How to cite LandSat 8?
Landsat products obtained via various sources (ex the EROS Center) can have their own requirements when using their products. For instance the EROS Data Citation page indicate that they are quite open regarding how their documents are cited and require the following acknowledgement only. For example

Acknowledgement:
Data available from the U.S. Geological Survey.
See USGS Visual Identity System Guidance (link is external) for further details.
Questions concerning the use or redistribution of USGS data should be directed to: ask@usgs.gov (link sends e-mail) or 1-888-ASK-USGS (1-888-275-8747).

Metadata files attached to images are often filled with useful information. Usual relevant information for a technical or scientific paper are the moment of acquisition, the scale, the bands, the name of the sensor, maybe the series of the product, maybe there is a serial number too, possibly a URL.
I have cited in a recent paper the following for Landsat 7, and did not went into much details because it was a reworked product (a special issued mosaic), which was convenient for our scope:
NRCan, Canada Centre for Remote Sensing. A complete orthorectified Landsat-7 mosaic
of the Canadian Arctic archipelago, August 2010. URL http://geogratis.gc.ca/api/en/nrcan-rncan/ess-sst/-/(urn:iso:series)a-complete-orthorectified-landsat-7-mosaic-of-the-canadian-arctic-archipelago