Science News, writing

The Largest Planet Found Around the Smallest(-type) Star

Your Random Science News Story for the Month of October

In a “grass is always greener on the other side of the fence” sense, astronomers just keep finding unusual rocky and gassy balls around other stars.1 Not too long ago, for example, there was a Jupiter-sized gas giant reported as being blacker than coal. There’s also

  • a planet (actually the solid, rocky core leftover from a gas giant) currently having its atmosphere stripped away at more than 35,000 km/hour because of the radiation from its host star
  • a planet that astronomers hypothesize has a core surrounded by a layer of water that acts both like a liquid and a gas (technically, it’s “supercritical”), and the whole thing is wrapped in a blanket of steam
  • a planet that glows a “dull magenta” because of all the heat its radiating leftover from its initial formation
  • a planet only a billion years younger than the universe itself

But now there’s a new ‘special’ exoplanet – one that seems to be too big given the type of star it orbits.

Exoplanet NGTS-1b - Version 1 - credit University of Warwick Mark Garlick
Artist’s impression, obviously. Our telescopes aren’t that good. 
Credit: University of Warwick/Mark Garlick

The paper announcing this discovery was published in Monthly Notices of the Royal Astronomical Society, and you can read it for free, here.

What happened?

Astronomers from the University of Warwick discovered the first planet within data provided by the Next Generation Transit Survey, or NGTS. They found this exoplanet (uncreatively dubbed NGTS-1b) using one of two classic exoplanet-finding methods. Specifically, they monitored the amount of light coming from its host star (uncreatively dubbed NGTS-1) over 100 nights — 118,498 images taken at 10 seconds exposure time a piece.

When a particular dip in a star’s light curve happens periodically, it indicates an object is (probably) regularly blocking some amount of that light. The bigger the planet, the more it blocks. The closer a planet to its star, the more it blocks. When this detection method was younger, we could only pick out Jupiter-sized planets orbiting their stars super closely (aka “Hot Jupiters”), but nowadays we can use this Transit Method to find Earth-sized rocks.2

NGTS-1b’s initial transit data was taken by the NGTS telescopes – 12 fully-automated 20 cm-aperture telescopes working as one at the ESO’s Paranal Observatory in Chile. Here’s a sample of the NGTS data over the course of one transit:

From Bayliss et al. 2017. You can see that only about 2.5% of the star’s light was blocked by the planet’s grazing transit. The black dots are the actual data, the vertical black lines indicating the amount of uncertainty in the measurement. The solid blue vertical lines indicate the ‘start’ and ‘end’ of the transit. The shades of red indicate the mathematical model predicting the smooth transit based on the actual data. 

Follow-up data was collected by the 1.2-meter Euler Telescope at La Silla Observatory – 50 exposures 3 minutes long over the course of 3 hours, all in one morning. One transit was observed:

(Note the much smaller error bars!)

The HARPS spectrograph on the ESO’s 3.6-m telescope (also at La Silla) was used to measure how much the planet was gravitationally tugging on its star, and therefore estimate its mass.

What’d they find?

NGTS-1b was measured to orbit its star once every 2.647 Earth days. Yes, that means it has to be super close – 3% the distance the Earth is from the Sun.

They actually had to estimate NGTS-1b’a radius using some bonus math and modeling, because from our perspective, the planet doesn’t pass entirely in front of the star – it grazes the star’s disk like the Moon does during a partial solar eclipse. The paper authors caution that their calculated radius value of 1.33 Jupiter radii “is not as robust as for most transiting gas giants in the literature, and simply represents our best estimate based on all available data for this system and the population gas giant planets in general”.

Its mass was calculated to be about 81% of Jupiter’s. While we’ve found plenty of exoplanets more massive than that, NGTS-1b is the most massive planet we’ve found around such an unmassive star (~62% the mass of our Sun).

So what?

NGTS-1 is one of the smallest and coolest types of stars, an M-dwarf.3  Proxima Centauri – the star closest to the Sun – is probably the most famous M-dwarf, but you might recall a news story about another M-dwarf earlier this year: TRAPPIST-1.

M-dwarfs are actually the most popular type of star in the galaxy; about 75% of all stars are M-dwarfs. That makes astronomers super keen to find planets around them, in attempts to better understand how most of the planetary formation in the Milky Way happens.

NGTS-1b is only the third gas-giant found orbiting an M-dwarf – the previous two are Kepler-45b (mass 50% Jupiter’s) and HATS-6b (mass 32% Jupiter’s).

The discovery of as large a planet as NGTS-1b around an M-dwarf throws a proverbial wrench into astronomers’ assumptions about planetary formation. Small stars, it was thought, simply didn’t have enough matter surrounding them to form such large planets.

But searching M-dwarfs for planets is a relatively new field, so further work is needed to truly gauge how frequent orbiting Hot Jupiters are. It’s currently estimated that 7% of planets around M-dwarfs have masses between 1 and 10 times that of Jupiter. After a lot more hard work combing through data provided by NGTS and other exoplanet survey projects, we’ll be able to adjust that number.

1. I mean, you have to replace grass with planetgreener with weirder, and fence with interstellar medium… 
2. It even works when there are multiple planets in the mix – here’s transit data for a system with seven total planets observed over 500 hours.
3. Its surface temperature is 3916 K; our own Sun’s surface temp is ~5800 K. That much cooler temperature means that most of an M-dwarf’s emitted photons are infrared (and therefore invisible to the human eye, but detectable by certain telescopes); they do emit some reddish light, though, so if you were anywhere near enough to it to see it with your eyeballs, it’d look reddish in color. 

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