One of the most well-studied stars, Vega, may have been hiding something from us for years — if confirmed, one of the hottest exoplanet candidates ever discovered.
The star, made famous in the 1997 science fiction movie Contact, has been a prime target for planetary scientists for decades. And to date, the search for an actual planet in the system has proven fruitless.
But a paper released earlier this month in The Astronomical Journal provides a tantalizing hint — it isn’t quite a smoking gun, but this may be the first evidence of a planet in orbit around Vega.
What’s new — Planetary scientists say there may in fact be a Super-Earth planet in close orbit around Vega. Here are some vital stats for the new, possible-planet:
- The planet, if it exists, orbits Vega in a mere 2.43 Earth days
- The planet appears to have a minimum mass of 20 Earths — a little more massive than Neptune
- If confirmed, the exoplanet would have the second hottest temperature ever recorded for an exoplanet (around 5400 degrees Fahrenheit), taking silver next to KELT-9b’s gold
If the Vega planet is there it — like KELT-9b — is a world in peril. Sam Quinn, an astronomer at the Harvard-Smithsonian Center for Astrophysics, tells Inverse its close proximity to Vega would make the world hellish — hotter than some stars.
“We would expect that it would be losing its atmosphere pretty rapidly,” Quinn says. “It would be pretty unlikely that it would hold on to its atmosphere this long.”
This may mean it was also once a larger world, which over time underwent tremendous atmospheric loss.
“It could be a rocky core leftover from a giant planet,” Quinn says, although there are many possible scenarios.
Here’s the background — Vega is a hot, bright, and large star, located about 27 light-years from Earth. It’s the fifth brightest star in the night sky. It’s between 400 to 500 million years old, weighs in at twice the mass of the Sun, and has about 2.3 times the radius.
Because of Vega’s size and relative proximity to Earth, it’s a well-studied star. But the star’s brightness, and the fact that it seems to rotate with one of its poles aimed toward Earth, means that detecting planets around it is difficult.
This isn’t for lack of trying. In 1983, NASA’s Infrared Astronomical Satellite picked up evidence of dust around the star — the building blocks of planets. Although the data didn’t provide conclusive evidence that planets had formed, it was some of the first hard evidence that stars beyond our solar system could form planets.
“It’s hard to know which you’re actually seeing.”
Charles Beichman, one of the co-authors on the 1984 study detailing the NASA 1983 observations, still works with IRAS data today. Beichman, who is now executive director of the NASA ExoPlanet Science Institute, recalls to Inverse how much the data surprised everyone at the time.
“That became an enormous surprise for everybody, and this debris very quickly reminded people that planets have to come from somewhere, and this solid material we showed had to be rocky material replenished from an asteroid belt or Kuiper belt,” Beichman says.
Subsequent observations have revealed gaps in the dust disk which hint at the presence of planets, as well as evidence for an asteroid belt, detailed in 2013. But few solid candidates for planets have turned up in the years we’ve been observing Vega. Until now.
What they did — Quinn’s collaborators — led by Colorado University Boulder undergraduate Spencer Hunt — collated 10 years of observations of Vega, looking for evidence of planets. The observations were radial velocity measurements. Radial velocity breaks down the light coming from a star into its spectra, and then looks for shifts in stellar activity relating to the tug of an orbiting planet.
After combing through the data, a consistent signal was found at 2.43 days. Because it doesn’t match the known rotational period of Vega itself (0.71 days), the signal is likely not from the star itself. Armed with this hint, the team then looked for other signs of a possible planet using other instruments in an attempt to corroborate the measurements, but an analysis of data from NASA’s Transiting Exoplanet Survey Satellite (TESS) showed that there were no transits detected. Transits occur when a planet passes in front of its home star and causes a small, but perceptible, dip in the light coming from the star. But because of Vega’s distinct orientation, the fact TESS didn’t detect a transit doesn’t rule out the presence of a planet.
