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This cartoon explains why the reported sizes of
some exoplanets may need to be revised in cases where there is a second
star in the system. Credits: NASA/JPL-Caltech. Larger labeled view
In the search for planets similar to our own, an important point of
comparison is the planet's density. A low density tells scientists a
planet is more likely to be gaseous like Jupiter, and a high density is
associated with rocky planets like Earth. But a new study suggests some
are less dense than previously thought because of a second, hidden star
in their systems.
As telescopes stare at particular patches of sky, they can't always
differentiate between one star and two. A system of two closely orbiting
stars may appear in images as a single point of light, even from
sophisticated observatories such as NASA's Kepler space telescope. This
can have significant consequences for determining the sizes of planets
that orbit just one of these stars, says a forthcoming study in the Astronomical Journal
by Elise Furlan of Caltech/IPAC-NExScI in Pasadena, California, and
Steve Howell at NASA's Ames Research Center in California's Silicon
"Our understanding of how many planets are small like Earth, and how
many are big like Jupiter, may change as we gain more information about
the stars they orbit," Furlan said. "You really have to know the star
well to get a good handle on the properties of its planets."
Some of the most well-studied planets outside our solar system -- or
exoplanets -- are known to orbit lone stars. We know Kepler-186f, an
Earth-size planet in the habitable zone of its star, orbits a star that
has no companion (the habitable zone is the distance at which a rocky
planet could support liquid water on its surface). TRAPPIST-1, the
ultra-cool dwarf star that is home to seven Earth-size planets, does not
have a companion either. That means there is no second star
complicating the estimation of the planets' diameters, and therefore
But other stars have a nearby companion, high-resolution imaging has
recently revealed. David Ciardi, chief scientist at the NASA Exoplanet
Science Institute (NExScI) at Caltech, led a large-scale effort to
follow up on stars that Kepler had studied using a variety of
ground-based telescopes. This, combined with other research, has
confirmed that many of the stars where Kepler found planets have binary
companions. In some cases, the diameters of the planets orbiting these
stars were calculated without taking the companion star into
consideration. That means estimates for their sizes should be smaller,
and their densities higher, than their true values.
Previous studies determined that roughly half of all the sun-like
stars in our sun's neighborhood have a companion within 10,000
astronomical units (an astronomical unit is equal to the average
distance between the sun and Earth, 93 million miles or 150 million
kilometers). Based on this, about 15 percent of stars in the Kepler
field could have a bright, close companion -- meaning planets around
these stars may be less dense than previously thought.
The Transit Problem for Binaries
When a telescope spots a planet crossing in front of its star -- an
event called a "transit" -- astronomers measure the resulting apparent
decrease in the star's brightness. The amount of light blocked during a
transit depends on the size of the planet -- the bigger the planet, the
more light it blocks, and the greater the dimming that is observed.
Scientists use this information to determine the radius -- half the
diameter -- of the planet.
If there are two stars in the system, the telescope measures the
combined light of both stars. But a planet orbiting one of these stars
will cause just one of them to dim. So, if you don't know that there is a
second star, you will underestimate the size of the planet.
For example, if a telescope observes that a star dims by 5 percent,
scientists would determine the transiting planet's size relative to that
one star. But if a second star adds its light, the planet must be
larger to cause the same amount of dimming.
If the planet orbits the brighter star in a binary pair, most of the
light in the system comes from that star anyway, so the second star
won't have a big effect on the planet's calculated size. But if the
planet orbits the fainter star, the larger, primary star contributes
more light to the system, and the correction to the calculated planet
radius can be large -- it could double, triple or increase even more.
This will affect how the planet's orbital distance is calculated, which
could impact whether the planet is found to be in the habitable zone.
If the stars are roughly equal in brightness, the "new" radius of the
planet is about 40 percent larger than if the light were assumed to
come from a single star. Because density is calculated using the cube of
the radius, this would mean a nearly three-fold decrease in density.
The impact of this correction is most significant for smaller planets
because it means a planet that had once been considered rocky could, in
fact, be gaseous.
The New Study
In the new study, Furlan and Howell focused on 50 planets in the
Kepler observatory's field of view whose masses and radii were
previously estimated. These planets all orbit stars that have stellar
companions within about 1,700 astronomical units. For 43 of the 50
planets, previous reports of their sizes did not take into account the
contribution of light from a second star. That means a revision to their
reported sizes is necessary.
In most cases, the change to the planets' reported sizes would be
small. Previous research showed that 24 of the 50 planets orbit the
bigger, brighter star in a binary pair. Moreover, Furlan and Howell
determined that 11 of these planets would be too large to be planets if
they orbited the fainter companion star. So, for 35 of the 50 planets,
the published sizes will not change substantially.
But for 15 of the planets, they could not determine whether they
orbit the fainter or the brighter star in a binary pair. For five of the
15 planets, the stars in question are of roughly equal brightness, so
their densities will decrease substantially regardless of which star
This effect of companion stars is important for scientists
characterizing planets discovered by Kepler, which has found thousands
of exoplanets. It will also be significant for NASA's upcoming
Transiting Exoplanet Survey Satellite (TESS) mission, which will look
for small planets around nearby, bright stars and small, cool stars.
"In further studies, we want to make sure we are observing the type
and size of planet we believe we are," Howell said. "Correct planet
sizes and densities are critical for future observations of high-value
planets by NASA's James Webb Space Telescope. In the big picture,
knowing which planets are small and rocky will help us understand how
likely we are to find planets the size of our own elsewhere in the