A team of astronomers, led by Andrew Vanderburg of the Harvard–Smithsonian Center for Astrophysics (CfA), has recently detected five new exoplanets circling a bright star designated HIP 41378, which lies some 380 light years away. The newly found alien worlds are larger than our planet, with sizes ranging from about 2.5 times the size of Earth to the size of Jupiter. The findings were presented in a paper published June 27 on the arXiv pre-print server.
The planetary system was spotted by NASA’s prolonged Kepler mission, known as K2. HIP 41378 was observed by K2 for a period of about 75 days between April 27, 2015 and July 10, 2015. These data, complemented by follow-up spectroscopic observations, allowed the scientists to detect transiting events and distinguish five planetary candidates. However, the team admits that it wasn’t an easy task to unveil the new exoworlds.
“Finding these planets was challenging in several ways. One challenge is in processing K2 data so that it is even possible to detect planets. Ever since the mechanical failure in 2013 that ended the original Kepler mission, K2 has been taking data while the telescope drifts back and forth, which causes problems with the data,” Vanderburg told Astrowatch.net.
However, he added that his team found a solution to the problem of the telescope drifting, allowing them to receive much ‘cleaner’ data. Overcoming the spacecraft’s instability issues, Martti Holst Kristiansen from the Technical University of Denmark, a member of the team, identified the five transits in diagrams, when closely inspecting the K2 data regarding HIP 41378.
“Once Andrew's corrected data was released we revisited our multi planet candidate and were finally able to distinguish the noise from genuine planets. The discovery of this remarkable system definitely had some challenges implicated,” Kristiansen told SpaceFlight Insider.
According to the research paper, the newly detected planetary system includes two sub-Neptune sized planets, designated HIP 41378 b and HIP 41378 c, having radius of about 2.9 and 2.56 Earth radii respectively. HIP 41378 b has the shortest orbital period of all the currently known planets in the system, circling its host star every 15.6 days. The other sub-Neptune exoplanet needs 16 days more to orbit HIP 41378.
HIP 41378 d is a Neptune sized planet, about four times larger than the Earth. Its orbital period is 157 days. The system also hosts a sub-Saturn sized exoworld, named HIP 41378 e, which has a radius of approximately 5.5 Earth radii and orbits its parent star every 131 days.
However, the planet that intrigues the researchers the most, is HIP 41378 f. With a radius ten times greater than the Earth’s, it is a Jupiter sized planet that needs almost a year to orbit the star.
“I am most excited by HIP 41378 f, which is a Jupiter-sized planet that probably orbits its star every 325 days. We don't know much about this planet yet, but I am excited for what we might learn in the future. It could be one of the first cool (temperature) planets that we study in depth. I hope that in the future, we can learn about its atmosphere and see what molecules make it up,” Vanderburg said.
According to the paper, HIP 41378 f is one of the first gas giants with a cool equilibrium temperature transiting a star bright enough for transit transmission spectroscopy. The team also noted that it could be possible to measure the planet’s oblateness in future studies, as its orbital period is long enough that its rotation will not have synchronized with its orbit.
“This planet is very interesting to me, because the process by which these massive planets are formed is not totally understood. How did this Jupiter form, and how did it get to its current orbit? Hopefully, someday soon we will know enough to answer that question,” said Juliette Becker of the University of Michigan, one of the co-authors of the paper.
The researchers underlined that we currently know very little about the masses of these planets, so that's one of the first things the team wants to learn about the system. They plan to use the radial velocity method to measure how much the star wobbles back and forth due to the planets' orbits.
“We do not yet have masses of these planets. One of the things we did in our paper is estimate how easy it would be to make measurements to find the masses, and we found that for a few of the planets, it should be possible to measure masses. This kind of measurement would be done using the radial velocity method,” Becker revealed.
Vanderburg is currently observing the HIP 41378 system with the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) on the Telescopio Nazionale Galileo on La Palma, Canary Islands, Spain. This instrument will allow the researchers to derive masses for one or more of the planets using very precise radial velocity measurements.
“So far we have accumulated 14 HARPS-N observations, but that is far too few to solve for masses. I expect it will take hundreds of observations to disentangle all the orbits. In the meantime HIP 41378 has slid behind the sun as the Earth moves around in its yearly orbit and won't be observable again for a couple of months,” said David Latham of CfA, a member of the team.
The scientists also suggested that there could be more planets lurking somewhere around HIP 41378, still undetected. As Vanderburg noted, K2 only observed this star for 80 days, and saw three planets transit the star only once.
“That means we either got lucky to catch all of the planets in the short time we were watching, or there are more planets that we would have found if we had just been observed for a bit longer,” he concluded.
The team hopes that future space observatories like NASA’s Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST) will bring more fascinating exoplanet discoveries like the detection of the HIP 41378 system. TESS is expected to cover nearly the whole sky, twenty times more than K2. It is designed to be able to find all the best planetary systems for follow-up observations, such as spectroscopic studies of the molecules in transiting planet atmospheres with JWST. HIP 41378 could be a preview of the type of discoveries the all-sky TESS survey will make routine.