In decades of research, NASA has visually confirmed 5,044 planets beyond our own solar system. Finding all these exoplanets has reshaped our understanding of the Milky Way. 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 alien life can thrive now, or may have thrived in the past.
These 5,000 “exoplanets” within a radius of 28,000 light-years from Earth – the farthest our telescopes can see planets with significant fidelity – may just be the tip of the cosmological iceberg. There are undoubtedly countless planets, potentially a hundred billion in our galaxy alone, that are too far away to be seen even with our best telescopes.
But it is possible that there are also hundreds of invisible hidden planets in star systems that are close enough that we can observe it directly. A team led by UCLA astronomer Thea Faridani has proposed a way to determine where these “hidden companions” exoplanets might be — without actually laying eyes on them. The technique involves calculating the possible effect of its gravity on the planets visible in its system.
Our current methods of planetary research – directly observing a planet through a telescope or looking for the silhouette of a planet as it intersects between us and its star – “can hide small and close planets or distant orbiting companions (large or small)” Faridani and her coauthors explained in a new peer-reviewed study that was accepted for publication in O Astrophysical Journal.
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The small, nearby planets are too small to be recorded as a silhouette as they pass in front of their star. Meanwhile, planets in distant orbit are until here of their stars that they are permanently shrouded in darkness. But “these planets can still exert dynamic influence on known planets,” wrote Faridani’s team.
A better understanding of this influence can help us determine which of the thousands of star systems within range of our telescopes are most likely to hide unseen companion planets. “Both nearby planets and more distant companions can be candidate planets to be found in follow-up observation campaigns, according to this method,” Lingfeng Wei, an astronomer at the University of California, San Diego and one of Faridani’s co-authors. , told The Daily Beast.
Any potential hidden planets in these systems’ water-friendly “habitable zone” — places close enough to be warmed by the star, but not so close to the dry star — 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 could not see but assumed were present. 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, contributing to the long-term stability of a star system.
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The new study combines and refines existing models into a helpful guide focused on a specific set of potential planets. Gravity’s effects are most obvious with nearby planets that orbit their stars (their “period”) in a day or less, compared to the 365 days it takes Earth to orbit the Sun. That’s because gravitational effects are stronger between objects that are closer together.
So the closest 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 ultrashort-period planets,” Faridani and co-author Smadar Naoz, an astronomer at UCLA, told The Daily Beast in a joint statement.
The math in Faridani’s study is… complicated. It takes into account the change in angular momentum – or the tendency of a rotating body to accelerate as a neighboring body decelerates. 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 guess whether the system includes invisible planets.
“This appears to be an interesting contribution,” Étienne Artigau, an astrophysicist at the Université de Montréal who was not involved in the study, told The Daily Beast. “The authors basically took a number of concepts that were already known and determined more general criteria that can be used by teams discovering planets.”
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To test their criteria, Faridani and his coauthors chose a handful of relatively closely related 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 about eight times more massive than our own planet. Faridani and company then added a hypothetical Earth-sized planet close to the star HD 15337 — and concluded that the system’s orbital dynamics made sense with the extra planet.
This does not mean that HD 15337 definitely has a hidden companion planet. But that means HD 15337 could have a hidden companion – and it should be at the top of the list for a closer look when our telescopes get better. “To confirm the existence of suspected exoplanets, we need high-resolution observations,” Wei said.
NASA’s new James Webb Space Telescope and other supersensitive orbital telescopes that are in development are “a good start,” Naoz and Faridani said. It may also be possible to “stack” observations, they added. That is, compile images from several different telescopes and other instruments. A telescope can be need where else is inaccurate. Grouping the images allows the instruments to offset each other.
With access to ever-improving research technology and a practical guide to the star systems most likely to hide planets, astronomers are ready to potentially add hundreds or even thousands of exoplanets to the current count, looking no further. And every new planet we confirm is a new planet that we can examine for signs of life.
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