Kepler-40b

Lore & Background

Kepler-40b was first detected as a transit event by NASA's Kepler spacecraft during its initial observations from May 13, 2009, to June 15, 2009. The light curve revealed four transits over 33.5 days and was made publicly available by the Kepler science team. A team of French and Swiss astronomers used the SOPHIE échelle spectrograph at the Haute-Provence Observatory in southern France to follow up on the candidate. After removing background light and ruling out alternative explanations such as a close binary star, they analyzed the host star's properties and confirmed the planet's existence. The discovery demonstrated that smaller telescopes like SOPHIE could effectively follow up space missions like Kepler, using the equivalent of one night on a 1.93-meter telescope to gather all necessary data.

The host star, Kepler-40, is an F-type evolved subgiant nearing the main sequence turn-off, meaning it is about to fuse the last of its hydrogen and become a red giant. It has 1.48 times the mass of the Sun, 2.13 times its radius, an effective temperature of 6510 K, and a metallicity 25.9% higher than the Sun's. The planet orbits every 6.87 days at an average distance of 0.081 AU, five times closer than Mercury is to the Sun, and has an orbital inclination of 89.7°, nearly edge-on as seen from Earth. Kepler-40b is 2.2 times Jupiter's mass and 1.17 times its radius, giving it a density of 1.68 g/cm³, similar to Neptune's. Its equilibrium temperature was initially estimated at 1620 K, thirteen times hotter than Jupiter's; a 2015 model suggested the nightside temperature could be even hotter at 2327 K.

Reader's Guide

The discovery of Kepler-40b was notable for several reasons. It was the sixth transiting planet found orbiting a star with a radius exceeding 1.8 solar radii, a category that includes stars like Kepler-5 and Kepler-7. More importantly, the confirmation of Kepler-40b highlighted the effectiveness of ground-based follow-up observations using moderate-sized telescopes. The team used the SOPHIE spectrograph on a 1.93-meter telescope for roughly one night of measurements to collect all the data needed to establish the planet's existence and parameters. This demonstrated that smaller instruments could validate candidates from space missions like Kepler, which was crucial for the efficiency of exoplanet discovery. The planet's parameters also provided an example of a hot Jupiter with a density similar to Neptune's, despite being much larger and more massive. The host star's evolved state—nearing the red giant phase—added context for studying planetary systems around aging stars. The discovery was published in the journal Astronomy and Astrophysics on January 4, 2011, after submission on September 15, 2010.

Did You Know?

• The planet's density of 1.68 g/cm³ is similar to that of Neptune (1.638 g/cm³).
• Kepler-40b orbits its star every 6.87 days, about thirteen times faster than Mercury orbits the Sun.

The Photometric Vision

The Kepler space telescope, launched by NASA in 2009 under the leadership of principal investigator William J. Borucki, was built around a single elegant idea: watch stars for tiny, periodic dips in brightness. Its sole scientific instrument was a photometer locked onto a fixed patch of the Milky Way, continuously tracking the luminosity of roughly 150,000 main-sequence stars. When a planet crossed in front of its host star from Earth's vantage point, the resulting shadow produced a measurable dimming signature in the data stream. Crucially, this transit method only works for systems whose orbital planes are tilted nearly edge-on toward us, meaning Kepler could never detect planets orbiting face-on. Despite that geometric constraint, the spacecraft surveyed 530,506 stars over its operational life and, as of mid-2023, had yielded 2,778 confirmed exoplanets. The telescope was sent into an Earth-trailing heliocentric orbit and carries the name of seventeenth-century astronomer Johannes Kepler, who famously held the opposite belief—that no other planetary systems existed beyond our own.

Two Wheels Down, A New Mission Born

The Kepler mission was originally designed for a three-and-a-half-year operational window, but greater-than-expected noise from both the stars and the spacecraft itself meant more time was needed to meet all science goals. In 2012, NASA initially planned to extend operations through 2016. Then came the crisis: on July 14, 2012, one of the four reaction wheels responsible for pointing the spacecraft seized up. For a while, the remaining three could carry the load. But on May 11, 2013, a second wheel failed, and the mission's ability to collect science data was effectively lost. On August 15, 2013, NASA publicly acknowledged that the two dead wheels could not be repaired. Rather than ending the mission outright, the agency solicited the broader space-science community for creative alternatives using the two surviving wheels and thrusters. The result was the K2 "Second Light" proposal, unveiled on November 18, 2013, which repurposed the crippled spacecraft to hunt for habitable-zone planets around smaller, dimmer red-dwarf stars. NASA approved the K2 extension on May 16, 2014, and the telescope went on to discover four additional confirmed exoplanets under the new campaign.

A Thousand Worlds and Counting

Kepler's scientific output reached a symbolic milestone on January 6, 2015, when NASA announced the confirmation of its 1,000th exoplanet. That same batch included four planets in habitable zones: Kepler-438b, Kepler-442b, and Kepler-452b were nearly Earth-sized and likely rocky, while Kepler-440b was classified as a super-Earth. The following year, on May 10, 2016, a single verification event added 1,284 new exoplanets to the catalog—the largest one-time confirmation in history. Beyond individual discoveries, Kepler data reshaped our statistical picture of the galaxy. In November 2013, astronomers used the mission's findings to estimate that as many as 40 billion rocky, Earth-sized planets might orbit in the habitable zones of Sun-like stars and red dwarfs across the Milky Way, with roughly 11 billion circling Sun-like stars specifically. The nearest such world could be as close as 3.7 parsecs, or about 12 light-years. Kepler's half-hour cadence measurements also proved invaluable for catching and studying supernovae. After fuel depletion ended the mission on October 30, 2018, NASA's TESS spacecraft, launched that same year, inherited the exoplanet-hunting torch.

The Hardware: A Giant Eye in Space

Weighing 1,039 kilograms, the Kepler spacecraft housed a Schmidt camera whose 0.95-meter front corrector plate fed light onto a 1.4-meter primary mirror—at launch, the largest mirror on any telescope operating outside Earth's orbit, a title briefly overtaken by the Herschel Space Observatory months later. The instrument's field of view spanned 115 square degrees, roughly the angular size of a fist held at arm's length, with 105 square degrees delivering science-grade quality. Rather than chasing sharp images, the photometer was deliberately given a soft focus to maximize photometric precision, targeting a combined differential photometric precision of 20 parts per million for a Sun-like star over a 6.5-hour integration. The focal plane array consisted of 42 CCD chips, each 50 by 25 millimeters with 2,200 by 1,024 pixels, totaling 94.6 megapixels—the largest camera system ever launched into space at the time. Heat pipes connected the array to an external radiator for cooling, and the chips were read out every 6.5 seconds to prevent saturation before being co-added on board for either 58.89 or 1,765.5 seconds. The project sat within NASA's Discovery Program, with Jet Propulsion Laboratory managing construction and initial operations while Ball Aerospace built the flight system.