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Starts With A Bang

Astronomers Go Nuts For Closest Exoplanet Directly Imaged Ever: COCONUTS-2b

At 35 light-years away, it’s also the 2nd coolest, 2nd widest planet ever found.

Despite discovering more than 4000 exoplanets, most remain obscure.

Although more than 4,000 confirmed exoplanets are known, with more than half of them uncovered by Kepler, directly imaging most of these exoplanets is not possible. Their small size and low luminosity, when coupled with the close separation distances from their parent stars, make that an impossibility. (NASA/AMES RESEARCH CENTER/JESSIE DOTSON AND WENDY STENZEL; MISSING EARTH-LIKE WORLDS BY E. SIEGEL)

Their nature — small, faint, and in tight orbits — prevent direct imaging.

An artist’s rendition of Proxima b orbiting Proxima Centauri. In order for direct imaging efforts to be successful, the planet must be well-enough separated from the parent star so that the parent star’s light can be blocked, but the reflected starlight from the planet will still be detected. (ESO/M. KORNMESSER)

The stellar glare simply overwhelms their planet’s reflected light.

Image of the exoplanet discovered in 2013, HD 95086 b. The star has been blocked out by a coronagraph and the diffraction patterns removed during data reduction. This detection showcases the present limits of exoplanet direct imaging, which require super-Jupiter masses and separation distances of many hundreds of AU from the parent star. (J. RAMEAU ET AL., ARXIV:1305.7428V1)

However, heat-generating exoplanets are special.

Three images of Jupiter show the gas giant in three different types of light — infrared, visible, and ultraviolet. The image on the left was taken in infrared by the Near-InfraRed Imager (NIRI) instrument at Gemini North in Hawaiʻi, the northern member of the international Gemini Observatory, a Program of NSF’s NOIRLab. The center image was taken in visible light by the Wide Field Camera 3 on the Hubble Space Telescope. The image on the right was taken in ultraviolet light by Hubble’s Wide Field Camera 3. All of the observations were taken on 11 January 2017. (INTERNATIONAL GEMINI OBSERVATORY/NOIRLAB/NSF/AURA/NASA/ESA, M.H. WONG AND I. DE PATER (UC BERKELEY) ET AL.)

Just like Jupiter, they reflect visible light, but emit their own infrared radiation.

Jupiter and its rings, bands and other heat-sensitive features in the infrared. Note how everything we observe, Jupiter’s bands, rings, and moons, all orbit in the same plane. This is a strong indication that they all formed at the same time: from the initial circumplanetary disk around Jupiter that dates to the formation of the Solar System. (TROCCHE100 AT THE ITALIAN WIKIPEDIA)

When well-separated from their parent stars, they yield to direct imaging.

The SPHERE instrument on ESO’s Very Large Telescope reveals a planet caught in the very act of formation around the young dwarf star PDS 70. This was the first planet found in the act of forming, revealed in 2018. The planet stands clearly out, visible as a bright point to the right of the centre of the image, which is blacked out by the coronagraph mask used to block the blinding light of the central star. There is a second planet, PDS 70c, farther out. These planets are both widely separated and very massive, emitting their own infrared radiation. (ESO/A. MÜLLER ET AL.)

The first one ever announced was Fomalhaut b, although its planetary nature is contentious.

An exoplanet detected around the star Fomalhaut, seen to move in multiple images over time. This kind of direct imaging is something Hubble and other telescopes can do for large worlds very distant from their parent star, but it doesn’t have the right instruments to do this for potentially habitable worlds. (NASA, ESA, AND P. KALAS, UNIVERSITY OF CALIFORNIA, BERKELEY AND SETI INSTITUTE)

Since then, a few dozen exoplanets have been directly imaged, all very massive.

Direct imaging of four planets orbiting the star HR 8799 129 light years away from Earth, a feat accomplished through the work of Jason Wang and Christian Marois. Typically, separation distances of many AU are required to directly image exoplanets, and only Jupiter-mass and larger exoplanets have been seen at present. (J. WANG (UC BERKELEY) & C. MAROIS (HERZBERG ASTROPHYSICS), NEXSS (NASA), KECK OBS.)

