Astronomers report dwarf star with unexpectedly giant planet

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Friday, November 3, 2017

In findings reported on Tuesday, an international team of astronomers discovered an exoplanet, called NGTS-1b, revolving around M-dwarf star NGTS-1, that the team said does not fit existing notions of how stars and planets form. NGTS-1b is a gas giant, similar to Jupiter and of comparable volume and mass, but its parent star is about half the diameter and mass of the Sun, making this the most massive planet orbiting an M-dwarf ever discovered.

NGTS-1b, about 600 light years from the Earth, is so extremely close to its star that a revolution around its star takes only about 2.6 Earth days — 2.647298 ± 0.000020 — and its surface temperature is about 800 K. The planet was discovered by observing periodic fluctuations in the star's apparent brightness as NGTS-1b passed in front of it. NGTS-1b's mass is less than Jupiter, about 0.812 MJ (mass of Jupiter), but it has greater volume, with radius about 1.33 RJ (radius of Jupiter). Its density was reported to be 0.42 g cm-3 (with error: +0.59 to -0.28), meaning Jupiter, whose density is 1.326 g cm-3, is likely around thrice as dense as the exoplanet.

"The discovery of NGTS-1b was a complete surprise to us," said lead author Doctor Daniel Bayliss of the University of Warwick, in England. "[S]uch massive planets were not thought to exist around such small stars. This is the first exoplanet we have found with our new NGTS facility and we are already challenging the received wisdom of how planets form."

The existing understanding of planetary formation, the team explained, says small stars can produce rocky planets but cannot pull in enough mass for larger, gaseous ones like NGTS-1b. Based on the observations, NGTS-1's radius is 0.573±0.077 R (solar radius) and using that value, astronomers calculated its weight to be about 0.598 M (solar mass). The star's density is almost ten times of the gas giant, 4.4 (with error: +2.4 to -1.4) g cm-3. In comparison, the Sun's density is 1.409 g cm-3.

The research team discovered NGTS-1b using the Next-Generation Transit Survey (NGTS), an array of telescopes physically located in Chile, which was designed to search for planets around bright stars. NGTS-1 is a relatively dim red M-dwarf, which are very common in the universe, of apparent magnitude 15.52 ± 0.08. University of Warwick professor Peter Wheatley, who runs the NGTS, said, "NGTS-1b was difficult to find, despite being a monster of a planet, because its parent star is small and faint. Small stars are actually the most common in the universe, so it is possible there are many of these giant planets waiting to be found."

To date, scientists have discovered only three gas giants which revolves around M-dwarf stars. The other two are Kepler-45b and HATS-6b which have masses 0.505 MJ and 0.32 MJ respectively.