Human beings have discovered over five thousand five hundred exoplanets. Over 80 percent of them have orbits shorter than 50 days which would place them at over twice as close to their star as Mercury is to the Sun. Some are even closer. Recent observations from TESS however have found one with an orbit of 82 days and another measured in hundreds of days.
Why are we finding so many toasty worlds, so close to the star? There are likely just as many planets farther out from their star that orbit with periods measured in hundreds of days, like the Earth does, but the reason we’ve seen so many of these types of planets comes down to the limits of our telescopes and how we are observing them.
Two of the most popular ways of finding exoplanets right now are the transit and radial velocity methods. To use them, we have to actually watch the planet move around the star or between us and the star, and waiting around for that to happen is expensive.
Telescopes are in high demand, and to stare at a star for hundreds of days hoping to see a planet go around is not practical with the way telescope time is allocated, so we are only getting good data from planets that whiz around their star in a short time. But of course astronomers realize that’s not the whole picture, there are no doubt many long period exoplanets out there, we just have to deploy an observing system capable of finding them. More on that in a minute.
Back to TESS: The Transiting Exoplanet Survey Satellite. It only looks at an area of the sky for about a month before it moves on to other parts of the sky. The idea was to sacrifice getting complete orbits of planets for partial ones, all the while imaging the entire sky and flagging potential transits as “Objects of Interest” for further study by other telescopes. TESS has amassed thousands of Objects of Interest, or TOI’s since its launch.
One of them, TOI-4600 is a rare system containing two long-period planets orbiting a nearby star that is 815 light years from Earth.
The TESS team discovered that the star hosts an inner planet with an orbit of 82 days, similar to that of Mercury, while a second outer planet circles every 482 days, placing its distance from its star somewhere between the orbits of Earth and Mars.
Both planets are likely gas giants, similar to Jupiter and Saturn, though the composition of the inner planet may be more of a mix of gas and ice. The two planets bridge the gap between “hot Jupiters” — the toasty, short-orbit planets that make up the majority of exoplanet discoveries — and the much colder, longer-period gas giants in our solar system.
Astronomers found the longer period planet by stacking up data from past searches of the sky to build up the light curve. TESS has scanned the entire sky once already. They also looked at the system using ground-based telescopes to get a fuller picture of the system.
What TESS is really good at though is for short searches over a wide area of sky. This method of observing is great for getting these short period exoplanets. An ideal scenario would be to look at the entire sky for a long time.
Luckily we have just such a program starting very soon with the Vera Rubin Telescope. It will survey the entire sky several times a week looking for all sorts of things, one of them being exoplanet transits. And it will do if for years, allowing astronomers to build a deep view of the heavens over a long time, seeing dips in brightness across the heavens.
Our window to more worlds around other stars is about to get a lot bigger.