OUR SEARCH FOR EXTRASOLAR PLANETS CONTENTS 2 The exoplanet next door What have we learned about Proxima Centauri’s Earth-mass exoplanet? 6 The hunt for Earth’s bigger cousins Super-Earths and sub-Neptunes may host conditions like our home planet — or not. 12 Why we haven’t found another Earth. Yet. Finding Earth-like planets is still a challenge for astronomers. NASA/AMES/JPL-CALTECH A supplement to Astronomy magazine Next-door planets THE EXOPLANET NEXT DOOR There’s no star closer to us than Proxima Centauri — and now we know it has an Earth-mass planet in its habitable zone. by John Wenz THE HUNT FOR EXOPLANETS HAS, the Kepler catalog, says. “Twenty years ago, in some ways, been about the hunt for we were finding the first exoplanets, and it an Earth-like planet — something warm was totally exciting,” he says. Then there where water could exist. Headlines tout was the Kepler space telescope, which each discovery as “the most Earth-like found thousands of planets, including planet yet.” Many of those planets are far some in the habitable zone, and some distant. within a few dozen light-years of us. A new discovery published August 24 in And now there’s a planet named Nature hits closer to home, though, with an Proxima Centauri b of 1.3 Earth masses An artist’s impression of the surface of Earth-mass planet in the habitable zone of right next door, zipping around its star in Proxima Centauri b, drawn as if it has an its star. What’s more, that star is Proxima 11.2 days. Its distance of around 4.3 million atmosphere. Whether or not an atmosphere Centauri, only 4.24 light-years away. miles (7 million kilometers) from its star exists is not yet known. ESO/M. KORNMESSER This is a landmark discovery: finding a may seem tiny, at less than one-fifth the planet orbiting the closest star to the Sun. distance between Mercury and the Sun, but And so far, this exoplanet shows some sim- Proxima Centauri is the runt of the litter in star). In order for instruments to directly European Southern Observatory’s HARPS favor of the solid, concrete evidence. other things that could generate a spurious ilarity to Earth, as it’s roughly the mass of the Alpha Centauri system. At a diameter image them, planets must be very young instrument, a spectroscope that measures The researchers also were tight-lipped Doppler signal at 11 days, but after looking our planet and in just the right place where, of around 125,000 miles (200,000km), it’s and still glowing, or else very large — not tiny wobbles in a star’s light. about their findings, awaiting the end of at all that, they concluded the variability in if it has an atmosphere, liquid water could only 1.43 times the diameter of Jupiter. possible with the 5-billlion-year-old “Instead of applying for a few nights the peer review process. But as their the data is best explained by the presence exist on the surface. So how was there a planet hiding Proxima b. over a semester and repeating the same intended press conference approached, an of a planet.” This is as in our backyard as it gets. around the closest star to us, just waiting to thing over the years, what we did here is anonymous ESO astronomer leaked the Part of the caginess may arise from a “I think it actually marks a transition,” be discovered? The simple answer: Finding How to find a planet (that try to convince ESO it was worth doing story to the German press, sending Pale 2012 detection of a planet around another Jeffrey Coughlin, a SETI Institute scientist a planet is really hard. Kepler found thou- doesn’t want to be found) intensive campaigns to monitor the star 60 Red Dot into damage control, with the star in the system, Alpha Centauri B. That not involved in the study who assembles sands of planets by staring at 145,000 stars That’s why the Pale Red Dot project, tasked days in a row, only 20 minutes per night,” team keeping a tighter lid on its findings as planet, aptly named Alpha Centauri Bb, in a minute region of the sky at the tail end with finding a planet around our nearest principal investigator Guillem Anglada- rumors swirled in the astronomy commu- was too hot to sustain life, but instantly of Cygnus, waiting for the 1 percent chance neighbor, had to turn to indirect — but reli- Escudé said in a press conference. nity. Nature was forced to address the became the closest planet to us by default. a planet would pass directly in front of a able — methods of detection. The research- The end result is a planet as close to us rumors in its materials to the press before Or it would have, if it actually existed. star and cause a dip in its light, in a ers chose radial velocity, a process that looks as any could be, outside of a similar discov- the official announcement. The detection was riddled with prob- method known as transiting. for shifts in a star’s light due to the tug of ery in the same system. Going back to the 2013 detection: the lems, drawn out from spurious data, and The problem with Proxima Centauri b a planet, sometimes called the Doppler The project team, however, has been evidence was initially weak. Subsequent ignored a low signal-to-noise ratio in is that it doesn’t transit — at least not from method. Gravity induces subtle movements cautious every step of the way. They observations bolstered the planet’s case, but search of a sensational new planet, the kind our vantage point. In order to witness a that cause the light of a star to move toward ignored weak evidence of the planet’s exis- it wasn’t until the most recent observation science fiction has long dreamed of. transit, the orbital plane of a planet must the blue end of the light spectrum, which tence stretching back to 2013 in favor of campaign that proof for a planet and not Instead of becoming an Earth-shattering be at or near our line of vision, but not all means it’s moving toward us, or the red end stronger observations over the subsequent some other phenomenon solidified. Given revelation, serious doubts were cast on the solar systems have the same orientation. A of the spectrum, which means it’s moving three years, factoring in other studies of the this history, the team has been methodical detection, which also used radial velocity. star might have all of its planets aligned at away. Based on those changes, researchers star stretching back 16 years. Though fairly every step of the way. “[Pale Red Dot] actually just said the a 90-degree angle from us, with the planets can calculate a mass estimate, and the fre- convinced it’s a planet (above 99 percent “The authors do a great job in their most likely explanation is the presence of a orbiting in such a way that they never pass quency gives an idea of the orbit. certain, in fact) based on their data, the analysis. They follow all protocol and all planet,” Seager says. “If you remember in front of their star, enabling our tele- The planet itself was found over a series researchers still refer to the planet as a can- standard techniques,” Sara Seager, an MIT Alpha Centauri Bb … I just think there’s a scopes to detect them. Some planets have of nights from January 19 to March 31, didate. A possible second planet in the sys- exoplanet researcher and astrobiologist, concern in the community that every This artist’s view shows how Proxima Centauri been found by direct imaging (that is, a 2016, during which Proxima was moni- tem with a 60-day or longer orbit only gets says. “And they do say that they looked at retraction looks bad, even though that one may look from just behind Centauri b. ESO/M. KORNMESSER planet appearing in a photo along with its tored closely for subtle variations on the a passing mention in the paper, ignored in all the different types of stellar activity and wasn’t officially retracted.” 2 ASTRONOMY • OUR SEARCH FOR EXTRASOLAR PLANETS © 2016 Kalmbach Publishing Co. This material may not be reproduced in any WWW.ASTRONOMY.COM 3 form without permission from the publisher. www.Astronomy.com This could be especially compounded In transit by the planet’s tidal locking to its parent The Pale Red Dot team found a planet. It The Proxima Centauri system star. Because of the small separation in the seems to be the right mass and the right system — the distance between Centauri b distance from its star to put something and its star is just 5 percent the distance of somewhat similar to Earth in our cosmic Habitable zone between Earth and the Sun — the same backyard. But reality is way more compli- side of the planet faces Proxima Centauri at cated than that, as seen by the aforemen- Proxima Centauri b all times, much as the same side of the tioned histories of Venus and Mars. Minimum mass: 1.27 Earth masses Astronomers need to observe the planet Orbital period: 11.19 days Moon faces Earth at all times. However, if Centauri b still has an in greater detail in order to further charac- Sun Proxima Centauri atmosphere, it could reach temperatures up terize it.