Hubble may have spotted a possible ‘exomoon’ | Science News for Students

Hubble may have spotted a possible ‘exomoon’

Unusually large, the moon tests theories on how such orbs form
Oct 17, 2018 — 6:45 am EST
an illustration of a dark exomoon orbiting a bright Jupiter-sized planet

The Hubble Space Telescope may have spotted a Neptune-sized moon orbiting a Jupiter-sized planet, as shown in this illustration.

Dan Durda

There may be a moon the size of Neptune far out in space. It appears to be orbiting a planet in another solar system. Such planets are known as exoplanets. So its moon would be exomoon. If it exists, this orb would challenge ideas of how moons are born.

David Kipping and Alex Teachey spotted signs of the exomoon with the Hubble Space Telescope. Both astronomers work at Columbia University in New York City. The pair pointed the telescope on the star Kepler 1625. Then it watched the star for 40 hours on October 28 and 29, 2017. Earlier, the Kepler space telescope had shown this star had a Jupiter-sized planet. It orbited its sun every 287 days. Scientists spotted the planet from dips in starlight reaching the telescope. Those dips were a darkening of starlight as the planet transited — passed in front of — the star.

But Teachey and Kipping saw something else in the Kepler data: signs of a second dimming. It happened either before or after the planet’s transit. And that second dimming is exactly what astronomers would expect if an exomoon were orbiting the planet.

The astronomers named this possible moon Kepler 1625b i. Informally, they call it Neptmoon. But there was always the chance it could simply be another planet. Or the dimming might reflect some activity on the star.  To be sure it really was a moon, the researchers needed more data.

That’s why they turned to Hubble. It is 3.8 times more sensitive to dips in light than Kepler is.

And Hubble also spotted the secondary dip in light after the planet’s 19 hour transit of its star. That transit started 77.8 minutes earlier than expected. This suggested something was tugging gravitationally on the planet. Kipping, Teachey and colleagues reported their finding October 3 in Science Advances.  

Both the dip in light and the early transit are consistent with there being a Neptmoon. Still, “we’re not cracking open champagne bottles just yet,” Teachey said in an Oct. 1 news conference. His team wants to check with Hubble again. Teachey said they might try in May 2019. That’s the next time the planet transits Kepler 1625.

René Heller is an astrophysicist at the Max Planck Institute for Solar System Research in Göttingen, Germany. He finds the new analysis of the transit data impressive. Still, he says, “I remain skeptical” that the exomoon exists. He, too, wants to see another transit and better observations of the star.

One reason to be cautious about the new data is how a moon as large as this one would form. In our solar system, moons have formed in only three ways. Some are knocked out of the body of a planet by an impact. Others form from gas and rock orbiting a planet. A moon might also be grabbed by the planet’s gravity. None of scenarios, however, should result in a moon as large as the new apparent exomoon.

“Kepler 1625b i, if real, would be about 10 times as massive as the mass of all moons and terrestrial planets in [our] solar system combined,” says Heller. “This suggests that this moon would have formed in a completely different way.”

Power Words

(more about Power Words)

astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

astrophysics     An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

colleague     Someone who works with another; a co-worker or team member.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

exomoon     A moon that orbits an exoplanet — which means it exists outside our solar system.

exoplanet     Short for extrasolar planet, it’s a planet that orbits a star outside our solar system.

gravity     The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

Jupiter     (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).

Kepler Space Telescope     A NASA mission to search for exoplanets — planets beyond the solar system — especially ones that might be Earth-like. The mission’s development began in 2002, by placing the first orders for the needed instruments that would be used. The mission was named for Johannes Kepler (1571 to 1630), the first person to describe the motions of planets about the sun so that their positions could be predicted accurately. The spacecraft carrying the telescope Kepler spacecraft lifted off March 6, 2009, at 10:49 p.m. from the Cape Canaveral Air Force Station in Florida. As of June 2017, it had turned up 2,335 confirmed exoplanets and almost 1,700 more possible ones.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

moon     The natural satellite of any planet.

Neptune     The furthest planet from the sun in our solar system. It is the fourth largest planet in the solar system.

orb     Something having a spherical shape.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.

scenario     A possible (or likely) sequence of events and how they might play out.

skeptical     Not easily convinced; having doubts or reservations.

solar system     The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

telescope     Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

terrestrial     Having to do with planet Earth, especially its land. Terra is Latin for Earth.

transit     (in astronomy) The passing of a planet, asteroid or comet across the face of a star, or of a moon across the face of a planet.


Journal:​ ​​ A. Teachey and D.M. Kipping. Evidence for a large exomoon orbiting Kepler 1625bScience Advances. Published online October 3, 2018. doi:10.1126/sciadv.aav1784.