The discovery of new exoplanets can help scientists understand how planets form and evolve.
Scientists from UNSW Sydney have located a potential new exoplanet – a planet that orbits a star outside of our solar system – using a technique known as 'transit timing variation'.
In research highlighted in a new paper, published today in The Astrophysical Journal, Scientia Senior Lecturer Ben Montet and PhD candidate Brendan McKee analysed changes in the timing of a known planet's transit across its star, to infer the presence of a second exoplanet.
After identifying an unusual trend in the movement of the hot Jupiter planet TOI-2818b, the team, from the UNSW School of Physics, ran a series of model simulations that pointed to the presence of a small planetary companion to the known hot Jupiter.
The new exoplanet is estimated to be 10-16 times the size of Earth, with a predicted orbital period less than 16 days.
"It's rare for hot Jupiters to have other planets near them," says Dr Montet. "So this new planet could have implications about how hot Jupiters form and in turn, help us to understand other solar systems."
Hunting for exoplanets
An exoplanet is any planet outside of our own solar system. Like the planets in our solar system orbit the sun, most exoplanets also orbit a star.
To date there are over 5500 known exoplanets confirmed by NASA, with trillions more predicted to exist within the Milky Way galaxy. Of the known exoplanets, there are approximately 500 known hot Jupiters – hot, gaseous exoplanets. Even lesser-known are companion planets to hot Jupiters – planets that orbit the same star as a hot Jupiter.
One method for hunting exoplanets, known as Transit Timing Variation (TTV), uses the movement of planets around their stars, which can affect the signal for the star's brightness.
"The planet passes in front of its star from where we see it on Earth, a bit like an eclipse, and it blocks some of its light," says Mr McKee. "And our records will show that the light emitted from the star will dip for a few hours as the planet travels in front of it. And we see those dips every single time a planet orbits."
Dr Montet describes the occurrence as like "the planet casting a shadow on the star, so it appears a little fainter".
Planets make good clocks, and an exoplanet's orbit around a star should remain stable, ensuring consistent timing between transits. "But if you have more than one planet at play, then the planets will pull each other with their gravity and make each other speed up and slow down a little bit," says Dr Montet. "This means the transits will arrive slightly earlier or later than normal, and you can use that to infer that another planet's causing these timing variations."
Modelling planetary movements
To start, Mr McKee went through three years of data from the TESS telescope, (Transiting Exoplanet Survey Satellite).
One known exoplanet is TOI-2818b, orbiting a star visible with a standard telescope located just over 1000 light-years away in the constellation Puppis. TOI-2818b, a hot Jupiter, was discovered via its transits. However, when analysing the data, Mr McKee noticed that its transit dips were not evenly spaced – they were occurring closer together over time.
If this planet were a clock, it wasn't keeping accurate time. Something was influencing its orbit, prompting Mr McKee and Dr Montet to investigate the mystery.
"The tricky thing is that there are a number of plausible explanations for why the planet is arriving early," says Mr McKee.
For example, the tides of a star can impact the gravitational pull on a planet, just like we see between the Moon and the Earth. When this is the case, the planet is typically spiraling inwards, about to get swallowed by the star, which would make the transits of the planet arrive earlier and earlier.
"So we had to work through all the other possible situations that could occur that would cause the same timing variations that we saw in the data," says Dr Montet. "But our tests and simulations suggested that none of the other explanations are physically possible. It would take wild new physics that is very implausible, so we were able to rule those out and say the only option left is that it has to be another planet.
What can this teach us about planet formation?
The first exoplanets were discovered in the mid 90s. While scientists haven't yet found an exoplanet that can support life like Earth, they have identified a number of Earth-sized rocky exoplanets, some of which are in the habitable zones of their stars, meaning they could potentially have water on their surface.
"There's a lot of questions about exoplanets that we haven't been able to answer yet," says Dr Montet. "Whenever we find planets, they throw up new puzzles about how they form, and hot Jupiters are a great example of that. Hot Jupiters were the first exoplanets we discovered, but we don't fully understand how they form or why they're there."
One way scientists think hot Jupiters may form, called dynamical or warm excitation, is chaotic and can make the system unstable, ejecting other planets out of the planetary system. The second possibility is called cold migration, a smoother process where the planet gradually drifts inward. "If this smooth method is common, we would expect to find hot Jupiters with companion planets. But if they typically lack companions, it suggests the chaotic scattering process is more frequent," says Dr Montet.
Current evidence points to a mix of both processes, but studying more hot Jupiters will help us determine which is more common.
Searching for more clues
Mr McKee and Dr Montet's work has pointed to the unusual transit of TOI-2818b being the result of a companion planet. However, many questions remain unanswered. "There are lots of factors that we don't know," says Mr McKee. "There are a couple of different features of the planet that are compatible with our simulations."
Further observations will help to narrow down exactly what kind of secondary planet is interfering. "The ESPRESSO instrument on the Chilean Very Large Telescope (VLT), run by the European Southern Observatory (ESO), will provide more data and help us to eliminate some of the possibilities when figuring out the features of the planet," says Dr Montet. "ESPRESSO data already was really important in eliminating some other exotic solutions, like a brown dwarf orbiting the star and tugging on the hot Jupiter. The VLT is the best-placed instrument we have to measure exactly where this hidden planet is.
"Every time we find new planetary systems around other stars, we're surprised that there are things that we did not envision, things that look nothing like our own solar system.
"Every time we think we really understand planet formation, we learn something new. And this is going to just keep happening over the next couple decades, as different missions come online and help us detect planets in new ways, using new techniques."
Dr Montet highlights the importance of a collaborative approach to exoplanet hunting. "There are many more planets than people, but the more people who are able to collaborate, from well-established facilities, to citizen scientists, we can narrow down answers to some of the most important questions and understand more about the universe."