The lightest one, 51 Eridani b, exceeds double Jupiter’s mass.

At 2.6 Jupiter masses, the exoplanet 51 Eridani b is the lowest-mass exoplanet directly imaged to date. Its 2015 discovery with the Gemini Planet Imager helps us in our understanding about how planets like Jupiter formed in our own Solar System some 4.5 billion years ago. (B. MACINTOSH ET AL., ARXIV:1508.03084)

But the closest one, newly discovered, is just 35 light-years away: COCONUTS-2b.

A variety of planets and brown dwarf stars that have been discovered to be in orbit around other stars. These widely-separated exoplanets emit mostly in the infrared, and are extremely low in their intrinsic luminosity. The newly discovered COCONUTS-2b is the second least luminous exoplanet known, behind only WD 0806–661B. (Z. ZHANG ET AL., ARXIV:2017.02805))

It was discovered back in 2011 by NASA’s WISE: a wide-field infrared telescope.

This all-sky view showcases the view of the sky as seen by NASA’s WISE telescope. The wide-field infrared survey explorer is well-suited for observing low-mass brown dwarfs, L- and T-tauri objects, and even super-Jupiters that emit their own infrared light. Some of these have since been identified as being exoplanets bound to other stars, including COCONUTS-2b. (NASA / JPL-CALTECH / WISE TEAM)

Recent work led to its identification as a widely-separated planet, bound to the dwarf star L 34–26.

The directly imaged exoplanet COCONUTS-2b orbits the M-dwarf star L 34–26, also known as COCONUTS-2a. These two objects are separated by a few thousand astronomical units and are located ~35 light-years away. They are bound together, although the parent star is so faint that Canopus would outshine it as viewed from COCONUTS-2b’s surface. (NASA/UNWISE & MELINA THÉVENOT)

It’s the second most well-separated exoplanet ever, behind TYC9486 b.

Many massive exoplanets are known to exist around other stars, but only a few have been directly imaged. COCONUTS-2b is one of only a handful of exoplanets with observed separation distances that exceed ~1000 AU, with only exoplanet TYC9486 b having a larger, wider separation. (Z. ZHANG ET AL., ARXIV:2017.02805)

It’s also the second faintest exoplanet ever found, behind WD 0806–661 B.

The white dwarf WD 0806−661 has a sub-brown dwarf companion. This image shows a near-infrared image from of ESO’s VLT/HAWK I (right) and the NASA/ESA Hubble Space Telescope WFC3 (left). The sub-brown dwarf was imaged by Hubble in 2014 and 2015 and the movement of this object is visible as there appear two points, one green and one purple (blue + red = purple) that represent the different filters and years the image was taken. (ESO’S VLT HAWK I & NASA/ESA HST WFC3)

The COol Companions ON Ultrawide orbiTS (COCONUTS) program is successfully identifying these massive, well-separated exoplanets.

An infrared image of the binary star and the newly discovered companion, but now viewed with special polarization filters that make dust discs and exoplanets visible. The companion seems to have its own dust disc. Note that only widely-separated objects that emit their own light can be seen in the glare of a luminous star at present. (C. GINSKI & SPHERE)

However, rocky exoplanets cannot be directly imaged yet.

If the Sun were located 10 parsecs (33 light years) away, not only would LUVOIR be able to directly image Jupiter and Earth, including taking their spectra, but even the planet Venus would yield to observations. (NASA / LUVOIR CONCEPT TEAM)

A properly equipped next-generation space telescope, like HabEx or LUVOIR, will someday reveal those worlds.

This artist’s concept shows the geometry of a space telescope aligned with a starshade, a technology used to block starlight in order to reveal the presence of planets orbiting that star. From tens of thousands of kilometers away, the starshade and the telescope must achieve and maintain perfect alignment to enable direct exoplanet imaging, but this is possible with current technology. (NASA/JPL-CALTECH)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


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