Twenty-two light-years away, a rocky world orbits a red dwarf. It\u2019s called LTT 1445Ac, and NASA\u2019s Transiting Exoplanet Survey Satellite (TESS) found it in 2022. However, TESS was unable to gauge the small planet\u2019s size. <\/p>\n
That\u2019s okay. The venerable Hubble took care of it. <\/p>\n
<\/p>\n TESS detects planets with the transit method. When an exoplanet transits in front of its star, it creates a measurable dip in light that TESS detects. But things have to line up right. The planet has to transit between TESS and the star. When that happens, TESS can not only detect the planet but also measure its size to some degree. <\/p>\n But LTT 1334Ac is kind of in no-man\u2019s-land from our perspective. The geometry is right for detection, but it\u2019s misaligned just enough that the planet may have performed a \u2018grazing transit.\u2019 That\u2019s when the planet transits in front of a portion of the star but not all of the star. That means that size measurements give an inaccurate lower limit for the planet\u2019s diameter. Unfortunately, TESS doesn\u2019t have the resolution to tell us whether LTT 1334Ac was performing a grazing transit or not. <\/p>\n But the workhorse Hubble Space Telescope does.<\/p>\n \u201cThere was a chance that this system has an unlucky geometry, and if that\u2019s the case, we wouldn\u2019t measure the right size. But with Hubble\u2019s capabilities, we nailed its diameter,\u201d said Emily Pass of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.<\/p>\n \u201cOur measurement is important because it tells us that this is likely a very nearby terrestrial planet.\u201d <\/p>\n Emily Pass, Harvard-Smithsonian CfA<\/cite><\/p><\/blockquote>\n<\/figure>\n Pass is the lead author of a new paper in The Astronomical Journal titled \u201cHST\/WFC3 Light Curve Supports a Terrestrial Composition for the Closest Exoplanet to Transit an M Dwarf.\u201d <\/p>\n \u201cPrevious studies of the exoplanet LTT 1445Ac concluded that the light curve from the Transiting Exoplanet Survey Satellite (TESS) was consistent with both grazing and nongrazing geometries. As a result, the radius and hence density of the planet remained unknown,\u201d the paper states. But not anymore.<\/p>\n Hubble determined that LTT 1334Ac does make a full transit across its star rather than a grazing transit. That means that the planet\u2019s actual diameter is 1.07 times Earth\u2019s diameter. It also confirms that the planet is rocky and has about the same surface gravity as Earth. <\/p>\n Grazing transits are an obstacle in exoplanet science. TESS is a powerful planet-finder, but it can\u2019t do everything. It can\u2019t always determine if an exoplanet is performing a grazing transit or a full transit. Astronomers are developing models of them to try to understand how to detect them better and how abundant grazing transits are. <\/p>\n In a 2022 paper, researchers wrote that even though they\u2019re rare due to geometric selection effects, \u201cGrazing transits present a special problem for statistical studies of exoplanets.\u201d They also explained that \u201cA failure to accurately model grazing transits can, therefore, lead to biased inferences even for cases where the planet is not actually on a grazing trajectory.\u201d<\/p>\n There\u2019s no full solution to grazing transits yet. But at least the Hubble can help in some cases. And though its observations determined the planet\u2019s size and its rocky makeup, habitability is out of the question. LTT 1334Ac\u2019s surface temperature is about 260 Celsius (500 F.) Water would boil away at that temperature, and life would have no chance. <\/p>\n There are two other planets in the system, both larger than LTT 1334Ac. There are also two more stars in the system, and all of them are red dwarfs. There\u2019s a lot going on in this system, but observations show that the entire system is likely co-planar. And the fact that LTT 1334Ac performs full transits means it\u2019s a great candidate for more study.<\/p>\n \u201cAs the nearest terrestrial exoplanet to transit an M dwarf (alongside LTT 1445 Ab), this planet is an exciting target for atmospheric characterization, particularly now that it is known to be nongrazing and its radius is therefore appropriately constrained,\u201d the paper states. <\/p>\n \u201cTransiting planets are exciting since we can characterize their atmospheres with spectroscopy, not only with Hubble but also with the James Webb Space Telescope,\u201d Pass said in a press release. \u201cOur measurement is important because it tells us that this is likely a very nearby terrestrial planet. We are looking forward to follow-on observations that will allow us to better understand the diversity of planets around other stars,\u201d said Pass.<\/p>\n This research highlights the synergy between different telescopes and how that synergy can lead to deeper understandings. It also shows that the Hubble is still a powerful science instrument. <\/p>\n \u201cHubble remains a key player in our characterization of exoplanets,\u201d said Professor Laura Kreidberg of the Max Planck Institute for Astronomy, who was not part of this study.\u00a0\u201cThere are precious few terrestrial planets that are close enough for us to learn about their atmospheres \u2014 at just 22 light years away, LTT 1445Ac is right next door in galactic terms, so it\u2019s one of the best planets in the sky to follow up and learn about its atmospheric properties.\u201d <\/p>\n The paper\u2019s authors agree. \u201cAs the nearest terrestrial exoplanet to transit an M dwarf (alongside LTT 1445 Ab), this planet is an exciting target for atmospheric characterization, particularly now that it is known to be nongrazing and its radius is therefore appropriately constrained,\u201d they conclude.<\/p>\n\n
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