The signal the team found is consistent but not strong, so the team is still calling it a candidate right now, awaiting other observations.
“It’s not the strongest signal in the world, so we want confirmation through other means before we call it a planet,” Quinn says.
The details of the possible planet are equally hazy. Radial velocity can be used to infer a minimum mass, for example, but the upper limits for the potential planet’s mass depend on a number of factors, including how the planet orbits on its star, which can vary based on inclination. That’s why the candidate is said to have a minimum mass of 20 Earths, but Quinn says there are scenarios under which it could be slightly more massive than Jupiter.
Beichman, who was not involved with the new study, says that the team’s caution is correct.
“I believe the limits that they set more than I believe this possible candidate, which is what they say,” he says. He adds the signal could turn out to be the result of yet-determined stellar activity.
“It’s hard to know which you’re actually seeing, and they’re very up front with this,” he says.
The data didn’t show any other strong planetary candidates at this time.
“This is the only candidate signal that we think may be a real companion,” Quinn says. “There are weaker signals at other periods that we think could be parts of stellar activity.”
However, planets could still exist further out from Vega, though the study puts constraints on the size of planets out up to distances of 15 times the distance of the Sun to Earth.
Of course, even if there were a planet at the right distance from Vega to have Earth-like temperatures, Vega is only expected to have a lifetime of 1 billion years before it exhausts its fuel, meaning such a planet — and life upon it — wouldn’t have long to take a foothold before it was destroyed. Vega will die in a similar way to the Sun — expanding its shell before collapsing back down into a dense object known as a white dwarf.
What’s next — Follow-up observations are needed to confirm if this planet is actually there. One potential way to do this is by using a spectrograph to look for light coming off of the planet. This could also show if there is an atmosphere and, if so, its composition.
While these kinds of observations are rare, they’re possible around young, hot planets — and the Vega candidate fits that bill. But they also need to be large for this to happen.
“That only works if it’s a giant planet,” Quinn says. “If it’s smaller and closer in size to Neptune … then we probably can’t detect it.”
One alternative is to collect more radial velocity data from Vega to strengthen the case for the planet.
“Another five years of taking data, if that bump becomes more and more significant rather than getting of washed away, then you go from a possible to a probable to a solid detection,” Beichman says.
In addition, the James Webb Space Telescope could take a look at Vega to find planets further out. Rather than look closely at the star, it could look at gaps in the star’s debris disk that, in other systems, have at times turned out to be planets. Beichman believes Webb will be able to actually capture images of the outer planets. In any case, we may be able to confirm planets around Vega in the next decade. It may just be a matter of when, where, and which ones.
Abstract — We present an analysis of 1524 spectra of Vega spanning 10 yr, in which we search for periodic radial-velocity variations. A signal with a periodicity of 0.676 day and a semi-amplitude of ~10 m s−1 is consistent with the rotation period measured over much shorter time spans by previous spectroscopic and spectropolarimetric studies, confirming the presence of surface features on this A0 star. The activity signal appears to evolve on long timescales, which may indicate the presence of failed fossil magnetic fields on Vega. TESS data reveal Vega's photometric rotational modulation for the first time, with a total amplitude of only 10 ppm. A comparison of the spectroscopic and photometric amplitudes suggests that the surface features may be dominated by bright plages rather than dark spots. For the shortest orbital periods, transit and radial-velocity injection recovery tests exclude the presence of transiting planets larger than 2 R⊕ and most non-transiting giant planets. At long periods, we combine our radial velocities with direct imaging from the literature to produce detection limits for Vegan planets and brown dwarfs out to distances of 15 au. Finally, we detect a candidate radial-velocity signal with a period of 2.43 days and a semi-amplitude of 6 m s−1. If caused by an orbiting companion, its minimum mass would be ~20 M⊕; because of Vega's pole-on orientation, this would correspond to a Jovian planet if the orbit is aligned with the stellar spin. We discuss the prospects for confirmation of this candidate planet.