Sky & Telescope - September 2016 - 8

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News Notes

EXOPLANETS I 1,284 Added to Kepler's Conﬁrmed Tally

Newly validated
Kepler planets
Previously
veriﬁed planets

800
600
400

13.7 - 22
(super-Jupiter)

8.3 - 13.7
(Jupiter)

5.1 - 8.3
(sub-Jupiter)

3.1 - 5.1
(Neptune)

1.9 - 3.1
(sub-Neptune)

1.2 - 1.9
(super-Earth)

0.7 - 1.2
(Earth)

200

0.5 - 0.7
(Mars)

Number of planets

1,000

Known Transiting Planets by Size
as of May 10, 2016

Earth diameters (planet class)

Green bars represent all previously veriﬁed exoplanets, including those
detected by other methods and telescopes. Red bars represent Kepler's
1,284 newly validated planets. Surprisingly, the spike in super-Earths and
sub-Neptunes is real - it's not an observational bias.
S&T: LEAH TISCIONE, SOURCE: NASA AMES / W. STENZEL

A new analysis by the Kepler team has doubled the total number of conﬁrmed exoplanets.
Over four years, the Kepler spacecraft stared at a patch of
sky straddling the Cygnus-Lyra border and tallied up 4,696
exoplanet candidates. But to conﬁrm that any one planet is
real requires diﬃcult, time-consuming follow-up observations,
often impossible for the smaller candidates.
Timothy Morton (Princeton University) and colleagues took
a diﬀerent tack. Instead of looking for additional evidence sup-

porting a given candidate's existence, they searched for signs
that it wasn't there. Their fully automated code ﬁrst examined
the transit light curve itself - a real planet will block the star's
light in a certain way that can sometimes be distinguished from
imposters, such as an eclipsing binary. Then the code factored
in how common imposters might be. For example, how common are binary stars, and how many lie in the direction Kepler
was looking?
The algorithm studied each planet candidate for several
minutes, then spit out a probability that the signal is a fake. Any
candidate with less than a 1% chance of being a false positive is
now considered real.
The new study, reported in the May 10th Astrophysical
Journal, doubles the list of conﬁrmed exoplanets from 1,041 to
2,325. That tally includes worlds found with all methods and
telescopes, not just Kepler's transit detections (see graph). The
algorithm also "passed" 651 earlier Kepler detections that had
already been conﬁrmed by some other method. And 428 candidates (generally those with large radii, whose signals are more
easily mimicked by other astrophysical sources; S&T: Apr. 2016,
p. 13) were ﬂagged as imposters.
Of the newly conﬁrmed planets, nine orbit in their stars'
putative habitable zones, regions in which a planet with an Earthlike atmosphere could sustain liquid water on its surface. That
raises to 21 the number of planets with diameters less than twice
Earth's that are orbiting in their stars' habitable zones.
Expect one last Kepler catalog next year, says mission scientist
Natalie Batalha (NASA Ames). The secondary K2 mission is still
going strong and will likely end in mid-2018, when the spacecraft
runs out of fuel for its attitude-control thrusters.
■ MONICA YOUNG

SUPERNOVAE I Brightest Explosion Resurges
Last year, ASASSN-15lh gained fame
as the most luminous supernova ever
discovered (S&T: Nov. 2015, p. 12). Now
almost a year later and against all odds,
the supernova has rebrightened.
Subo Dong (Peking University, China)
and colleagues discovered ASASSN-15lh
on June 14, 2015, using the All-Sky Automated Survey for Supernovae (ASASSN).
Its peak power was more than twice that
of any previously known stellar explosion. Usually, much of a supernova's
initial glow actually comes from the
8

September 2016 sky & telescope

decay of radioactive nickel, created in
abundance near the core. What's weird
about ASASSN-15lh and a few others like
it is that they're so bright, they'd need an
awful lot of nickel to explain their glow.
Then, three months after it began
fading, the supernova changed its mind.
Over some 40 days, its ultraviolet radiation
increased ﬁvefold before plateauing for
another couple of months and ﬁnally dropping away again. Meanwhile, radiation at
visible wavelengths ignored this transformation and continued to fade unabated.

A born-again supernova isn't unheard
of. But typically when that happens, the
blast has run into nearby gas ejected by
the star before it exploded. ASASSN-15lh
doesn't display any of the emission lines
you'd expect in its spectra if this were the
case, explains Diego Godoy-Rivera (Ohio
State University) and others in a preprint
posted to the open-access site arXiv.org
on May 2nd.
The creation of a magnetar - until
now the most successful explanation for
the supernova's oddities - also struggles
to explain ASASSN-15lh's behavior.
■ MONICA YOUNG

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Sky & Telescope - September 2016

Table of Contents for the Digital Edition of Sky & Telescope - September 2016