Behold the lightest planet ever imaged by a telescope: an extremely young, Jovian-like planet that’s twice the size of Jupiter. Astronomers detected it through visible light, which is an extraordinary feat for a planet of this nature.
Normally, exoplanets are indirectly detected when they pass in front of their host star. The temporary dimming of the parent star allows scientists to count the number of planets in a stellar system, and determine a planet’s radius and mass. But the transit method, as it’s called, doesn’t reveal much about a planet’s composition. Hence the importance of direct imaging, where a star’s light is fed into a spectrograph and used to determine, for instance, what substances are present in an object’s atmosphere.
What’s more, direct imaging techniques are also useful for seeing planets far from their parent star, whereas transit or Doppler techniques are best at detecting planets close to their star. And as Stanford University astronomer Bruce Macintosh told me, direct imaging will never catch up with Kepler’s thousands of planets, “but we can study the planets we do at amazing levels of detail.”
See the Light
By using the recently commissioned Gemini Planet Imager (GPI), Macintosh, the lead author of the new study, was able to spot the Jupiter-like planet, called 51 Eridani b, even though it’s a million times dimmer than its host star. What’s more, Macintosh and his colleagues were able to detect copious amounts of methane, which is unusual for an object like this. In fact, 51 Eridani b exhibits the strongest methane signature ever detected in an exoplanet, a feature that may provide clues about its formation. Their study now appears at the journal Science.
Discovery image of the planet 51 Eridani b with the Gemini Planet Imager taken in the near- infrared light on December 18, 2014. The bright central star has been mostly removed to enable the detection of the exoplanet one million times fainter. (Credits: J. Rameau (UdeM) and C. Marois (NRC Herzberg))
The GPI, a high contrast imaging instrument built for the Gemini South Telescope in Chile, was designed to find planets exactly like this—young planets orbiting bright stars. Previous direct imaging telescopes only allowed astronomers to detect exoplanets at relatively wide separations from their host star, typically at distances greater than 10 AU (were 1 AU = average distance of the Earth to the Sun). But with GPI, astronomers can pinpoint objects as close as 5 AU.
“Right now the only kind of planets we can see with direct imaging are giant planets (above a Jupiter mass), and most importantly, young planets,” Macintosh told io9. “When planets form, a lot of energy is released, so the planet is hot and then gradually cools down as it radiates away that energy. Young planets might have temperatures of 500-2000 degrees Kelvin, so they shine in the infrared, where they might only be 100,000 to a million times fainter than their star.”
To pinpoint 51 Eridani b, the astronomers used a process called adaptive optics to sharpen the image of the parent star, and then block out the excess starlight. The residual light was then analyzed, with the brightest spots indicating a potential planet.
Ruling out Artifacts
Prior to this discovery, the lowest-mass objects ever directly imaged by a telescope were on the order of five-times the mass of Jupiter. At twice the mass of Jupiter, 51 Eridani b is considered the lowest-mass object ever directly imaged by a telescope.
The left side of the animation shows the GPI images of the nearby star 51 Eridani in order of increasing wavelength from 1.5 to 1.8 microns. The images have been processed to suppress the light from 51 Eridani, revealing the exoplanet 51 Eridani b (indicated) which is approximately a million times fainter than the parent star. (Credit: Robert De Rosa (UC Berkeley), Christian Marois (NRC Herzberg, University of Victoria)
Given the precise nature of the discovery, I asked Macintosh how he was sure the remaining incoming light wasn’t indicative of some sort of visual artifact, or some other celestial object or configuration.
“It’s a complicated process,” he replied. “In the case of GPI, one very powerful tool we have is GPI doesn’t just take images, but spectra of everything in its field of view—our instrument makes a ‘data cube’ where for every pixel we get the brightness at a bunch of different wavelengths. So we get the spectrum instantly, and 51 Eri has a very distinctive spectrum, showing strong methane absorption signatures that an artifact wouldn’t have.”
Macintosh said his team worried about whether they found a background star that was just aligned by chance.
“Normally you then have to wait a year after you think you see a planet to make sure it’s moving together with/around the star, but we didn’t want to wait,” he says. “Fortunately, we had the unique spectrum, and could calculate the odds of such an object (a brown dwarf) with strong methane appearing so close to the star at about a million to one, so we’re pretty sure that didn’t happen.”
Hot, Young and Full of Gas
At 100 light-years away from our Sun, 51 Eridani b is relatively nearby. It’s currently in orbit around its parent star at a distance of about 13 to 15 AU. By comparison, our Jupiter is 5.2 AU from the Sun, but it likely formed much further away.
Remarkably, the planet is just 20 million-years-old, which is a scant tick of time in cosmological terms. To put its extreme youth into perspective, it formed 40 million years after the dinosaurs went extinct. The planet features an effective temperature of 800 degrees Fahrenheit (~600-750 K), but it’s actually one of the coldest exoplanets ever imaged via visible light. Previously detected Jovian-like planets feature temperatures around 1,200 degrees Fahrenheit.
This said, it’s not the youngest Jupiter-like planet discovered.
“There are younger planets, but they’re much more massive (5-10 jupiter masses) and also in much wider orbits (50+AU),” says Macintosh. “So this is not quite the youngest planet but it is the most Jupiter like. The 10 Jupiter mass objects at 50-100 AU maybe didn’t form like Jupiter did—this one has a good chance it did.”
Indeed, astronomers say this planet is very much what Jupiter was like in its infancy.
“In the atmospheres of the cold giant planets of our solar system, carbon is found as methane, unlike most exoplanets, where carbon has mostly been found in the form of carbon monoxide,” noted NASA AMES astrophysicist Mark Marley in a statement. “Since the atmosphere of 51 Eri b is also methane rich, it signifies that this planet is well on its way to becoming a cousin of our own familiar Jupiter.”
What’s more, the “coldness” of 51 Eridani b suggests it formed through a core build-up process, and not from quickly collapsing material. Consequently, this young Jovian may be understood as a “bridge” between hotter planets with wider orbits and those more like Jupiter.
“51 Eri b is the first one that’s cold enough and close enough to the star that it could have indeed formed right where it is the ‘old-fashioned way,’” adds Macintosh. “This planet really could have formed the same way Jupiter did—the whole solar system could be a lot like ours.”
Read the entire study at Science: “Discovery and spectroscopy of the young Jovian planet 51 Eri b with the Gemini Planet Imager”.