s gravity pulls on it.
The result is a set of criteria that astronomers can apply to a distant star system to make an educated guess whether the system includes unseen planets.
“It seems like an interesting contribution,” Étienne Artigau, a University of Montreal astrophysicist who is not involved in the study, told The Daily Beast. “The authors essentially pulled together a number of already known concepts and determined more general criteria that can be used by teams discovering planets.”
To test their criteria, Faridani and their co-authors chose a handful of relatively close systems and checked their calculations. They analyzed HD 15337, a system just 146 light-years from Earth that has an orange dwarf star and two planets, both about eight times more massive than our own planet. Faridani and company then added a hypothetical Earth-sized planet near the star HD 15337 and concluded that the system’s orbital dynamics made sense with the extra planet.
That doesn’t mean that HD 15337 definitely has a hidden companion planet. But it does means HD 15337 could have a hidden companion – and should be high on the list for further consideration once our telescopes get better. “To confirm the existence of suspicious exoplanets, we need higher resolution observations,” Wei said.
NASA’s new James Webb Space Telescope and other super-sensitive orbital telescopes in development are “a good start,” Naoz and Faridani said. It might also be possible to “stack” the observations, they added. In other words, compile images from a bunch of different telescopes and other instruments. A telescope could be accurate where is another imprecise. Grouping the images allows the instruments to compensate for each other.
With access to increasingly better survey technology and a handy guide to the star systems most likely to hide planets, astronomers are poised to potentially add hundreds, if not thousands, of exoplanets to the current count, without looking any further. And each new planet that we confirm is a new planet that we could scan for signs of life.
