The planet candidate KOI-456.04 strikes me as significant not so much because of the similarity of its orbit with that of Earth (a 378 day orbital period around a star much like the Sun), but because of the methods used to identify its possible presence. Make no mistake, this is still very much a planet candidate, as co-authors René Heller and Michael Hippke are at pains to explain, noting that systematic measurement errors cannot be ruled out, though they estimate an 85 percent likelihood that it is there.
We don’t have many examples of small planets potentially in the habitable zone of a star like ours, and this is what has received the most media attention. So let’s look at this aspect of the story quickly, because I want to move past it. If this candidate is confirmed, it looks to be less than twice the radius of the Earth, receiving about 93 percent of Earth’s insolation from its star. Make assumptions about its atmosphere and you can arrive at a surface temperature averaging 5℃, 10 degrees lower than Earth’s mean temperature.
Image: Most of the exoplanets from the Kepler mission are the size of Neptune and in relatively close orbits around their host stars, where temperatures on these planets would be far too hot for liquid surface water (third panel from above). Almost all of the Earth-sized planets known to have potentially Earth-like surface temperatures are in orbit around red dwarf stars, which do not emit visible light but infrared radiation instead (bottom panel). The Earth is in the right distance from the Sun to have surface temperatures required for the existence of liquid water. The newly discovered planet candidate KOI-456.04 and its star Kepler-160 (second panel from above) have similarities to Earth and Sun (top panel). MPS / René Heller.
But what I want to dwell on is the methodology used to study this system. Heller (Max Planck Institute for Solar System Research) and Hippke (Sonneberg Observatory, Germany) are joined here by colleagues at the University of Göttingen, UC-Santa Cruz and NASA Ames in a new look at archival data from Kepler on the star Kepler-160 in Lyra, which was observed by the mission between 2009 and 2013. The star is similar to the Sun in mass and radius and previously known to have two confirmed planets.
The new work analyzes transit timing variations in the orbital period of the planet Kepler-160c suggestive of a third planet. They find Kepler-160d, a third world that is disturbing the orbit of Kepler-160c. This is a planet without any transits that is thus only indirectly confirmed.
The intriguing candidate, potentially the fourth planet here, is KOI-456.04, which appears to be 1.9 Earth radii in an orbital period of 378 days. The Max Planck Institute for Solar System Research (MPS) happens to be building the PLATO Data Center, and the suggestion is that the PLATO mission, to be launched in 2026, will have the chance to confirm this interesting object of interest and study it in much greater detail.
Heller and Hippke have been developing their exoplanet detection pipeline in several recent papers, studying twelve detrending algorithms for stellar light curves in detail. ‘Detrending’ refers to eliminating noise within transit data to cull out evidence for a planet. The results pointed to a detrending algorithm available in the open source package called Wōtan, used in combination with a transit search algorithm known as ‘transit least-squares’ as the most accurate choice. Heller and Hippke developed TLS specifically to look for smaller planets by modeling stellar limb darkening (see Dataset Mining Reveals New Planets for more on this).
What emerges is a more precise model of the brightness variations seen in a transit event, one that the duo believe improves upon the more established ‘box-like’ approximation known as the ‘box-fitting least square’ (BLS) algorithm. The latter is somewhat faster in computational terms, but the Wotan/TLS combination is in the authors’ view more sensitive. I talked to both Heller and Hippke about the new paper via email and asked Hippke about the advantages of their method. His response:
I… believe that Wōtan+tls are the leading toolset in finding new transiting exoplanets. You gain about 10% sensitivity going from BLS to TLS. In other words, at the same false alarm rate (e.g., 1%) you get 10% more planets from TLS. Naturally these are at the small end of the size distribution (you find large planets as easily with BLS). Smaller planets are usually more interesting because rocky planets are believed to be < ~ 2 Earth radii.
The dominating noise source in transit observations is in many cases stellar variability, which is why Heller and Hippke tested a dozen detrending methods, all of which are available through Wōtan (TLS is likewise an open source tool). According to Hippke, the Wōtan methods are more important for more active stars — remember that M-dwarfs can be quite active in comparison to G-class stars like the Sun. Young stars just at the end of planet formation are likewise active, making them interesting targets for using the Wōtan tools to achieve optimal detrending for exoplanet detection.
Heller told me that the team is 100% sure that Kepler-160d exists — this is the non-transiting world found through using transit timing variations of Kepler-160c. But what of the planet in the orbit roughly similar to that of the Earth around the Sun?
Our statistical analysis gives us 85% confidence that the signal belongs to a transiting planet. But 99% would be needed to call this a planet. In this case, this object would be called Kepler-160e. For now, it is not. So this object is transiting (I mean, if it is real in the first place), but we are less certain than for the non-transiting planet Kepler-160d that it actually exists. And so KOI-456.04 remains a candidate unless someone can show that it exists with more than 99% certainty.
Thus the tantalizing ‘world’ in the Earth-like orbit remains a Kepler Object of Interest (KOI), an object that cannot be currently validated or falsified, but one that will doubtless be on the target list for the PLATO exoplanet mission. The larger story is that the tools Heller and Hippke have deployed show the promise of pulling 10% more planets (and smaller ones at that) out of the raw data, which makes analysis of ongoing observations as well as reanalysis of older datasets more accurate. It will be fascinating to watch as the computational methods on display in this paper are applied to other known exoplanet systems, with their validity then put to the test by future space- and ground-based observatories.
The paper is Heller et al., “Transit least-squares survey. III. A 1.9 R⊕ transit candidate in the habitable zone of Kepler-160 and a nontransiting planet characterized by transit-timing variations,” Astronomy & Astrophysics, Volume 638, id. A10 (June, 2020). Abstract/preprint.