Astronomers discover a bloated ‘hot Jupiter’ exoplanet

An international team of astronomers has discovered a new bloated exoplanet “hot Jupiter” as part of the Next Generation Transit Survey (NGTS). The newly discovered extraterrestrial world, designated NGTS-21b, is about 30% larger and more than twice as massive as Jupiter. The result was detailed in an article published Oct. 3 on arXiv.org.

The so-called “hot Jupiters” are similar in their properties to the largest planet in the solar system, but have orbital periods of less than 10 days. Such exoplanets have high surface temperatures because they orbit their parent stars very closely.

Now a group of astronomers led by Douglas R. Alves of the University of Chile in Santiago, Chile, reports the discovery of a new extrasolar planet of this type. Observing a metal-poor K dwarf star called NGTS-21 with telescopes at ESO’s Paranal Observatory in Chile, they identified a strong transit signal. The planetary nature of this signal was confirmed by follow-up observations.

“NGTS-21 was observed during the 2018 campaign from March 24 to November 7, where 9157 images were captured over 150 nights with an exposure time of 10 seconds per image…. A strong signal was detected at 1,543 days and a validation process began to either confirm the signal as likely passing hot Jupiter or to reject it as false positive evidence,” the researchers explained.

NGTS-21b has a radius of 1.33 Jupiter radii and a mass of 2.36 Jupiter masses, giving a density at the 1.25 g/cm level³. The planet orbits its host approximately every 37 hours at a distance of about 3.5 million kilometers. As a result, NGTS-21b is exceptionally hot — its equilibrium temperature is estimated at around 1,357K.

The scientists emphasized that the parameters of NGTS-21b indicate that it is a bloated, massive, hot Jupiter. This is due to the fact that some giant planets get bigger when their parent stars are at the end of their lives, and NGTS-21 is estimated to be 10 billion years old.

The bloated planets have been known to astronomers for almost two decades, but it’s still unclear what causes the bloating processes. In general, the possible explanations could be assigned to two theories – scientists believe that the inflation is caused by energy deposition of the host star or by inhibited cooling of the planet.

“Inflation-free planetary structure models suggest that the planet’s atmosphere is inflated by about 21%, while inflation models predict a radius consistent with observations, thus pointing to stellar radiation as the likely origin of NGTS-21b’s radius inflation,” the astronomers wrote and tried to find the most plausible explanation for the inflation of the newly discovered alien world.

The paper’s authors also revealed fundamental properties of the parent star NGTS-21. According to the study, it is a metal-poor dwarf of spectral type K3V, located about 2,090 light-years from Earth. The star is 14% smaller and almost 30% less massive than the Sun. The effective temperature of NGTS-21 was measured at 4,660K.

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